Talk:Acid dissociation constant

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Former featured article candidate Acid dissociation constant is a former featured article candidate. Please view the links under Article milestones below to see why the nomination failed. For older candidates, please check the archive.
Article milestones
Date Process Result
July 30, 2008 Peer review Reviewed
October 25, 2008 Featured article candidate Not promoted
December 6, 2008 Featured article candidate Not promoted
Current status: Former featured article candidate
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All discussions prior to FAC have been archived. Petergans (talk) 09:18, 30 October 2008 (UTC)

Equilibrium symbol[edit]

There is some discussion at Wikipedia talk:Accessibility#Equilibrium_symbol that you may find of interest. --Itub (talk) 15:53, 17 October 2008 (UTC)

Thanks for this info. In fact I already use a picture in the documentation for my programs (HTML, Dreamweaver). BUT, is not the only <math> ...</math> in the article, so if it's not rendered correctly then there may be problems elesewhere. Unless we get a FAC comment to the contrary I suggest things are left as they are. Petergans (talk) 08:19, 19 October 2008 (UTC)
I've changed my mind on this and have replaced with ⇌ throughout. However, I think the alignment is not quite right. In
acid + base ⇌ conjugate ...
I suggest the space between the horizontal lines should line up with the (lower case) text characters. Petergans (talk) 17:20, 29 October 2008 (UTC)

Comments brought over from FAC[edit]

I was in the middle of updating my comments at FAC when Sandy archived the nomination, meaning that it will not be promoted to FAC this time around. I thought it might be useful to leave my remaining comments here. I will only add comments here on issues that you have responded to but for which I still feel there is an issue.

  • The "struck O" notation is in error in a couple of places in that you have "O" instead of "O". I changed all the struck Os to thetas because of this, before I figured out that the problem was just the sequence of tags.
  • I still don't think the table of dielectric constants needs to be there.
  • With regard to the listy section; yes, I think separate paragraphs might work. The {{main}} templates do need to go, though.

If you would like an outside reviewer for any further changes, please just let me know. I will respond if I have time. Mike Christie (talk) 23:18, 25 October 2008 (UTC)


Here are some follow up notes. I will try to add a little each evening; I am also looking through the changes made since FAC.

The reason for the repeated link was that carbonic acid is much more informative in regard to the chemistry and role of the pK values than is acid-base homeostasis. Therefore I've taken out the first link, rather than the second as suggested. Petergans (talk) 08:33, 30 October 2008 (UTC)
Yes, I saw that, and guessed that's why you linked the way you did. I think that's OK. The relevant guideline is here, and strictly interpreted we would only link the first occurrence. I think we can make the argument that the links are necessary to help the reader because of the state of the target articles. Mike Christie (talk) 22:53, 30 October 2008 (UTC)
I've re-worded the links to carbonic acid molecule and carbonic acid so the reader will see that the first link refers to the structure of the molecule and the second to its chemistry. It so happens that both aspects are contained in the same article, but it is less obvious that this is overlinking.Petergans (talk) 08:43, 31 October 2008 (UTC)
  • I've switched some en dashes to hyphens. I think most should be uncontroversial; see WP:MOSDASH if you want the gory details but I did check some in Google Books scans of chemistry texts to be sure I was following standard practice. One I am not sure of is "L-(+)–Ascorbic Acid"; the first dash should be a hyphen, but is that second dash actually a minus sign? I think it is and have changed it accordingly; if it is not, please let me know or change it back. Here is the diff of the change to all the dashes. I don't know if you're aware, but the dashes that appears under the edit window as an "Insert" symbol choice are an en dash and an em dash respectively; there's a minus sign further to the right. I tend to use the entity names: "&mdash;", "&ndash;" and "&minus;" myself, to make it easier to actually see which is which when editing. However, this can be quite distracting when you have equations, as you do.

More tomorrow, if I have time. Mike Christie (talk) 01:31, 30 October 2008 (UTC)

The one in ascorbic acid should be a hyphen and I've changed it accordingly. --Itub (talk) 06:35, 30 October 2008 (UTC)
OK, thanks. I'll assume the others are OK unless I hear otherwise. Mike Christie (talk) 22:53, 30 October 2008 (UTC)

Notes Oct 30th[edit]

More notes.

  • In the section on monoprotic acids, this appears: "The extent of dissociation and pH of a solution of a monoprotic acid can be easily calculated when the pKa and analytical concentration of the acid are known. See ICE table for details." I think it would flow more smoothly if we could avoid the "see" construction by integrating the reference to the ICE table into the text. How about: "When the pKa and analytical concentration of the acid are known, the extent of dissociation and pH of a solution of a monoprotic acid can be easily calculated using an ICE table."?
  • Single sentence paragraphs are somewhat jerky to read. The suggested revision above would make that paragraph a single sentence. I think it could be moved up and appended to the paragraph that currently ends "classified as strong bases."
Both agreed. BTW, ICE table is one of many articles linked from this page that I have revised. Gibbs free energy, however, was not one of them, which is why the notation is different there. Petergans (talk) 09:59, 31 October 2008 (UTC)
OK, that explains the notational discrepancy. Mike Christie (talk) 21:22, 1 November 2008 (UTC)
  • In the section on "Acidity in nonaqueous solutions" you mention methanol as an alternative solvent. The following discussion does not mention methanol again. Is methanol of sufficient interest as a solvent in this discussion to warrant its inclusion in the table of dielectric constants, or to add a column for it in the tables of pKa values?
Methanol is important but I don't have any pK data to hand. The research paper [1] could be cited but, as the title suggests, it is not specifically about pKa values; it is about the determination of stability constants of coordination complexes in which pKa values are needed as subsidiary information, an application which is mentioned in the Importance... section. I may be able to fish out some pKa values from my database. Petergans (talk) 09:59, 31 October 2008 (UTC) In the database I found a few horribly complicated molecules have been done in pure methanol, but more publications referred to mixed methanol/water systems. Unfortunately, my database does not cover molecules of pharmaceutical interest, which would be the main area of application. Petergans (talk) 10:55, 31 October 2008 (UTC)
The question is if another expert in this field were to read this section of the article, would they have a legitimate complaint that the article does not treat methanol as thoroughly as it should? I'll take your word for it either way on this; I have no way of knowing. Mike Christie (talk) 21:22, 1 November 2008 (UTC)
The sentence referring to methanol has been removed. Other solvents could have been mentioned too, but determination of pKa in those solvents is a minority sport. Petergans (talk) 12:14, 2 November 2008 (UTC)
OK. I've struck this comment on the assumption that it's OK not to mention methanol. Mike Christie (talk) 14:52, 2 November 2008 (UTC)
  • It would be nice to avoid this single-sentence paragraph: "In solvents of low dielectric constant ions tend to associate forming ion pairs and clusters, which complicates the interpretation of pKa values". Is there more that can be said here? I'm not knowledgeable about chemistry, so these suggestions may not be suitable, but some additional questions that could be answered in this paragraph occurred to me: What would be a reason for using a solvent of low dielectric constant despite this problem? What is a typical example of such a solvent in wide use? Do the complications prevent the measurement of pKa completely? If not, are there techniques that could be mentioned?
The sentence has been removed, as it is of marginal significance. Low dielectric constant is much more significant for metal ion coordination complexes, which is not the topic here. Petergans (talk) 10:55, 31 October 2008 (UTC)
  • Instead of "temperature /Kelvin" I would suggest either "temperature in degrees Kelvin" or "temperature in Kelvin", whichever would sound the most natural to someone with a chemistry background. The current version may be standard usage but looks odd to a lay reader.
OK Petergans (talk) 09:59, 31 October 2008 (UTC)

More later, or possibly over the weekend -- I am travelling tomorrow and may not be editing much till Saturday or Sunday, though I may get a short time online tomorrow night. Mike Christie (talk) 23:35, 30 October 2008 (UTC)

Notes Nov 1st[edit]

  • The caption to the image in the lead starts "The weak acid acetic acid donates a proton to water in an equilibrium reaction to give the acetate ion and the hydronium ion". Two adjacent links that run blue text together are discouraged, and I think "acetic acid" is more useful to state first here, so how about: "Acetic acid, a weak acid, donates a proton to water in an equilibrium reaction to give the acetate ion and the hydronium ion"?
Accepted.Petergans (talk) 12:14, 2 November 2008 (UTC)
  • I also think you can cut the word "Key:" from the caption; shorter is better for captions, and it is evident that the following text provides a key.
OK Petergans (talk) 12:14, 2 November 2008 (UTC)
  • The "See also" section links to several articles that are already linked in the body of this article. See WP:SEEALSO for the guideline on this; generally there's no need to have a link here if it appears in the body. There is an exception for editorial discretion, if necessary; are any of these needed? Personally I tend to think that anything important enough to link here should probably be linked somewhere in the article, so I usually try to eliminate these sections. The ones that are repeated in the article are isoelectric point, Henderson–Hasselbalch equation, Determination of equilibrium constants and Hammett equation. Dissociation constant appears in the navbox near the top (which I will comment on in a bit; I think it might be better as a template). I'd suggest deleting all the above links from the "See also" section. That leaves two: Hydrolysis of metal salts and QSAR. Is there a natural place where these could be linked from the body of the article?
Removed all but one of the links. The remaining one is a related topic which is not suitable for linking from this article. Hammett equation and Taft equation are close relatives, see below. Petergans (talk) 12:14, 2 November 2008 (UTC)
OK. Dissociation constant is in fact linked from one of the navboxes, so it could be cut too, but I see the logic in leaving it there. If that navbox were to end up at the bottom of the article as a template I'd suggest cutting it in that case, as the two links would be quite close to each other then. Mike Christie (talk) 14:56, 2 November 2008 (UTC)
  • The external links in the "External links" section should each have a short description so that a reader can understand why they might wish to go to that site. The layout guideline for links is here.
Done Petergans (talk) 12:14, 2 November 2008 (UTC)
  • There are two navboxes: one in the lead, and one early in the first section. WP:ACCESS recommends that a navbox in the lead comes before the text, to make it easier to read. Graham87, who is blind and uses a screen reader, commented on the accessibility of the text at this talk page section at Sandy's request. You can see his comments there. How about making one of the navboxes an end-of-page template, along the lines of the "Solutions" template already at the end? More than one template at the end is OK. I would also suggest making the navbox that is not already a template into a template, just to make it easier to edit. Graham didn't say that the location of the navbox after the intro text is a problem, but it might be best to try it in both locations and move it to comply with the guideline unless it causes problems.

-- Mike Christie (talk) 22:49, 1 November 2008 (UTC)

The first navbox has been removed. All the essential links contained in it either were in the article or have now been added, except for dissociation constant and proton affinity which are related articles, but which are not on topic here. Petergans (talk) 16:03, 2 November 2008 (UTC)
OK. Graham's remaining comments were that it would be good to be able to hide the navbox, and that the equilibrium symbol should use Template:Eqm. On the first point I have left a note with another editor who is expert on templates to see if it is easy to make Template:Chemical equilibria hideable. On the second point, he doesn't give a reason, but I would guess it's because the image you're using (Image:Equilibrium.svg) can't be read easily by speech synthesizers. Is there any reason not to use Template:Eqm? Mike Christie (talk) 17:23, 2 November 2008 (UTC)
Geometry guy just made the template hideable, so that resolves that question. I think it should start visible, as it is now, but I will check with Graham and see if there is a strong preference for it to start hidden for any reason. Mike Christie (talk) 17:50, 2 November 2008 (UTC)
I see most of the harpoons have been replaced with {{Eqm}}. I have replaced the remaining two. I don't have time to do more tonight, and may not have much time tomorrow, but will see what I can work in. Mike Christie (talk) 02:35, 4 November 2008 (UTC)
  • I've added a link to the Taft equation and have revised that article. It contained an elementary error of the kind that gives WP the reputation of being "patchy" in quality of articles. The error was to use k, not K, for an equilibrium constant. This might seem trivial but it is not. k is used to represent a kinetic rate constant. What made be blow my top was the fact that this error was copied over from the IUPAC gold book,[2] which many WP editors believe is gospel! Petergans (talk) 12:14, 2 November 2008 (UTC)
  1. ^ Gibson, G. (2003). "Potentiometric titration of metal ions in methanol". J. Chem.-Rev. Can. Chim. 81: 495–504. doi:10.1139/v03-035.  Unknown parameter |coauthors= ignored (|author= suggested) (help)
  2. ^ Definition IUPAC Gold Book 1996 Article
    • The article on the Taft equation was actually correct; this equation is used for calculating rate constants. Perhaps, like other free energy relations, it is also used with equilibrium constants, but it is certainly used with rate constants and I think that was the original use (the rates of a certain hydrolysis reaction). I looked at several books and all most used the lowercase k. --Itub (talk) 13:55, 2 November 2008 (UTC)
      • Thanks for this correction. The article did not say that k is a rate constant, I just assumed it was an equilibrium constant. I'll revert and remove Taft from this article. The comment about Gold Book still stands as that was the source of the unexplained equation. Petergans (talk) 16:03, 2 November 2008 (UTC)

Notes Nov 2nd[edit]

  • The images are all copyright compliant; at least one is missing a description but that doesn't have to be fixed for this article. However, the graphs all need a source of some kind. The source doesn't have to supply the identical graph, or the exact numbers used in the construction of this graph; it just has to give an interested reader a place to go to see that the graph presented matches a reliable source. A reference to a handbook of data, or to a textbook that contains similar graphs, would suffice. The image articles can contain the source information themselves -- it doesn't have to be added to this article.
I think the graphs you to which you refer are ones that I created myself in EXCEL or, for speciation diagrams, with my own program HySS. I have previously tried to to modify the source information for an image, but could not see how to do it. What is the procedure? Petergans (talk) 16:03, 2 November 2008 (UTC)
There's no requirement to do it a special way; I'd suggest you just hit "Edit this page" when looking at the image and add a "==References==" or "==Sources==" section, and add the sources in whatever way you like. Here is an image that I added references to recently; this one actually needed three or four references. Mike Christie (talk) 17:45, 2 November 2008 (UTC)
Done Petergans (talk) 09:46, 3 November 2008 (UTC)
  • Some of the references don't have page numbers or chapter references so that the reader can locate the relevant information. In some cases this may be legitimate if the reference is to a discussion that is carried on through an entire book or article, but otherwise a more specific location is needed. I looked at this version (specified in case you add or change references, since I want to refer to the footnotes by number). The following references seem to me to need a page number or range, chapter number, or section number: 5, 7, 10, 13, 23, 25, 34, 35, 36, 37, 38, 40, 41, 42, 43 and 44.

-- Mike Christie (talk) 15:27, 2 November 2008 (UTC)

Fixed all but 34 to 44 (current 36 to 46). These last are not references to any specific material cited in the article. Rather, they are a kind of additional reading in relation to the applications to which pKa values have been put. I think they come under the "legimitimate" category mentioned above. The book titles indicate that the whole book is concerned with a particular topic. Petergans (talk) 11:12, 3 November 2008 (UTC)
All appear to be fixed except reference 13, March & Smith's Advanced Organic Chemistry, which appears to be used just for the fact that acetonitrile is used to obtain pKa values for organic compounds. Mike Christie (talk) 11:49, 4 November 2008 (UTC)

Template to category?[edit]

As this article has evolved the templates have become less and less important. I now suggest that the remaining template:Chemical equilibria be replaced by a new category: Equilibrium chemistry. This category will contain all the topics now linked in template:Chemical equilibria and many more, as detailed in the provisional list below. Unfortunately I don't know how to create a category, so if this suggestion is acceptable, can some please help to create it.

name: Equilibrium chemistry

sub-category of : Chemistry

home chemical equilibria

categories: pH indicators, Acid-base chemistry, Thermodynamics, Separation processes, Solutions

acid-base extraction*acid-base homeostasis*acid–base reaction*acid dissociation constant*binding constant*Brønsted–Lowry acid-base theory*buffer solution*dissociation constant*determination of equilibrium constants*distribution coefficient*distribution ratio*equilibrium constant*equilibrium stage*equilibrium unfolding*Gibbs free energy*Haber Bosch process*haemoglobin*Henderson–Hasselbalch equation*hydrolysis*ICE table*law of mass action*Le Chatelier's principle*liquid-liquid extraction*Partition coefficient*phase diagram*phase rule*Protein pKa calculations*reversible reaction*reaction quotient*self-ionization of water*solubility equilibrium*relative volatility*sulfuric acid*stability constant*theoretical plate*thermodynamic equilibrium*vapor-liquid equilibrium*van 't Hoff equation

Petergans (talk) 10:22, 4 November 2008 (UTC)

I can help with this. If you haven't yet seen it, you may be interested in this, which gives a lot of background information and lists the pros and cons on three methods of organizing information: lists, categories and templates. One point it makes is that it is often useful to have both categories and templates on the same topic; they are not mutually exclusive. Mike Christie (talk) 11:56, 4 November 2008 (UTC)
I would like to go ahead with creating the category. We can decide later if the navigation template: chemical equilibria should also be retained. It was originally created by Happy Camper for chemical equilibria. I feel that is is not qute right for this article because it contains a number of links which are irrelevant to the topic. What is your opinion Ivan (Itub)? Any other opinions? Petergans (talk) 14:13, 4 November 2008 (UTC)
I'm generally not a big fan of these navigation templates, except for very well defined sets of topics (otherwise they end up getting bloated). I think a category should be enough in this case. The relevant articles are already linked when they appear in the text, and if someone wants a comprehensive list they can go to the category. --Itub (talk) 15:03, 4 November 2008 (UTC)
Agreed, nem. con. Have you the time to create the category? The provisional list above is already in alphabetical order. It would be better if a chemist did it, don't you think?.Petergans (talk)
I'm not sure about the template idea, because equilibrium chemistry is so large a field, pretty much all of chemistry. I'm not hostile to the idea of a category, but I'd like to see a little more discussion and a more concrete proposal as to where it fits with the other categories. Physchim62 (talk) 16:52, 5 November 2008 (UTC)
The main reason for suggesting this category is that equilibrium chemistry is under-represented in WP. It covers a very broad range of disciplines and so will be complementary to existing categories which tend to be restricted to one discipline. The articles I have collected together in the provisional list above all mention equilibrium or pK explicitly. But look at articles like the chelate effect and molecular recognition to see how badly quantitative equilibrium chemistry is sometimes treated. No article even exists dealing with the stability constants of metal complexes. These are examples of things on my to do list. The equilibrium chemistry category will draw attention to the fact that such diverse topics have a fundamental property in common. Petergans (talk) 08:45, 6 November 2008 (UTC)

(reset indent) Nav templates IMO are more user-accessible than categories, since they give a certain structure. Also, not all articles need be listed. Some nav templates I've done include:

Some relevant discussion here: Wikipedia_talk:WikiProject_Chemistry/Archive_16#.7B.7BReaction_mechanisms.7D.7D

On the other hand, categories will just give a hodge-podge of all articles listing that topic. Creating a cat is simple: simply add [[Category:Acid dissociation constant]] to all articles which you want. Go to Category:Acid dissociation constant, add a {{main}} and that's about it. --Rifleman 82 (talk) 17:30, 5 November 2008 (UTC)

Notes Nov 5th[edit]

I think the "Importance of pKa values" section could be improved by trying to make the sentences flow more from each other. I will work on this in a sandbox and post a draft here, if I can come up with anything better than what is there already.

Sorry I haven't had much time to review the article over the last couple of days; I may have a little more time tomorrow but I do owe another article a GA review. I should have more time over the weekend. I think it's now quite close to a renomination for FA but I'd like to give it a couple more reads through first. Mike Christie (talk) 03:44, 6 November 2008 (UTC)

In Section "Factors that determine the relative strengths of acids", the sentence

"Proton sponge, 1,8-Bis(dimethylamino)naphthalene, has a pKa value of 12.1. It is one of the strongest amine bases known. The high basicity is attributed to the relief of strain upon protonation and strong internal hydrogen bonding."

seems wrong. At the very least, it needs to be referenced. Physchim62 (talk) 08:51, 6 November 2008 (UTC)

What part of it seems wrong? It sounds plausible enough to me, and it agrees with what the proton sponge article says, which references for all these claims. I haven't checked the references, though, but I'm sure one or two of them could be useful for this article. --Itub (talk) 09:51, 6 November 2008 (UTC)
Checking again, the incorrect part seems to be that the conjugate acid has a pKa of 12.1, which makes it a stronger base than trimethylamine for example (and much stronger than aniline). The reference is in Proton Sponge, and I would transfer it except that I would like comment on the exact nomenclature that we're using here. This is a topic which is very "nomenclature-sensitive", so I shan't change without being sure that I'm not introducing a new style! Physchim62 (talk) 13:38, 6 November 2008 (UTC)
Also, I don't see where the strain is in unprotonated Proton Sponge: it has limited rotational freedom, yes, but that isn't strain (merely an entropy problem in its synthesis), and that is not relieved on protonation, quite the reverse! Physchim62 (talk) 13:47, 6 November 2008 (UTC)
The content here was essentially copied from proton sponge so any error here is also an error there. Given that the original literature is cited there I very much doubt if there is an error. Yes, proton sponge is a stronger base than trimethylamine. Petergans (talk) 14:36, 6 November 2008 (UTC)

Notes Nov 9th[edit]

Here are notes on all the remaining issues with the article that I can see. In these suggested rewrites I haven't preserved links because I wanted to be able to work with the text more easily; if we agree on the sequence of sentences I will be glad to make the parallel changes to the article text.

Importance section[edit]

Here’s an attempted rewrite of the "Importance" section. I’m posting it here rather than to the article because of the likelihood that in this rewrite I may have made subject-matter errors. What I’m trying to do is make the text flow more naturally. Comments?

A knowledge of pKa values is important for the quantitative treatment of systems involving acid–base equilibria in solution. Many applications exist in biochemistry; for example, the pKa values of proteins and amino acid side chains are of major importance for the activity of enzymes and the stability of proteins. Protein pKa values cannot always be measured directly, but may be calculated using theoretical methods. Buffer solutions are used extensively in biochemistry to provide solutions at or near the physiological pH for the study of biochemical reactions; the design of these solutions depends on a knowledge of the pKa values of their components. Important buffer solutions include MOPS, which provides a solution with pH 7.2, and tricine, which is used in gel electrophoresis. Buffering is an essential part of acid base physiology including acid-base homeostasis and is key to understanding disorders such as acid-base imbalance.
Buffer solutions also play a key role in analytic chemistry. They are used whenever there is a need to fix the pH of a solution at a particular value. Compared with an aqueous solution, the pH of a buffer solution is relatively insensitive to the addition of a small amount of strong acid or strong base. The buffer capacity of a simple buffer solution is largest when pH = pKa. In acid-base extraction, the efficiency of extraction of a compound into an organic phase, such as ether, can be optimized by adjusting the pH of the aqueous phase using an appropriate buffer. At the optimum pH, the concentration of the electrically neutral species is maximized; such a species is more soluble in organic solvents having a low dielectric constant than it is in water. This technique is used for the purification of weak acids and bases.
In pharmacology, ionization of a compound alters its physical behavior and macro properties such as solubility and lipophilicity (log p). For example, ionization of any compound will increase the solubility in water, but decrease the lipophilicity. This is exploited in drug development to increase the concentration of a compound in the blood by adjusting the pKa of an ionizable group.
Knowledge of pKa values is important for the understanding of coordination complexes, which are formed by the interaction of a metal ion, Mm+, acting as a Lewis acid, with a ligand, L, acting as a Lewis base. The ligand may also undergo protonation reactions, so the formation of a complex in aqueous solution could be represented, symbolically by the reaction
[M(H2O)n]m+ +LH ⇌ [M(H2O)n−1L](m−1)+ + H3O+
To determine the equilibrium constant for this reaction, in which the ligand loses a proton, the pKa of the protonated ligand must be known. In practice, the ligand may be polyprotic; for example EDTA4− can accept four protons; in that case, all pKa values must be known. In addition, the metal ion is subject to hydrolysis, that is, it behaves as a weak acid, so the pK values for the hydrolysis reactions must also be known.
Assessing the hazard associated with an acid or base may require a knowledge of pKa values. For example, hydrogen cyanide is a very toxic gas, because the cyanide ion inhibits the iron-containing enzyme cytochrome c oxidase. Hydrogen cyanide is a weak acid in aqueous solution with a pKa of about 9. In strongly alkaline solutions, above pH 11, say, it follows that sodium cyanide is "fully dissociated" so the hazard due to the hydrogen cyanide gas is much reduced. An acidic solution, on the other hand, is very hazardous because all the cyanide is in its acid form. Ingestion of cyanide by mouth is potentially fatal, independently of pH, because of the reaction with cytochrome c oxidase.
In environmental science acid–base equilibria are important for rivers and lakes. For example, humic acids are important components of natural waters. Another example occurs in chemical oceanography: free copper(II) is toxic to phytoplankton but when copper(II) is complexed to naturally-ocurring organic ligands it is not toxic. Petergans (talk) 11:37, 10 November 2008 (UTC)
The design of a universal pH indicator requires a mixture of indicators whose adjacent pKa values differ by about two. This is because the transition range of a pH indicator is about pKa ± 1: this is the range over which the color is intermediate between the colors of the acidic and basic forms of the indicator.

(end of rewrite)

I cut two sentences because I did not understand their relevance to this article or the adjacent material well enough to include them in the rewritten version:

  • The isoelectric point of a given molecule is a function of its pK values, so different molecules have different isoelectric points."Isoelectric focussing is a technique used for separation of proteins by 2-D gel polyacrylamide gel electrophoresis." Does isoelectric focusing use pKa values, or just make use of buffer solutions? If the latter I’m not clear why we need to mention it here.
Suggested addition in magenta. The application depends on the pK values of different proteins being different. Petergans (talk) 11:37, 10 November 2008 (UTC)
  • "The reaction that converts adenosine triphosphate to adenosine diphosphate is very pH sensitive." Same question: does this need to be mentioned? If so, can we specifically say what it is about pKa values that is critical here?
This sentence was added by Eaglefalconn, who presumably had a reference for it. The statement is true but needs a citation which I cannot provide just now. Probably best to remove it. Petergans (talk) 11:37, 10 November 2008 (UTC)

In addition I would like to expand the "environmental science" paragraph, or cut it, or combine it with another paragraph. I don’t see an easy combination target. Is this something for which an example can be given of a specific situation where pKa values are used? Or can more be written about this?

I've suggested a couple of examples. Stumm, W. (1996). Water chemistry.  Unknown parameter |coauthors= ignored (|author= suggested) (help) (ref 46) has lots more, but I don't have a copy of it to hand. Likewise, with Millero's book - I know that this is an active research topic at the Universities of Miami and Messina, amongst others. The emphasis is usually on metal-ion complexation.
You've done a splendid job on the rewrite. Shall I incorporate it into the article? Petergans (talk) 11:37, 10 November 2008 (UTC)
Thank you; please go ahead! Mike Christie (talk) 11:43, 10 November 2008 (UTC)
I've gone ahead and redone this section based on the discussion above. Could you just check that the environmental science section is correctly cited? I had to move the references to where I thought they should be, so it would be good if you could confirm. I also don't see that we have a citation for the comments about isoelectric point and isoelectric focusing; or can that be covered by one of the two existing gel electrophoresis citations, further up? Mike Christie (talk) 00:07, 11 November 2008 (UTC)


The lead does not appear to discuss the "Factors that determine the relative strengths of acids" section, or the "Experimental determination of pKa values" section. (See WP:LEAD.) I’ve added some material to the draft below to try to correct this, and cut somewhat elsewhere to compensate. As above I have put the rewrite here for fear of technical/subject matter mistakes. The "factors" section includes a couple of short sentences but I felt unable to summarize accurately the information about Pauling’s second rule or the subsection on thermodynamics; perhaps another more knowledgeable editor could fill in the missing information.

An acid dissociation constant, Ka, (also known as acidity constant, or acid-ionization constant) is a quantitative measure of the strength of an acid in solution: the larger the value the stronger the acid and the more the acid is dissociated, at a given concentration, into its conjugate base and the hydrogen ion. The strength of a base is nowadays quantified by the Ka value of its conjugate acid.
Ka is an equilibrium constant. For an equilibrium between a generic acid, HA, and its conjugate base, A−, HA ⇌ A− + H+, Ka is defined, subject to certain conditions, as
where [HA], [A−] and [H+] are equilibrium concentrations of the reactants. The term acid dissociation constant is also used for pKa, which is equal to −log10 Ka. pKa is proportional to the standard Gibbs free energy change for the reaction. While the standard enthalpy change for a weak acid dissociation reaction may be positive (endothermic reaction) or negative (exothermic reaction), the standard entropy change is always negative. pKa values for endothermic reactions increase with increasing temperature; the opposite is true for exothermic reactions. This is in accord with Le Chatelier's principle.
In aqueous solutions, acids that release a single proton are partially dissociated to an appreciable extent in the pH range pKa ± 2. The actual extent of the dissociation can be calculated, using the pKa value, if the analytical concentration of the acid and pH (or analytical concentration of added mineral acid) are known. Acidic behavior can also be characterized in non-aqueous solutions. Factors that determine the magnitude of pKa values include Pauling's rules and, for organic acids and bases, inductive effects and mesomeric effects; these effects are summarized in the Hammett equation. Structural effects, such as inter-molecular hydrogen bonding, can also be important. pKa can be experimentally determined by potentiometric (pH) titration, but for values of pKa less than about 2 or more than about 11 spectrophotometric or NMR measurements may be required.
A knowledge of pKa values is essential for the understanding of the behaviour of acids and bases in solution. For example, many compounds used for medication are weak acids or bases, so a knowledge of the pKa and log p values is essential for an understanding of how the compound enters (or does not enter) the blood stream. There are many other applications, including aquatic chemistry, chemical oceanography, buffer solutions, acid-base homeostasis, protein chemistry and enzyme kinetics. A knowledge of pKa values is also a prerequisite for a quantitative understanding of the interaction between acids or bases and metal ions to form complexes in solution.

(end of rewrite)

I suggest cutting the following:

  • "The term pKb is used in relation to bases, though pKb has faded from modern use due to the easy relationship available between pKb and pKa, the strength of its conjugate acid." It doesn’t seem important enough relative to the other items which do warrant mention in the lead.
True, but something needs to be said about bases. I hope my sentence does it.
  • "such as Michaelis–Menten kinetics, which involve a pre-equilibrium step": I suggest cutting this just to shorten the last paragraph, which is longer, relative to the body section it relates to, than some of the other lead material.
Your changes are fine. I've added a summary of the thermodynamic aspect and edited some of the technical stuff. Petergans (talk) 12:48, 10 November 2008 (UTC)
Looks good. I have to head off to work now, so won't be editing again till this evening; I think your version is fine to go ahead and edit into the article. Mike Christie (talk) 12:53, 10 November 2008 (UTC)
I have made these changes, with some tweaks. I added a few links to the new material but it might be good to glance over the lead and see if anything else should be linked. Mike Christie (talk) 00:28, 11 November 2008 (UTC)

Other issues[edit]

The following list contains everything else I know of that needs to be addressed for FA.

  • The article is inconsistent in its use of American English versus British English. For example, it has both "-ize" and "-ise" endings; "behaviour" is a British spelling but "gray" is the preferred American spelling. It doesn’t matter which is chosen, since this is not a topic which has a regional connection, but it does need to be consistent through the article. I’ll be happy to go through and make the necessary changes if you can tell me which you prefer – American or British English.
I'll go for British, of course! Petergans (talk) 15:40, 10 November 2008 (UTC)
  • I have a couple of questions about the units style. The relevant style guide is WP:UNITS; I will fix a few things myself. The guidelines I am referring to are these:
    • The first instance of each unit needs to be spelled out in full and linked; it can be abbreviated throughout the article thereafter.

Done Petergans (talk) 15:40, 10 November 2008 (UTC)

    • Where a unit is not sufficiently familiar that the abbreviation will be quickly recognized by readers, the abbreviation should be shown after the first use – e.g. "decimeter (dm)". One possible exception: M stands for Molar concentration I presume? If so, since the M is not an abbreviation, I think it would suffice just to link the first occurrence of the "M" to the molar concentration article.
    • A nonbreaking space goes between the number and the unit; the code is "&nbsp;"
    • The product of units is indicated by a middle dot: "&middot;".
    • There's an instance of "300 /M" but it is definitional and I think is OK.
This is a quotation so, MUST stay as it is. Petergans (talk) 15:40, 10 November 2008 (UTC)
    • There's a mix of exponential and division notation in this: "ΔHO /kJ mol−1"; it looks like a standard usage, but I'd like to ask if it would be as readable to the target audience if converted to be consistently exponential. If it should stay as it is for readability I think that's fine.
In this case I think WP should give a lead and use this notation which is precise, unambiguous and, once the convention is understood, clear. Otherwise, how is the general public ever to come to terms with standard scientific notation? In text the slash can be replaced by "in" which effectively has the same meaning as "divided by". Petergans (talk) 15:40, 10 November 2008 (UTC)
There appears to be a conflict between the WP and IUPAC recommendations for composite units. In this table, numerical values have been obtained by dividing (slash) the physical quantity, ΔHO, by the unit of measurement, kJ mol−1. "kJ per mole" is hardly ever used in chemistry texts. The slash convention is particularly valuable when a number has been multiplied or divided by a factor since the old usage of ,e.g. times 1000, can be ambiguous. In this case the quantity in Joules per mole has been divided by 1000. The unit kJ mol−1 is standard in thermodynamics. Middle dot in units is obsolete in chemistry. Petergans (talk) 13:35, 10 November 2008 (UTC)
  • Why is "Bis" capitalized in the "factors" section?

Done Petergans (talk) 15:40, 10 November 2008 (UTC)

  • "The total volume of added strong base should be small compared to the initial volume of to keep the ionic strength nearly constant." A word is missing.

Done Petergans (talk) 15:40, 10 November 2008 (UTC)

That’s everything I could see to fix. Mike Christie (talk) 21:11, 9 November 2008 (UTC)

A few comments on the comments!
  • I'll let Peter choose for the spelling, but remember that British spelling is not always consistent between -ize and -ise (people at WP:FAC will probably be aware of this).
  • M is a very common abbreviation for mol dm–3. It should probably be defined at its first usage and restricted to article text (not tables or formulae).
  • Use of a middle dot to create a product of units is very rare in scientific articles on Wikipedia, and almost unknown in other scientific contexts. We should use a thin space, but a nonbreaking space is quite normal given current browser limitations.
  • Division of a quantity by a unit, as in "ΔHo /kJ mol−1" is normal scientific practice for tables and graph axes, as per ISO 31-0: it avoids having to repeat the unit on each line.
Physchim62 (talk) 13:17, 10 November 2008 (UTC)
Comments on the comments on the comments: I agree with the first three points (of course, ize/ise should be consistent within the article even if British English is chosen). Regarding the fourth point: it is of course correct, but putting the units in parentheses is also very common even among scientists and I think is much more intuitive to non-scientists. I suggest putting the units in parentheses. --Itub (talk) 13:25, 10 November 2008 (UTC)

Remaining issues[edit]

I've been back through the talk page looking for outstanding issues. I thought it would be more convenient if I summarised the situation here, rather than making people read the long text above.

  • Reference 13 still doesn't have a page number; it does look like it needs one.

DonePetergans (talk) 13:17, 11 November 2008 (UTC)

  • The changes to the lead and to the importance section have been done but should be reviewed to make sure I didn't mess anything up. This includes checking for additional possible links in the new material, mostly in the lead, and checking that the references for environmental science are correct.

DonePetergans (talk) 13:17, 11 November 2008 (UTC)

  • I moved some text around in the bit on isolectric focusing, and it has no reference as it stands; that should be fixed.

DonePetergans (talk) 13:17, 11 November 2008 (UTC)

  • I've converted to British English throughout. However, I am unreliable on this issue as I've spent decades on both sides of the Atlantic, so it would be best for someone else to check for any remaining American English usages.
  • I don't have a strong opinion on the remaining units usage issues; I agree with the suggestion that using parentheses rather than a slash for dividing by units in tables and graphs might be easier for lay readers. I understood the usage once it was pointed out but the parens would be clear too. Anyway, I think this is up to the regular editors here. There might be some discussion of this at FAC.

I stopped using the (unit) convention because it can be ambigous if a numerical factor is involved, though that is not an issue in this article. Petergans (talk) 13:17, 11 November 2008 (UTC)

There are also two points I've asked for external advice on. One is a stylistic issue: the formatting of "see also" and "external links" sections. See enzyme kinetics, for example, which uses em dashes; I've found that most featured articles just use bare links without explanation, which I think is suboptimal. I've left a query with a style expert but have not heard back. We could renominate at FAC without resolving this.

The other issue is the lack of citations for long sections of the article. For example, the section on equilibrium constants has essentially no citations for the majority of the text; the references are apparently to specific points. I suspect that this is because nothing in this section would be regarded as even faintly necessary to cite in a professional journal; it would be common knowledge. Here it's necessary to go further, but I am not sure what the current style is and have asked the FAC director for an opinion. For example, it might be the case that a given text that covers the entire area could simply be cited, once, at the end of the section. Or it might be more usual to cite at once at the end of each paragraph. I'll post a note here when I hear back.

I'll come back tomorrow evening and look through one more time. Does anyone else think there's anything else that needs working on before we go back to FAC? Mike Christie (talk) 02:15, 11 November 2008 (UTC)

  • I have commented out the navigation template as I feel that the article has evolved so much since it was introduced that it is now redundant. Is there a consensus for this?
  • Creation of an Equilibrium chemistry category. This is a separate issue from the one above. Looking through articles to which it might apply, the majority give Physical chemistry or Analytical chemistry as the category (with no consistency!). I suggest that an equilibrium chemistry category would be more appropriate. When the Thermodynamics category is present the new category should be added. See Portal:Chemistry/Categories for a list of categories currently in use in chemistry. Equilibrium chemistry would fit nicely into this list. Petergans (talk) 20:52, 11 November 2008 (UTC)
  • I do not understand why the {{Main|Determination of equilibrium constants}} template was removed from the section on Experimental determination ... There is now no link to that article, which does in fact give a lot more detail. What should be done? Petergans (talk) 13:17, 11 November 2008 (UTC)

The points above are now mostly taken care of; I haven't heard back from the FAC director on the sources but will ask someone else if I don't hear tomorrow. For the {{main}} template issue: the reason I removed it is that it's intended for summary style. It indicates that the entire linked articles would be inserted at that point if it were not for length constraints. Hence it is used for articles that are of necessity too broad in scope to fully cover their material; it permits long sections to be summarized and removed to subsidiary articles. See chemistry for multiple examples of this usage. In this article it doesn't seem to be appropriate because the section on equilibrium constants wouldn't be expected to include the entire article on determination of those constants -- some of what's in that article doesn't specifically relate to the acid dissociation constant. If there is no natural place to link to the "determination" article from the existing text, I'd suggest either looking for a natural paraphrase which would leave us with text that could be used for the link, or else adding it to the "See also" section at the end.

If you're OK with that, then there's not much else to do. I think it's important to get another opinion on the references, since that could lead to quick opposes if we don't understand the usual citation style. Other than that the remaining topics of discussion, such as units and the navbar, seem to be the sort of thing that the editors here can agree on and which aren't likely to provoke disagreement at FAC. I will try to get back here in the next day or two with a response on the references. Mike Christie (talk) 03:26, 12 November 2008 (UTC)

  • I have put an unlinked category in my sandbox. If someone will set up the skeleton category for me, I will hopefully be able to fill in the details. This will make it easier to assess the idea. Petergans (talk) 10:16, 12 November 2008 (UTC)
  • I'm afraid I've never created a category or done anything with them, so I can't help; I took a look at WP:CAT but I'd have to do some experimenting to figure out the right way to do it. I'll leave a note with a helpful admin I know who I'm sure will know more than I do on this. Mike Christie (talk) 00:39, 13 November 2008 (UTC)
    Category pages can be edited and created like any other page. However the list of articles in the category is generated automatically by the software from the articles which actually are in the category. Geometry guy 20:02, 13 November 2008 (UTC)


A couple of editors have responded at WT:FAC and said that they recommend putting a relevant reference at the end of each paragraph that is currently uncited. This does not apply to the lead, in which the common practice is to cite only controversial information or direct quotes. Because of the equations, it's not completely easy to tell where a paragraph begins, but I can see perhaps two or three citations needed in the Definitions section; four needed (all four paragraphs) in the first part of the "Equilibrium constants" section; three for "monoprotic acid" (the first one can be placed at "the following conclusions can be drawn", thus covering the list below it; and so forth. I would think this would often be citations to whole chapters, not page references.

I did also notice that the note about free copper (II) in chemical oceanography has a comment in the text saying "I got this snippet from a conference booklet, not from the cited book". It does still need to be cited; can we cite the conference booklet? Not the best possible source, but it seems likely to be reliable so it will do. Mike Christie (talk) 10:49, 13 November 2008 (UTC)

In answer to the last point: F.J. Millero, "The speciation of metals in natural waters", PL4 in XIV Italian-Spanish Congress on the Thermodynamics of Metal Complexes, Capri, 2003 (Plenary lecure 4 - I don't know how to format this citation as there is no publisher or page number). Maybe it would be simpler to pretend it came from ref. 49?

On the general question of references, it appears that the scientists are under-represented on the panel of FAC editors, so may I make the following observations?

  • Regarding Definitions and Equilibrium constant, this is all common knowledge. There is nothing controversial here: it's all been accepted for more than 50 years. Expositions can be found in numerous text books of which references 1, 6, 8, 11, 12, 13 and 18 are typical examples. Between them they cover all of the material in these sections.
  • References to chemistry text-books are less than perfect. The really popular books go through many editions and change with each edition, obviously. For example, the current edition of Atkins, Pysical Chemistry is the 8th. Whereas edition 1 had a section "pK and pH" (p363) this has disappeared in edition 8.
  • Chemistry books always have a table of contents and an index which makes it easy to find specific items, even when no page number is cited.
  • On the specific section Equilibrium constant, this is essentially a highly condensed summary of chapter 2 of Rossotti & Rossotti, reference 5. What good would it do to cite specific page numbers for each paragraph? This book, though old, has never been bettered for its thoroughness and theoretical rigour.
  • Part of the present dilemma arises from an intrisic ambiguity in WP regarding references - are they for verifiabilty or do they point the reader towards futher information, or both?

It is clearly important that this article conforms to house-style, but did I not read somewhere that application of WP rules should always be tempered with common sense? Petergans (talk) 21:11, 13 November 2008 (UTC)

Yes, common sense trumps most rules. I should start by saying I hold no particular brief for the house style here; I merely know how to conform to it, and in the articles I've worked on have not found them unreasonable. The way house style requires scientific articles to be cited does seem less than ideal, I agree. I also take your point about the ambiguity regarding references. My own feeling on that is that they're primarily for verifiability, but since WP is for a general readership the bar for what a reader can be expected to know has to be set quite low.
A featured article candidacy is not usually a good place to argue for changes in the house style, though. I'd suggest the following.
  • Do the minimum necessary to conform; there's no need to cite specific page numbers, for example, if an entire book can be used to source a section. I'd put a cite to that book at the end of each paragraph. If appropriate, you could add text to the reference that comments that the material is distributed through chapters X and Y, or something to that effect.
  • Nominate at FAC, adding, if you like, a comment to the effect that the citations seem necessary to comply with WP:MOS but seem out of place to someone with any knowledge in the area. Alternatively, nominate without the citations, but in that case I would definitely add a note in the nomination that you can add citations to any paragraphs thought to need them. You might also comment in that case that the sources listed in "further reading" cover all the relevant material (assuming that's the case).
  • Follow the FAC (a successful FAC, let's hope) with discussion at WT:CITE of what should be required for scientific citations. The FAC under your belt, combined with your academic and scientific experience, won't hurt your ability to argue your case. I know the topic has come up before; you might ask User:Geometry guy about this -- I believe he is also a UK academic (in mathematics) and might be aware of prior discussions.
There's a fair amount of inertia in things such as WP:CITE, and personally I tend not to spend much energy on arguing on WP about that sort of thing. I get my enjoyment from researching my own articles, and from reviewing others -- I really enjoyed reviewing this article, for example, and feel as though I've learned quite a bit from it. You're evidently enormously valuable to WP as a content contributor, and I'd hate to see you distracted by discussions about house style when you could be bringing a dozen core chemistry articles up to featured status. How about tackling chemistry itself? It was a featured article candidate last year (see here) but that may have been a disruptive nomination, judging by the comments. Bringing one of the core topics to FA status would be tremendously valuable. Anyway, those are my thoughts. Mike Christie (talk) 03:34, 14 November 2008 (UTC)
As an ex-academic, my primary concern is with content and I try to avoid involvement in discussions about style. Chemistry is a mess and deserved to be rejected for FAC. It needs a major re-write but my experience is that many WP editors object to major re-writes. My next immediate project is to clean up pH, which is also a mess. After that I'm thinking of creating Stability constants of complexes as this constitutes a big hole in WP coverage of solution chemistry. (Comment added after Are we ready?, below)Petergans (talk) 11:20, 14 November 2008 (UTC)
Regarding citing common scientific knowledge: there has been extensive discussion about this by the members of various science wikiprojects, and the consensus guideline is Wikipedia:Scientific citation guidelines. Basically, something that is expected to be included in nearly every college textbook and class doesn't need to be cited in a per-sentence basis, but a general reference such as a textbook or monograph should be given near the beginning or end of the paragraph (in this case I'd say even section?) --Itub (talk) 07:33, 14 November 2008 (UTC)
WP:SCG is indeed the guideline to follow and the place to discuss the issue. I don't have time to comment further, but would like to draw attention to a misconception: "WP is for a general readership the bar for what a reader can be expected to know has to be set quite low". No! "Wikipedia is an encyclopedia incorporating elements of general encyclopedias, specialized encyclopedias, and almanacs." It is for a varied, not a general audience. What the reader can be expected to know depends upon the topic/article, and within each topic/article, the level of detail. Geometry guy 08:10, 14 November 2008 (UTC)
I'd completely forgotten about the existence of WP:SCG, and it's kind of embarrassing to get corrected by the very first sentence of the five pillars. In my defense, the debates I recall on scientific citation included demands for frequent citation because of the presumption that general readers would need those citations. However, I notice a couple of interesting things in WP:SCG: the example text on aldols does not have any cites in the second paragraph; and the guideline comments that general reference cites can be given on the talk page, not the main article. I've never seen the latter done, but it appears to be an option. Mike Christie (talk) 11:22, 14 November 2008 (UTC)
This misconception is widespread, so I flag it whenever I see it. In your defense, I would point to WP:MTAA, but on reading this guideline, I found it to be, erm, suboptimally written. Much better reading, in my view, is this essay by Markus Poessel.
WP:SCG is also suboptimal. The reason for this can easily be seen from its origins. In September 2006 (before my wikitime), the GA project launched its first sweep of GAs to counterbalance the relaxed and enthusiastic early promotion of articles. The delisting of multiple math and science articles without inline citations created a furore, and WP:SCG emerged out of this.
I support the basic principle that knowledge that can be found in any decent textbook is uncontroversial, unlikely to be challenged, and therefore does not need to be cited on a sentence-by-sentence basis. The solution I prefer is to have a general footnote at the beginning of the section along the lines of "The material in this section can be found in standard textbooks such as X and Y".
However, I think WP:SCG needs the attention of a wide range of Wikipedians, as it hasn't really received it so far. Geometry guy 20:33, 14 November 2008 (UTC)

Are we ready?[edit]

Many thanks to Mike Christie for all the time and hard work he has put in, especially on such alien subject matter. The text now runs much more smoothly.

On the matter of references, I have now included the chapter titles in some of them. Hopefully this helps in regard to matters of "common knowledge". I've only done it for books that I have copies of, so it may be useful to add some more chapter titles. Petergans (talk) 11:08, 16 November 2008 (UTC)

I invite interested editors to do a last read-through/edit/comment before nominating again for FAC. Petergans (talk) 10:58, 14 November 2008 (UTC)

The category:Equilibrium chemistry has been created. In the end it turned out very easy to do.
  1. Create the category page and put in the template:catmain with the name of the principal file. This is optional.
  2. Add the link [[category:Equilibrium chemistry]] to each page (e.g. acid dissociation constant) that is to be included in the category. This automatically creates a link which will be seen when the category:equilibrium chemistry page is opened. Petergans (talk) 14:10, 14 November 2008 (UTC)

Small changes in section headings and ALT text for LaTeX equations and images[edit]

L'Aquatique and I added ALT text to the LaTeX-formatted equations and to the first image of the lead to make them accessible to people who use screen readers, per this discussion on the Talk page of the accessibility guidelines. Do they seem reasonable and accurate to everyone here? You'll find them buried in the <math> and Image: tags in editing mode. If they're OK, we should probably add ALT text to the remaining images.

We also changed a few of the section headings slightly to remove the formatted text pKa in the section headings and to avoid repetition of the article title in the section headings, per this discussion on the accessibility talk page and per MOS:HEAD. Hopefully, everyone will be fine with those as well. Proteins (talk) 21:35, 21 November 2008 (UTC)

I'm not happy about removing pKa from headings. pKa is is standard notation in chemistry for these quantities. The heading to section 7 does not mean the same as before. The subscript is important to differentiate pKa from pKb. pK by itself is used when the context makes it clear that that subscript a is implied, as in the phrase "the pK of the acid is ..". There are three alternatives for section headings: i) accept that standard chemical notation should be used, ii) use the text "pKa" or pK or iii) spell it out - "acid dissociation constant".
"acid dissociation constant" may be the article title, but it is also the name of the quantity being discussed, so some repetition is inevitable. Petergans (talk) 00:11, 22 November 2008 (UTC)

First, am I correct in understanding that you approve of the ALT text messages in the equations and lead image? Would you like me to add it for the remaining images, or perhaps you would like to do it yourself? The goal is to describe each image so that visually impaired readers can appreciate its content as well. As it is, blind readers hear the figure caption repeated twice, as you can tell by turning off the images on your browser. Some captions do describe their image, but many just give its significance, like delivering the "punchline" of a joke without its setup.

As for the section headings, I'm fine with using pKa myself, although it seems to run afoul of the consensus on accessibility to the blind and for style. You can find more information about those concerns by following the links to discussions and guidelines above.

Although they may seem unfamiliar at first, I think the section headings aren't made significantly worse by eliminating the article title from them. MOS:HEAD is only a guideline, but you know the old saw about customs having more force than law; people will ask you why it isn't being followed. For example, if this were Martin Luther, people would ask you to replace a section heading "Luther's education" with "Education". If you think about it, it's not that ambiguous whose education is being discussed, although the form may be unfamiliar. I think we agree that "acid dissociation constant", its abbreviation Ka and the equivalent form pKa are the same for the purposes of MOS:HEAD, but you could argue that point, I suppose. I modified section heading 7 so that I think you'll agree with it.

I see that the article is up for FAC, so I'll read through it carefully and give my review this weekend. A few suggestions from my brief scan:

  • a longer discussion of the sigma factors for electronic induction, with a short table illustrating the most outstanding/common functional groups? Given all the other tables, a short table of a few rows might not be out of place. You mention the Hammett equation, but it's only one sentence, and I'm afraid that people who don't know it already won't get much out of it. Perhaps you think the topic is too technical? You might consider illustrating it with a homologous series, such as methanol to methanol substituted with halides to phenol to phenol substituted with other electron-withdrawing groups? BTW, is there a discussion of resonance affecting pKa values?
I've expanded the bit about the Hammett equation. I started with a table of values, but decided in favour of a more general discussion. There is a table of original values in hammett equation. I've also added a more up-to-date reference. Petergans (talk) 16:24, 23 November 2008 (UTC)
  • Do you think an extra "Notes" column could be added to the pKa tables so that non-(bio)chemists don't have to follow hyperlinks to know what glycine is or why it has two titratable groups? Right now, the tables seem uncomfortably similar to those found in the CRC manual or a textbook, and people might criticize the article along the lines of WP:NOTTEXTBOOK and WP:IINFO. You might consider adding a molecular diagram of each acid, perhaps with the titratable proton highlighted in red? Visual aids might help you get your point across to non-chemists.
  • pKa shifts in proteins? The maleic/fumaric example of pH shifts due to hydrogen bonding is good – I use it in my own lectures – but it would be nice to mention their role in enzymatic catalysis and protein stability. On the other hand, perhaps that's too parochial.

Good luck with the FAC! Proteins (talk) 13:47, 22 November 2008 (UTC)

You make a lot of good points. Firstly let me explain a misunderstanding. I thought that the changes you made were part of preparing the article for FAC, which is why I responded here, rather than editing the article. I need to have a definitive answer on the use of subscripts in section headings.
Unfortunately, I can't give you a definitive answer, especially given that I'm a relative newbie on Wikipedia. I believe that it's OK per WP:ACCESS; I personally checked that pKa in section headings is read correctly by JAWS and Fire Vox, and Graham87, who knows much more, found the same. I think the greatest opposition will come from people interpreting MOS:HEAD strictly, in particular its discouragement of "special characters" and the repetition of the article title. I posted a few questions at WT:MOS, so you should have your answer in a day or two. Proteins (talk) 16:26, 22 November 2008 (UTC)
The ALT text additions were by and large OK, though I have edited them for scientific content. I would prefer that you added the alt text elsewhere, too, as I think that the outsider's description is likely to be more informative than mine as I am too close to the subject matter.
Great! I'll be happy to add the ALT text. Proteins (talk) 16:26, 22 November 2008 (UTC)
I will consider the other points over the weekend. I did consider putting structural formulas alongside pK table entries for the reasons that you outline, but decided against this idea as it would have increased the row heights too much. I'll do some trials in my sandbox. Regarding the effect of hydrogen bonding on protein pK values, please feel free to add something yourself, with references. I am not familiar with this area. Petergans (talk) 14:43, 22 November 2008 (UTC)
I'll think it through and try to find some good examples that aren't too obscure. The earliest examples in protein science would probably be the pKa shifts found first spectrophotometrically and enzymatically in ribonuclease A in the 1950's, which are associated with Tyr-Asp hydrogen bonds and with the catalytic histidines. That example might allow us to talk more about how NMR is used to determine the pKa's of individual sidechains, and maybe pD? A more modern example might be more illuminating and more fun, however. Proteins (talk) 16:26, 22 November 2008 (UTC)

Equilibrium constant[edit]

There are a couple of very minor problems in the definition of an equilibrium constant. Firstly, I personally have never seen the abbreviation Kt for the thermodynamic equilibrium constant. From International Union of Pure and Applied Chemistry (1993). Quantities, Units and Symbols in Physical Chemistry, 2nd edition, Oxford: Blackwell Science. ISBN 0-632-03583-8. p. 50. Electronic version., "ISO and the IUPAC Thermodynamics Commission recommend the symbol Ko and the name 'standard equilibrium constant', but some thermodynamicists prefer the symbol K and the name 'thermodynamic equilibrium constant'."

As you say, there is no consensus for this notation. In the present context I think that the important thing is to differentiate clearly between a constant based on activities and one based on concentrations, but I don't like either Ko or K. What do you suggest?Petergans (talk) 15:23, 22 November 2008 (UTC)

Secondly, activities (and therefore Ko) are indeed dimensionless, because they are defined by dividing by the standard state: ie a(H+) = c(H+)γ(H+)/co. In standard notation, the first two equations should be:

Only in this way can you keep Ko as a dimensionless quantity suitable for taking a logarithm thereof. Physchim62 (talk) 12:38, 22 November 2008 (UTC)

This is a valid point of view, but I have taken another, equally valid one, namely, that the standard state is defined by setting activity to one. On your system the activity coefficient is dimensionless, on mine it has the dimension of inverse concentration. I have tried to simplify this section as much as possible for the sake of the general reader. Otherwise we have to get into a detailed discussion of activity and standard states, which I suggest is too complicated for this article. I have tried to improve the article activity coefficient to reflect this issue. Maybe that is the place where the detailed discussion should go? As mentioned in that article, when the concentrations are expressed as mole fractions the question of dimension does not arise. That's why I have side-stepped the issue. I've also side-stepped the question of molar and molal concentrations. Petergans (talk) 15:23, 22 November 2008 (UTC)
Hmm, that's not actually the definition of standard state, or of activity coefficient… I'm looking to do a minor blitz on standard state, so I'll take a look at activity coefficient as well! Might it not be simply to start out by saying: "The acid dissociation constant is the equilibrium constant for the reaction HA ⇌ H+ + A." That definition has two main advantages: it is independent of the solvent used, and it closely mirrors the way Ka values are measured in practice. It's disadvantages are that it appears to give a quantity with dimensions of concentration (a common problem for equilibrium constants) and it does not mirror the actually Brønsted–Lowry equilibrium, which would be HA + S ⇌ HS+ + A. Those advantages and disadvantages could be easily mentioned, so long as we point out that the conventional definition really is Ka = [H+][A]/[HA], this isn't just some simplification we've made up! Physchim62 (talk) 13:22, 23 November 2008 (UTC)

I am following a long-established convention. For example Rossotti & Rossotti (1961), equation 2-1 (mass-balance) reads

implying that concentration = activity / activity coefficient. Regarding the definition of Ka, my objective has been to show what approximations are involved when the simplified formula is used, because sometimes they can come back to bite the unwary. Petergans (talk) 16:46, 23 November 2008 (UTC)

The long-established convention is that Ka = [H+][A]/[HA], or, as IUPAC prefers these days (2006 Technical Report, to give a dimensionless Ka), Ka = [H+][A]/[HA]co [1]. It is the thermodynamic dissociation constants which are "unconventional", and this indeed causes some confusion (eg, at strong acid). Physchim62 (talk) 20:03, 23 November 2008 (UTC)
The current definition of the Henry's law activity coefficient based on amount concentrations (from the IUPAC Green Book) is ac,B = γc,BcB/co. This gives dimensionless activity coefficients (as the term "coefficient" would imply), as well as dimensionless activities (as has long been conventional). Physchim62 (talk) 20:30, 23 November 2008 (UTC)
It appears that the confusion arises from IUPAC having changed its recommendations. I have a copy of the Compendium of Analytical Nomenclature 2nd. edition (1987) which recommends the symbol y, not γ for activity coefficient. Can we anyway agree that the current text is sufficient for its purpose, which is to emphasise the fact that when K defined as a concentration quotient it is implicit that Γ is a constant?
The reference [2] is very useful and I will add it in the appropriate place. Petergans (talk) 10:16, 24 November 2008 (UTC)
I had a go at rewriting this section, and I agree that I'm not really adding anything important beyond what is already there. The IUPAC report is the only IUPAC recommendation I've been able to find which risks a definition of Ka: luckily it agrees with the article! IUPAC has certainly changed its definitions in the past: I've found a 1976 recommendation which has activity coefficients defined as you define them above, and the symbolism changed with the approval of ISO 31. The 3rd edition of the "Orange Book" uses γ for the activity coefficient (see section 3.3). Physchim62 (talk) 13:54, 24 November 2008 (UTC)
I've tried modifying your sandbox, but am not convinced that the result is any better than the current text. It is a little more rigorous at the expense of being more difficult to read. I'd like to keep the current text in spite of the fact that it is out of step with current IUPAC recommendations, because it is simple, clear and logical. Petergans (talk) 16:36, 24 November 2008 (UTC)
The alt text is good if a bit too descriptive in places. Ideally it shouldn't give any more information than the picture or equation. So I don't know why we should note that H2O is water, or the gamma is a Greek letter, in the alt text; this information is not available in the graphics or the equations. Graham87 06:06, 26 November 2008 (UTC)
I haven't completed a review of the alt text, but I agree, it should only say what the image or equation says. Petergans (talk) 08:37, 26 November 2008 (UTC)

Tim's inadvertant errors[edit]

Having criticized User:TimVickers for this set of edits – made entirely in good faith and with the objective of improving the lead – intellectual honesty requires me to be specific, to see if editors can get ideas for improving the understandability of the article. IMHO:—

  • Ka is not a physical constant in the sense of the speed of light or the rest mass of an electron. Ka are fairly constant, constant enough to be useful but variable enough to catch the unwary. Ko values are only "constants" because they include experimentally-determined 'fiddle-factors' (better known as activity coefficients).
  • [minor point] I'm not sure if Ka "measures" or "describes" the strength of an acid. Similarly for bases, where the phrasing grated even more for me.
  • All solvents will allow acid–base chemistry to occur. Believe me, I've done acid–base chemistry in hexane, usually using butyllithium as the base and sometimes using toluene as the acid (great for safely cleaning your syringes!). The pKa of hexane is estimated to be around 45, although it can't be determined (see below).
  • Ka values are never measured, they are determined: that means they are calculated on the basis of a number of other measurements. If pKa falls between 2 and 11, I suppose you could, just about, determine it with a single measurement: in practce, this is never done. Moral: there is a difference between measurement and determination.
  • You can investigate the formation of complexes without knowing anything about pKa values, I've had several students who've done it, at least one of whom has published papers on their results. The trick is to make your (neutral) complexes in methanol, using triethylamine to mop up any excess hydrogen ions (which should never really be called "protons" in a condensed phase, even if we all do it). So I wouldn't really call that knowledge "vital". On the other hand, if The Boss tells you to transfer your nice reaction in methanol to 6 M aqueous nitric acid then you need to think very seriously about pKa values! (not my personal experience, but a real one all the same) That is the difference between a "quantitative understanding" and a mere "investigation".
  • [additional point] Ligands are, almost by definition, bases: there is one well-known example of a ligand which is a Lewis base without being a Brønsted–Lowry base, but I feel that it is the exception that proves the rule! However this particular misphrasing was there before Tim's edits, and I include it here merely for completeness.

Physchim62 (talk) 00:25, 27 November 2008 (UTC)

While one can make a distinction between measurement and determination, I think you are being too rigid with respect to this point. Perhaps most things in science are not really "measured" directly, and yet scientists write all the time about measuring things that are arguably determined. Just to give one example look at the title of this IUPAC report: The Measurement of pH - Definition, Standards and Procedures. Yet one does not measure pH directly but determines it based on other measurements, theories, and conventions, even if all the logic is hidden inside an easy-to-use pH meter. --Itub (talk) 10:11, 7 December 2008 (UTC)


Collins English dictionary: Parenthesis A phrase, often explanatory or qualifying, inserted into a passage with which it is not grammatically connected. In "A generic acid, HA, .." HA is grammatically connected. It is neither explanatory nor qualifying. It defines the symbol to be used for a generic acid in the equilibrium expression which follows. Petergans (talk) 11:54, 29 November 2008 (UTC)

Acid strength[edit]

Im sorry chaps, the sentence " A high value indicates that an acid is stronger, because the acid produces more hydrogen ions in solution" is incorrect. From pH#Calculation of pH for weak and strong acids

pH = ½( pKa - log c)

where c is the acid concentration. This equation clearly indicates that the pH of a solution of a weak acid depends on the acid's concentration, which is obvious when you think about it. The ICE table calculation implies it, too. That's why I wrote, some time ago, "at a given concentration".

The introduction of corrosiveness in the lead is not right, because no mention of it is made in the body of the article.

Where on earth did the phrase "because they tend towards infinity" come from? I don't understand the phrase or what it is meant to convey. What has infinity got to do with thermodynamic quantites?

There are not changes for the better. Petergans (talk) 14:31, 2 December 2008 (UTC)

Further material[edit]

During the helter-skelter of the last few days I did not have time to include any of the following material in the applications section. It was sent to me by Ivo Leito after some e-mail correspondence between us. I will not incorporate them myself, for the reasons given in the next section. I hope someone else will do so.

  • Organic synthesis
pKa values of acids and bases are very important in organic synthesis. Many of synthetically useful reactions proceed via acid anions (most notably carbanions – anions of CH acids). Such reactions are catalyzed by bases that generate the anions by deprotonating the corresponding acids. In order to generate a sufficient amount of the anion the pKa value of the catalyst-base has to be of the same order of magnitude or higher than the pKa value of the acid, which is to be deprotonated. At the same time at too high basicity of the catalyst competing side-reactions may occur – different other compounds in the solution may be deprotonated and the generated anions will compete with the desired anions. Mostly these reactions are carried out in nonaqueous solvents and the pKa values of relevance are those determined in the corresponding solvents.
Reference: March's Advanced Organic Chemistry: Reactions, Mechanisms, and Structure, 6th Edition, Michael B. Smith, Jerry March, Wiley, 2007.
  • Liquid chromatography
pKa values of acids and bases are very important in liquid chromatography. In reversed-phase partition chromatography – the most common mode of analytical liquid chromatography– usually polar buffered water-organic mobile phases and nonpolar stationary phases are used. In such system nonpolar molecules are retained more strongly than polar ones. Partition properties of the analyzed substances depend strongly on their ionization states. Neutral molecules are less polar than their ionized counterparts and are thus more strongly retained by the stationary phase. Therefore neutral forms of acids and bases are retained more strongly than their ionized forms. Mobile phase pH enables to control the extent of protonation-deprotonation of acids and bases and varying the pH allows to vary retention times of acids and bases and provides an additional possibility for optimizing chromatographic separations.
Reference: Practical HPLC Method Development, 2nd edition, Lloyd R. Snyder, Joseph J. Kirkland, Joseph L. Glajch, Wiley, 1997.

Petergans (talk) 18:50, 7 December 2008 (UTC)

FAC not promoted[edit]

I am truly sorry that it has come to this after all the time and effort that has been put in, not only to writing the article, but also to replying to the concerns expressed at FAC. I offer my sincere thanks to all who have participated.

It appears that consensus is impossible between us professionals and those reviewers at FAC who, notwithstanding their ignorance of the subject matter, are prepared to instruct us on how to present our material. For this reason I do not intend to spend any more time on this article and do not intend to re-submit as FAC. Petergans (talk) 11:03, 7 December 2008 (UTC)

I'm very sorry as well, not least because of your disappointment. I too honor the tremendous work that you and others have lavished on the article thus far, and I hope that all of us can do a better job in the future at forestalling such disappointments.
I urge you to consider, however, a strength of Wikipedia: that knowledgeable authors can get feedback — sometimes painfully blunt or poorly informed feedback — from their potential readers on the intelligibility of their prose. We don't write for ourselves, but to help others understand and appreciate new things. True, there's no pleasing everyone; but neither should we ignore good-faith criticisms, right?
I have not forgotten my pledge to help with the article, and I suspect several others will be committed to improving it as well. There are many paths by which we can reach our common goals; perhaps a fresh path is called for? You may like what you see, once we've made some progress, and I hope you'll consider rejoining us. Proteins (talk) 11:51, 7 December 2008 (UTC)
Citric acid speciation.png

Fair enough. I feel that I've given all that I can give and it's time for others to pick up the baton. You mentioned Linus as a model. Well he was a genius and if I had a fraction his talent I, too, would be able to write like an angel :-). BTW I still have the EXCEL files from which some of the diagrams were made. The problem with the pictures appears to be associated with the reduction to thumbnail size as the originals do not look so poor (example at the right). Indeed, HySS (freeware) was designed to produce diagrams of publication quality at the width of half a JACS column. Petergans (talk) 14:11, 7 December 2008 (UTC)

I too think that it is a shame that the article was not promoted. However, in the light of aim of Wikipedia (being an anccylopedia) my opinion is the exact opposite of the of Petergans. Yes I see that the reviewers and the topic experts/professionals did not get to an agreement. I think it the aim of the review is twofold. 1) Make sure that the content is relevant and of high quality (this is not what the discussion is about) 2) To make sure the content experts present the content in a way that is understandable and jargon free for the interested intelligent reader with some background knowledge. For this 2nd aim lay-reviewers are not merely equal, but actually better suited than content experts, as content experts may have mistaken ideas about background knowledge. In my view, the reviewers tried their best to get to an agreement how the content presentation maybe improved, which was not taken sufficiently serious by the content experts. This means in my opinion that the decision not to promote was justified as the article was no compelling or even brilliant prose aimed at the moderately knowledgable Wikipedia user. Arnoutf (talk) 17:20, 1 January 2009 (UTC)
The real dilemma here is that people who know little or nothing about the subject thought that the presentation could be improved without having any idea as to how to do it. The prose may not be "brilliant" but I don't think a better concise presentation of the subject matter can be found anywhere, not in any text-book or monograph or review. We wanted to give present the topic with full scientific rigour, not as a dumbed-down version. This may not appeal to "moderately knowledgeable" people, but my concept of an encyclopaedia is that it covers all knowledge. This means that it has to cover topics that require some previous specialist knowledge and indeed WP does do so, in all the sciences. The pity is that none of those articles is ever likely to make the grade for FAC, for the reasons so clearly expressed above. Petergans (talk) 22:56, 1 January 2009 (UTC)
I agree it is a very fine line between dumbing down and writing an accessible text. I agree that this is very difficult, and may result in many very good articles never reaching FAC. The difference between very good and featured should count however, and I think that in the science articles this line is guarded very thoroughly. On the upside this also means that science FA are likely to be really worthy of that title (I have seen some sports or computer games articles making FA wich I would rate at B-class myself....). Arnoutf (talk) 10:42, 2 January 2009 (UTC)
Well, I don't see how anyone can make an objective judgement concerning the quality of something they don't understand. Petergans (talk) 20:04, 2 January 2009 (UTC)
What I think can be done, for someone who has enough of the prerequisite knowledge to be able to learn from the article, is to judge how well it says what it says (as opposed to judging whether what it says is correct). That still leaves the risk of a non-expert rephrasing something and unwittingly losing accuracy. I think it would be good for those such as myself who are not expert in the topic to propose wording changes here on the talk page, rather than in the article, as it is evident that sometimes those changes have caused unintended changes in the meaning. Mike Christie (talk) 21:50, 2 January 2009 (UTC)

Base edits[edit]

I hastily attempted to explain the base section without thinking it through. What I have realized will help in explaining the Kb though. After reflection Kb involves two equilibria instead of the simple one that Ka involves, since it not just involves the formation of the conjugate acid (as I was thinking) but the disassociation of water (I realized the mistake in my math was stupidly making the OH times the H equal to water when multiplying by 1/Kw is required to convert the two).

I am suggesting something like this: Kb represents the ratio of the associated acid to water (i.e. If the Ka of an acid is 1.0 x 10-7 then Kb will be 1 x 10-7 (much less associated acid than water) because Kw (which is the Ka of water) is much lower than the Ka of the acid.)--Jorfer (talk) 01:50, 12 December 2008 (UTC)

Jef, I don't wish to be unkind, and I hope you will take this as constructive comment. The truth is, as shown by your own admission that you had not thought through what you had written, that you are out of your depth here. There is no shame in that. What is shameful is that you won't take on board comments from people like Itub. Regarding pKb all that needs to be said is that the sum of pKb of a base and pKa of its conjugate acid is equal to pKw. Nowadays people usually determine protonation constants so pKb is only of historical interest and does not merit a long discussion.
Please don't edit stuff you do not thoroughly understand. Petergans (talk) 10:35, 12 December 2008 (UTC)
OK, I won't make it a long discussion, just a simple explanation. I know that my editing is not perfect but I am confident other editors will hold me accountable. Making mistakes is part of the process of making an article the best it can be.--Jorfer (talk) 21:02, 14 December 2008 (UTC)
Your edits simply do not make sense - they are not improvements or clarifications. You don't know enough about the subject. Your lack of experience is painfully obvious. Please turn your attention to something you really know about. We have not made up the present text. It is waht you will find in many text-books. Please leave well alone. Petergans (talk) 20:10, 15 December 2008 (UTC)
I feel my most recent attempt is valid (though I admit that my first attempt was mistaken and my second attempt was not good either). Some explanation is needed, and since no one else has been willing to, I attempted to. I would not have done this if I was not confident it would quickly result in an accurate description through collaboration. I messaged Proteins so he can weigh in.--Jorfer (talk) 01:13, 16 December 2008 (UTC)
I suggest that when Proteins is free to collaborate with Petergans on the article, that would be a good time for those who are non-experts to pay attention. Towards the end of that process I think the experts would benefit from hearing which parts of the article are easy for non-specialists to understand. I agree that there have been outbreaks of testiness here, but my experience working with Petergans on this article convinces me that a productive collaboration between experts and non-experts is possible. The article does need to be made as clear as it can be to non-specialists. I believe Petergans, Itub and the others are acting completely in good faith when they revert edits which they say are creating errors and inaccuracies. I'll keep the article on my watchlist, and I hope Proteins can find the time to work on it at some point. Mike Christie (talk) 18:57, 4 January 2009 (UTC)
I concur with Mike. I made a sequence of edits to the article on 19 December addressing some of the concerns raised in my FAC review. These edits have not been reverted. Geometry guy 19:08, 4 January 2009 (UTC)

Petergans and anonymous IP edits[edit]

I want other editors to weigh in on Petergan's removal of wording I included to broaden access, on what seems to me to be his use of an anonymous IP as a hostile sockpuppet, and on his strikethrough edit (all of which can be seen by looking at the history).--Jorfer (talk) 23:52, 2 January 2009 (UTC)

This is a disgraceful personal attack. I NEVER use an anonymous IP. Petergans (talk) 09:45, 3 January 2009 (UTC)
Whatever disagreements you may have with Petergans, as far as I have seen he is unfailingly polite and I doubt very much that that hostile IP is him. Regarding the edits themselves, in my opinion he is correct in removing the inaccuracies and misconceptions that you have introduced as a result of your limited understanding of the topic. I suppose that's the reason the IP removed them too, although with a less civil edit summary. --Itub (talk) 17:19, 3 January 2009 (UTC)
If it wasn't for this edit, I probably wouldn't be accusing him of such. Since IP edits are few and far between on this page, and since the IP is exhibiting extreme anger, it does not appear to be someone detached from the article. These are the first edits this IP has made and it does not appear to be dynamic.--Jorfer (talk) 22:22, 3 January 2009 (UTC)
I agree that it seems plausible that the three edits by were made by someone connected closely with the article, perhaps Petergans. However, the Wikimedia software logs out users after an hour of inactivity, and it is easy to make an edit accidentally while not logged in, thinking that one is logged in. In particular, it is completely inappropriate to accuse an editor of sockpuppetry, when an honest mistake is just as likely an explanation. Further, this is an issue that is best raised on user talk, not article talk. In any case, assumption of good faith is the best way to de-escalate, rather than escalate a disagreement. Geometry guy 22:51, 3 January 2009 (UTC)
Whether it was intentional or not was not the point. The point was Petergans' behavior with this article.--Jorfer (talk) 23:18, 3 January 2009 (UTC)
The IP is an ISP based in New Jersey, whereas Petergans is based in Leeds, so even the presumption of the identity of the IP is inappropriate. Geometry guy 23:29, 3 January 2009 (UTC)
Fair enough, I was hoping someone else could check since I am not an administrator. Just a random coincidental (since it occurred right after Petergans edit) IP interjection...weird but OK. That only leaves the strikethrough edit and argument on several attempts at opening access that have been denied.--Jorfer (talk) 05:18, 4 January 2009 (UTC)

My problem with the most recent edits was the removal of this which serves to increase access:

The graphs on the right have been created based on the definition of Ka at the top of the page and each species pKa values to aid in explaining the mathematical implications of pKa on speciation (the formation of species).

--Jorfer (talk) 22:54, 3 January 2009 (UTC)

This was removed because it is incorrect and confusing. Petergans (talk) 10:31, 4 January 2009 (UTC)

Solution equilibria[edit]

I have just seen that stability constants of complexes has been promoted for Did You Know and will appear on the front page in the next day or two. This article is complementary to ADC. It rounds off the project, which I began last May, to bring solution equilibria in WP up to a decent standard, from the deplorable state that it was in before. The main changes I have made are:

Regarding the last two comments in base edits above, if and when user:proteins is willing to revise the article I will be happy to cooperate with him. Until that time I'll keep watching ADC, but will only intervene in exceptional circumstances. Petergans (talk) 16:27, 7 January 2009 (UTC)

Intro Length[edit]

Jeeze. Theres just no pleasing everyone. (See copious argument about the lead during FAC). EagleFalconn (talk) 07:13, 20 January 2009 (UTC)

Standard free energy change definition[edit]

The article says...

Note that the standard free energy change for the reaction is for the changes from the reactants in their standard states to the products in their standard states.

This is not consistent with my (somewhat limited) intuition. Am I incorrect in thinking that the standard free energy change for the reaction is for the change from all reactants and products in their standard states to the equilibrium mixture of reactants and products? Awaspaas (talk) 04:08, 8 September 2009 (UTC)

Standard free energies are given as if the reaction goes to completion I believe. This is of course a theoretical free energy change as even reactions like the combination of Sodium and Chlorine leaves Schottky defects and Frenkel defects in the latice structure though they are considered to go to completion. This theoretical value is still useful as it can be adjusted for actual yield of the product and can be used for very close approximations where the reaction is considered to go to completion.--Jorfer (talk) 05:39, 8 September 2009 (UTC)
Now that I think of it, those two circumstances may be referring to the same thing, but I'm not sure. If we have a spontaneous reaction of a reactant becoming a product where the product is favored, then the theoretical conversion from pure reactant to pure product would be accompanied by a decrease in free energy. Likewise if a mixture of reactant and product in their standard states were allowed to reach equilibrium, the reaction would proceed in the forward direction to produce more product and consume more reactant, again accompanied by a reduction in free energy. But are those two reductions in free energy the same value? Awaspaas (talk) 21:32, 10 September 2009 (UTC)
You are right that any reaction will eventually go to completion given enough time, and thus will equal the value for Free Energy if standard conditions remain. The key word is given enough time; due to kinetics that can be a near infinite period of time. However, the values are given per mole of reacted material (rather than total material), so the values are to be adjusted for actual yield, making it useful not just in theory but in practice.--Jorfer (talk) 22:40, 10 September 2009 (UTC)
If you are calculating a ΔrG from tabulated values of ΔfG, then you are calculating it for 100% completion. If you get a value which is very close to zero, you know that the reaction will be an equilibrium: on the other hand, if ΔrG is lower than about −60 kJ/mol (−15 kcal/mol) you pretty safe ignoring any equilibrium effects. The equibrium pressure of free chlorine above solid sodium chloride (ΔfG = −384 kJ/mol) is completely negligible! Table salt simply doesn't spontaneously revert to sodium and chlorine! Physchim62 (talk) 10:52, 11 September 2009 (UTC)

The numerical value for the standard Gibbs energy change is based on the complete conversion of one equation mole from the left side to the right side of the equation. One equation mole means that if the equation contains a stoichiometry coefficient other than one for either a reactant or product, that number of moles of that reactant must be converted or product formed. For reactions in solution, if the reaction is reversible, the concentration of reactants and products must remain at standard state throughout the reaction process. There are various hypothetical ways of achieving this result. 1) You have an Atlantic ocean of reactant and product, so that the conversion of 1 mole does not significantly change concentrations. or 2) You constantly replenish reactant and remove excess product to maintain everything at standard state, or 3) you allow the reaction to proceed converting an infinitesimally small amount of reactant to product such that the concentration change on either side of the equation is negligible, and factor the measured result to obtain kJ/mol. For biochemical reactions, ΔG°' is typically determined by measuring equilibrium constants and using -ΔG°' = RT ln K. (talk) 06:01, 27 February 2011 (UTC)

Ionic Strength[edit]

Is there a particular reason why the effect of ionic strength on the the pKa is not dealt with directly? And presentation of the "Debye-Huckel" relationship that can be used to provide the apparent pKa based on the pKa and ionic strength? --Dr DBW (talk) 23:50, 8 October 2009 (UTC)

It is not encyclopedic to demonstrate all forms of a mathematical equation. Wikipedia is not a textbook.--Jorfer (talk) 22:13, 10 October 2009 (UTC)
It is not a simple matter. I am currently working on this: see Bromley equation, Davies equation and SIT theory which I have written. It remain to write Pitzer equation but I don't know when I will have the time to do it. Petergans (talk) 08:09, 12 October 2009 (UTC)
Good stuff :thumbsup: Just wanted to see if there was something I was missing to why it wasn't in there, and seems rather fundamental (to me anyone, but hey, that is where my interst lays) --Dr DBW (talk) 00:18, 14 October 2009 (UTC)

standards states in mixed solvents[edit]

I noticed that in the section discussing mixed solvents (water/meoh for example) for those species which are not normally soluble in water, the it was stated that there is currently no method to mix standard states. This does not reflect the current literature, as the Born Radius model has been used to adjust the standard states for at least 20-years. See for example literature by Chen and Mock. (talk) 14:37, 12 October 2009 (UTC)elephantwalker

polyprotic acid: fractional formation[edit]

Unregistered user:Quantumkinetics added the following explanation for the speciation diagram.

The plots on the right were generated by calculating the percent formation of each species in solution. These values are also known as fractional composition values, α (alpha). In general, the fractional composition of a n -protic acid that has been deprotonated i -times is given as a function of pH (or [H+]) by the equation:

where K0 = 1 and the other K-terms are the dissociation constants for the acid. So for a generic diprotic acid, 3 species are present in solution: H2A, HA-, and A2-. The fractional composition values can be calculated as shown:

It is more complicated than it needs to be because a product such as is simply the cumulative constant βj and the denominator is, of course, the same in all expressions of fractional formation . Moreover, the equations do not apply at high pH where the self-dissociation of water must also be taken into account.

Since the specition diagrams were not calculated in this way, the expressions have been removed to this discussion to avoid giving a misleading impression. Petergans (talk) 08:14, 13 June 2010 (UTC)

More on unfavorable entropy contribution[edit]

  1. First a note to Petergans: Yes, Levoslashx's edit today is incorrect. But I think a detailed edit summary (35 words) deserves more than a one-word dismissal as "ignorance". This is a good-faith edit, which a confused student could make. Perhaps an explanation of the error can help clarify the article for others.
  2. The source of his/her error seems to be that the table has a column heading (-)T ΔS, which does have positive values. I will change the signs to list (+)T ΔS with negative values.
  3. The edit summary asks "If the enthalpy is positive, and the entropy contribution was unfavorable, how could it be spontaneous?" The answer is that these are standard-state values (indicated by the superscript StrikeO.png, valid when reactants and products are all at unit activity (1 mol/L for ideal solution). This is noted in the last paragraph of the Thermodynamics section. In the standard state it is true that the ionizations are not spontaneous. However they do have small non-zero equilibrium constants, meaning that they are spontaneous when the ion concentrations are sufficiently small (Q < K).
  4. Finally the physical reason for the negative entropy of ionization is that the ions formed tend to organize the solvent water molecules around them, which decreases the ionization entropy of the system. Dirac66 (talk) 16:40, 26 February 2011 (UTC)
  1. I posted a detailed explanation on User talk:Levoslashx at the time of the reversion.
  2. I was following the widely used convention as in the cited references.
  3. Of course there is spontaneous dissociation when HA is added to water. The significance of the equilibrium constant is that it allows the extent of dissociation to be calculated, as in ice table. What does "However they do have small non-zero equilibrium constants" mean? Equilibrium constants are, by definition, independent of concentration, apart from some possible variation due to changes in the quotient of activity coefficients.
  4. I am aware of the explanation, but could not find a citation to verify it. Petergans (talk) 17:32, 26 February 2011 (UTC)
  1. OK, good.
  2. Yes, I know many authors list (-)TΔS as it is easier to put only one minus sign in the table. But I think this way is clearer as it is harder for a reader to miss a whole column of minus signs than to miss one as Levoslashx seems to have.
  3. Here I did not mean to suggest that K depends on concentrations, but rather that the spontaneity (sign of ΔG) depends on concentrations through ΔG = ΔGo + RT ln Q. My answer #3 above is too long for this article, but after the words "dissociation of a weak acid is not a spontaneous process" we could add "when reactants and products are in their standard states", or just "in the standard state".
  4. Atkins' Physical Chemistry (8th edn p.94) mentions that ion entropies vary in relation to their degree of ordering the surrounding water. I will try to write a sentence or two based on this. Dirac66 (talk) 23:14, 26 February 2011 (UTC)

I have added a ΔG column to the table, and reverted to tabulating minus T ΔS. It should be much clearer now that ΔG = ΔH - TΔS. Petergans (talk) 16:12, 28 February 2011 (UTC)

Measurement of pKa of strong(ish) acids[edit]

I'm having trouble with this paragraph, which may have become garbled by contributions from different editors:

"In water, measurable pKa values range from about −2 for a strong acid to about 12 for a very weak acid (or strong base). All acids with a pKa value of less than −2 are more than 99% dissociated at pH 0 (1 M acid). This is known as solvent leveling since all such acids are brought to the same level of being strong acids, regardless of their pKa values. Likewise, all bases with a pKa value larger than the upper limit are more than 99% protonated at all attainable pH values and are classified as strong bases.[3]"

"measurable pKa values range from about −2 for a strong acid" I would have thought that the measurable range in water would be +2 to 12[**]. At low pH we have the problem of what is meant by pH itself, plus the buffering due to H2O accepting protons from the strong acid, although there are equations[*] that adjust for water ionization. Likewise at very high pH, H20 buffers by loss of protons.

"All acids with a pKa value of less than −2 are more than 99% dissociated at pH 0 (1 M acid). This is known as solvent leveling since all such acids are brought to the same level of being strong acids, regardless of their pKa values." These two sentences seem either to be non sequitur. or don't explain the connection completely.

Can I suggest the following rewrite?

In water, readily measurable pKa values range from just below 2 for a strongish acid to slightly greater than 12 for a very weak acid (conjugate of a strongish base). For stronger acids, H2O itself acts as a buffer, accepting H+ from the strong acid, and limiting the change in pH. Differences in definition of pH in terms of activity versus concentration also make measurement difficult when [H+] is high. As a result, acids with very negative pKa do not lower the pH below zero in aqueous solution even though an acid with pKa -2 should be 99% dissociated at pH 0. This is known as solvent leveling since all such acids are brought to the same level of being strong acids, regardless of their pKa values. Very low pKas can be determined using polar solvents other than water. A similar buffering effect due to donation of H+ from H2O occurs at pH 14, limiting the measurable pKa of very weak acids (conjugate acids of strong bases).

[*] I encountered this equation many many years ago when I taught phys chem for biochemists. As I recall, the textbook was Chang which I no longer have in my possession.

[**] I think the workable range is more like 1.5 - 12.5 by maybe we don't want to get into fractional values. (talk) 19:44, 26 February 2011 (UTC)

This does seem clearer. Two suggestions for alternate wording:
  1. "strongish" is not an English word. From memory, the usual term is "fairly strong".
  2. "buffer". I don't recall seeing "buffer" used in this context, although I suppose the effect is analogous to buffering. Usually one says that H3O+ is the strongest acid that can exist in water, since any stronger acid transfers a proton to H2O and forms H3O+. And similarly OH- is the strongest base that can exist in water. Dirac66 (talk) 23:33, 26 February 2011 (UTC)
It is indeed buffer action in that H2O is mopping up H+ from the strong acid, and limiting further change in pH. It's unfamiliar in that we don't usually think of buffers having pKa 0 or pKa 14, nor do we normally expect buffers to be present at 55 mol L-1. The relevant buffer equations are
pKa = 0
H3O+ ⇌ H2O + H+
H+ here represents protons from the strong acid; addition of strong acid drives the reaction right to left.
pKa = 14
H2O ⇌ OH- + H+
H+ here represents protons captured by the strong base; addition of strong base drives the reaction left to right. (talk) 02:09, 27 February 2011 (UTC)
The usual definition of a buffer solution is a conjugate acid-base couple. So to be consistent we should say that the buffer at pH 0 is the couple H2O - H3O+, rather than only H2O. And similarly the buffer at pH 14 is the couple H2O - OH-. Dirac66 (talk) 02:45, 27 February 2011 (UTC)
Yes, that's it exactly. (talk) 06:04, 27 February 2011 (UTC)
buffer capacity as a function of p{H]

It is true that the buffer capacity of water increases steeply at high hydrogen ion concentrations, see diagram at right. (I will add this and the equation on which it is based to buffer solution). Nevertheless, there is a fundamental flaw in the arguments above, which is, that all pK determinations are based on pH measurements. At very high or low hydrogen ion concentrations pK values are obtained as concentration quotients from spectrophotometric measurements. The hydrogen ion concentration is calculated from its experimentally known analytical concentration. There is always the possibility of confusion when the word pH is used. The glass electrode can be used to measure H+ activity or concentration depending on how it is calibrated (buffer solutions or acid/base titration). When calibrated in concentration the measurement should be written as p[H] but this convention is not universally accepted.

The lower limit of -2 is a theoretical limit which is based on the calculated extent of dissociation of an acid in 1M H+ (concentration) solution. It is stated to be approximate. BTW I deprecate the use of weasel words such as "readily", "strongish" in this context, as pK is a quantitative measure, not a qualitative one. The term "strong acid" comes to us from the past, as an acid which is "fully dissociated". Quantitatively there is no such thing as 100% dissociation, so we have to interpret "fully" as meaning that the undissociated acid concentration is below the limit of measurability. I will add the missing link to solvent leveling which should help to explain that sentence. Petergans (talk) 10:38, 27 February 2011 (UTC)

Um, why does your graph have a local maximum at pH 7? I learned that pure water has a negligible buffer capacity, since there is no conjugate acid-base couple which is effective at pH 7. Dirac66 (talk) 14:16, 27 February 2011 (UTC)
What's being shown is the buffering capacity as represented by the lowest concentration of a member of a buffering couple. At pH 7, this is 10-7 mol/L. At pH 6 [H+] is 10-6 mol/L but [OH-] is 10-8 mol/L. Hence buffering capacity seems to go down. At low pH, the H2O / H3O+ couple starts to dominate, so buffering capacity starts to rise again. Same happens at high pH when the OH- / H2O couple dominates. The objection might be that OH- / H+ are not members of a buffer couple, so why don't we treat H2O / H3O+ as the operative buffer couple at pH 6? (talk) 22:33, 27 February 2011 (UTC)
Unless what is depicted is the buffer capacity of a pH 7 buffer solution rather than pure H2O. (talk) 23:38, 27 February 2011 (UTC)

Just so. See buffer solution for the updated explanation. Petergans (talk) 13:57, 28 February 2011 (UTC)

For HI entropy favors dissociation[edit]

From W.E.Dasent "Inorganic Energetics" (2d edn 1982, Cambridge Univ Press) p.168:

Reaction HX(aq) → H+(aq) + X-(aq) X = F Cl Br I
ΔH° / kJ -16 -57 -65 -62
(-)TΔS° / kJ (*) +30 +10 +4 -3
ΔG° / kJ +14 -47 -61 -65
Larger monoatomic anions are less effective in ordering solvent, and this trend contributes to strengthening the acid. For HI Ka is estimated at 1011. I will think about how to put this into the article.
(*) I have put a minus before the entropy term for consistency with the article. However Dasent is one source who gives the values of (+)TΔS°. Dirac66 (talk) 01:04, 1 March 2011 (UTC)
My last attempted post seems to have disappeared, in which I explained the addition of a ΔG column and reversion of sign for TΔS. The idea is to show the components of ΔG that are added together. Showing ΔG explicitly makes this clear.
The Dasent example is dubious since K has not been measured except for HF. In any case, I did not include any comment on the fact that ΔS is negative because it would have been based on speculation. Though it is well-founded it's not the same as experimental evidence. The problem is that simulation of ionic solutions is extremely complicated. M.R. Wright, "An introduction to aqueous electrolyte solutions, Wiley, 2007 has a good discussion of computer simulations for studying solvation, chapter 13, pp 542-560, but I think it's a bit off-topic for this article. Petergans (talk) 09:34, 1 March 2011 (UTC)
Sorry - I confused you by moving your last post to the end of the second previous section because it was part of that discussion. I did note the move in my edit summary.
It is true that Dasent's values for HCl, HBr and HI are only theoretical estimates, so the conclusion that TΔS is positive (or (-)TΔS is negative if you prefer) is not very definitive. But we can use such data for HCl, HBr, HI to say that the entropy contribution is less important for anions of large atoms, and that it has even been estimated to be of opposite sign in the case of HI. Dirac66 (talk) 14:28, 1 March 2011 (UTC)

I have removed the reference to spontaneous, because it was wrong to include it (my mistake!). A reaction is spontaneous if ΔG is negative, not ΔGStrikeO.png. The discussion of entropy looks just about right as it is. I have not been able to find anything more specific in the books available to me here. Petergans (talk) 07:50, 2 March 2011 (UTC)

On borate ionization, hydrolysis issue[edit]

It may be that this citn. will be good enough, but something deeper would be nice:

A. Earnshaw & Norman Greenwood, 1997. Chemistry of the Elements, 2nd Edn. Butterworth-Heinemann, pp. 203-205. [ ISBN13 978-0750633659 ]

Prof D. —Preceding unsigned comment added by Meduban (talkcontribs) 17:37, 13 May 2011 (UTC)

The boric acid article refers this fact to W.L. Jolly, Modern Inorganic Chemistry, McGraw-Hill 1984 (p.198), which I have just rechecked. The conclusion is based on the Raman spectrum of B(OH)4- observed in strongly alkaline solutions. Yes, the source should be given in this article also, so I'll copy it in. I don't have a copy of Earnshaw + Greenwood to check, but please add it too if it provides additional support for the statement. Dirac66 (talk) 19:30, 13 May 2011 (UTC)

"(B(OH)3) acts as a weak acid, and is sometimes described as a proton donor ". This is incorrect - candidate for my howler list?. It is described as behaving like a proton donor. Petergans (talk) 08:07, 14 May 2011 (UTC)

"Behaving like a proton donor" is more correct but possibly unclear to nonexpert readers. How about "behaving like a proton donor in increasing the concentration of H3O+"?
Also I have found two more sources for the formation of B(OH)4-. Cotton and Wilkinson "Advanced Inorganic Chemistry" 5th edn p.169, and Housecroft and Sharpe "Inorganic Chemistry" 2nd edn p.315. Jolly is still the best source however because it indicates that the conclusion is based on the Raman spectrum. Dirac66 (talk) 17:16, 14 May 2011 (UTC)
This is a very old story, predating C&W and H&S by at least half a century, i.e. my student days -:(. I don't know where the original proposal came from, but I guess it was based on thermochemical evidence about bond energies and the calculated OH bond dissociation energy. Petergans (talk) 09:29, 15 May 2011 (UTC)
OK then, I have inserted "confirmed by Raman spectroscopy" here and at boric acid. This removes the suggestion that Raman was the first evidence, while still citing Jolly as a reliable source. Dirac66 (talk) 14:08, 15 May 2011 (UTC)

pH (Help for the Layman)[edit]

Please let me know if there is a more appropriate forum for my issue: I'm not suggesting dumbing this article down, just adding something to make it useful to the layman. I have tried to understand this article a couple times, but I don't see any way to relate it to pH. The average education includes an introduction to the pH scale, and most people can understand the comparative acidity or alkalinity of a substance by its location on that scale. Since the infobox on such articles as Citric Acid provides pKa values rather than pH, and the Acidity link comes to this page, I think it behooves us to at least offer a way for the layman to convert pKa into pH, assuming that's even possible. If not, then I guess I'll see if there's a way to have the pH available in addition to the pKa.

Your average Joe just wants to know, "Is citric acid more acid than acetic acid?", or something like that. Wikipedia should be able to provide that without sacrificing the technical creamy-center goodness. Thank you for your forbearance. Keyesc (talk) 02:11, 8 December 2011 (UTC)

Briefly, the pH of an acid solution depends on both the pKa of the acid AND its concentration. So the comparison would have to be done at equal concentrations for example. We could however add a section explaining how the pH is calculated for an example or two. Dirac66 (talk) 03:49, 8 December 2011 (UTC)

The smaller the pKa, the more acidic the compound. pH is dependent on concentration, so it is pretty much meaningless on its own. The concentration of H+ ions for a 1 M solution of a weak acid (HA) in water can be calculated:

Making the assumption that all the H+ ions are derived from the acid, HA, we can approximate [H+] = [A<sup.>-]. Rearranging, we can derive an expression for the concentration of H+ ions, [H+]

pH is simply -log [H+]

Do note that many assumptions are used here. The bottom line is that pH and pKa is not trivially convertible. pH defines the acidity of a specific solution, whereas pKa is a measure of how ionizable any hydrogen substituent is (one molecule has one pKa for each hydrogen). --Rifleman 82 (talk) 03:56, 8 December 2011 (UTC)

Okay, thanks for this. We definitely need to include a section on pH prominently here, since this is the only measure of acidity your average reader will ever have met. Maybe a second or fourth sentence should be added along the lines of 'Unlike pH, which is a measure of the acidity of a particular concentration of an acid, pKa measures the acidity of a chemical directly - so hydrofluoric acid always has a pKa of 3.17, but if it is diluted then the pH of the solution will depend on how much water it is mixed with.' -- (talk) 18:43, 17 June 2014 (UTC)

Actually, the concept of pH has a much wider application than simple acids. It's not something that can be simplified down to a sentence or two. For example, the pH of a solution of a base can be calculated from pKb which is related to pKa ... Also, salts like Al2(SO4)3 and FeCl3 dissolve in water to give acidic solutions. I think that what the layman needs is a qualitative exposition of the idea of acidity, but this is not the place for that. This article is specifically about acid dissociation constants. By their very nature they are quantitative, not qualitative. The article on pH is more directed towards the layman.Petergans (talk) 22:01, 20 June 2014 (UTC)

pH/pKa inconsistency in phosphoric acid[edit]

Just as a small comment: in acid dissociation, when the acid and conjugate base are equal in concentration, then pH = pKa. for the graph depicting phosphoric acid dissociation, this is incorrect; the first pKa equals 2.15, and in the graph, the 50/50 distribution quantities is not at that value. any suggestions? (talk) 14:16, 25 September 2014 (UTC)

Yes, you are correct. The graph appears to show pKa1 at about 3.1, so it needs to be redrawn. Dirac66 (talk) 03:12, 26 September 2014 (UTC)
I agree. It looks as though I used a different value of pK1 in the table and when creating the graph.I'll have to check which is the "correct" value to use. I will attend to this in the next few days. Petergans (talk) 10:00, 26 September 2014 (UTC)


Perhaps this article should be completely re-written as it is terribly unencyclopedic and requires pre-existing specialist or expert knowledge of chemistry to comprehend and is of little if any educational value to the casual reader. Compare to the "internal combustion" article, while it describes a complex system of many interacting physical and chemical processes, it is concise and accessible to the intended audience and 25% shorter than this article. Just as it would be inappropriate for a chemist or chemistry student to rely on a general knowledge encyclopedia as a primary source for information on a chemistry topic, the reverse is true, as a lay person seeking basic information on a topic consults an encyclopedia rather than a detailed technical textbook . That is where this article belongs in a text book or a professional reference. (talk) 06:01, 8 December 2011 (UTC)Moi

I think you are mistaken in thinking that pH is a property specific to a compound, because it isn't. pKa is the correct measure of the acidity of a hydrogen substituent of a compound, and that's exactly what is said in the first sentence of the article. This topic is pretty basic, and it might be covered at the upper secondary school level, or in the first year of undergraduate chemistry. This topic is perhaps the foundation for a topic like the Henderson-Hasselbach equation, for example. There is a little math but that is quite unavoidable. --Rifleman 82 (talk) 06:16, 8 December 2011 (UTC)

Actually, yes I was under that incorrect pH assumption, perhaps the lead could explain the concept in a more simplified rudimentary style and include an explanation on the differences of the concept of pH and pKa. The extent of my formal science education was an introductory Physics 101 class so I was until now ignorant of the acidity constant until reaching this article. P.S. Thank you for proving wrong the stereotype commonly held by many people who regard infantrymen as having have a rather limited intellectual capacity. :) (talk) 07:09, 8 December 2011 (UTC)Moi

The fundamental difference between the IC engine a pKa value is that an acid dissociation constant is a quantitative measure of a molecular property, so a qualitative presentation cannot convey the essence of the concept.
Secondly, there is an inherent contradiction between making something of this nature comprehensible to the layman and using proper scientific rigour. I opt for scientific rigour. In my view, the alternative is waffle and Wikipedia has far too much of that already.Petergans (talk) 11:39, 8 December 2011 (UTC)

Calculation of the Ka value[edit]

For example:

Ka(HCl) = [Cl
]÷[HCl] = (Density(Cl
)÷Molar mass(Cl
)÷Molar mass(H+
))÷(Density(HCl)÷Molar mass(HCl)) ≈ (?g/L÷35.45g/mol)×(?g/L÷1.01g/mol)÷(1.49g/L÷36.46g/mol) ≈ ?mol/L×?mol/L÷0.04mol/L

But how to get the Density(Cl
) and Density(H+
)? Or is it possible to get the [Cl
] and [H+
] directly?

Thanks. (talk) 13:01, 27 May 2014 (UTC)

No, it is not possible to measure the density of each component of a solution. The classical method determines the ion concentrations by measuring the electrical conductivity of the solution, knowing the molar conductivity. For HCl, the ion concentration is about equal to the acid concentration, which shows that the acid is 100% dissociated and is a strong acid. For a weak acid such as CH3COOH, the conductivity gives the ion concentrations which allow Ka to be calculated as in your first line. Other methods can also be used to measure the concentrations, such as spectrophotometry. Dirac66 (talk) 19:12, 27 May 2014 (UTC)
Perhaps a link between molar conductivity and activity coefficient is required for strong electrolytes and the hypotthesis of full dissociation is not tenable.-- (talk) 15:59, 26 September 2014 (UTC)

Standard state symbol[edit]

I switched from a graphics-file symbol to a unicode character that looked about the same, in order to give better display when the font-size or background-color is not what is hardcoded into the graphics file. I chose the character based on the character-equivalents noted in the file itself by User:Pieter Kuiper and in keeping with what Standard state says IUPAC recommends. I called it a plimsoll since that was an easy thing to say and link in my edit-summary, but that is maybe an older or typographic term not commonly used in chem and/or wouldn't technically have the same bar length as the chemistry symbol. But call it what you like..."ominus" ( seems to be used in chem/physics style-guides for LaTeX), "strikethrough-O" (File:StrikeO.png is file I swapped out) or U+29B5 Circle with horizontal bar) (that's the unicode char I swapped in)...that it's a valid symbol here is supported by WP:RS. @Petergans:, what are the specific differences in the symbol you are seeing and what is the basis for choosing? DMacks (talk) 15:06, 30 April 2015 (UTC)

Unfortunately the symbol inserted by DMacks appears not to be supported by all browsers. Both my Firefox and my Explorer show it as a blank square (not circle), suggesting that the browsers do not recognize the symbol. From his comments I think DMacks sees it (presumably in a different browser) as a barred circle of slightly more appropriate proportions than the symbol used by Petergans. This may be true in some browsers, but in the interests of having a symbol which everyone can see, I am going to revert to Petergans' version of a plimsoll. (Such problems did not arise with paper encyclopedias :-)) Dirac66 (talk) 15:20, 30 April 2015 (UTC)
How does o look for you? DMacks (talk) 15:40, 30 April 2015 (UTC)
Yes, that is indeed visible in both Firefox and Explorer, presumably because it is just Wikipedia source code without the special Unicode character. Dirac66 (talk) 16:22, 30 April 2015 (UTC)
I also saw the blank square on my browser. The symbol o is too small. 0 or O may be OK for text, but neither corresponds exactly to the symbol used in books. I considered them, but decided in the end on the graphic. Incidentally, this symbol may appear in many articles where thermodynamic properties are discussed. The representation in equations,e.g. , is not wholly satisfactory either. Petergans (talk) 18:51, 30 April 2015 (UTC)

This was settled last year, but yesterday editor seems to have reinserted the symbols which are illegible in Firefox and Explorer into the Thermodynamics section. In the interests of having a Wikipedia article which is legible in all browsers, I will restore the StrikeO.png symbols, without reverting his/her edits entirely as they do contain other useful changes. Dirac66 (talk) 23:47, 2 April 2016 (UTC)


In the section on self-ionization of water the reaction is given as

H2O ⇌ OH + H+

Why H+ and not H3O+? The answer is that the aquation of the proton is not an equilibrium process - it goes to 100% completion. It is therefore irrelevant to Kw, which is an equilibrium constant. When the proton is involved in chemical equilibria in water it is a universal convention to represent it as H+, even though the species in solution is the hydrated hydronium ion. Petergans (talk) 08:59, 7 August 2015 (UTC)

H3O+ is a more correct (though not exact) description of the cation in water than is H+, so the equation 2 H2O ⇌ OH + H3O+ is really more correct. It is true that a common convention is to simplify the equation as you have done, both because it is simpler and for historical reasons: Arrhenius believed the cation was H+ before Bronsted modified it. But there is not a universal convention to always write it that way. Of the two general chemistry books on my desk, Whitten et al (4th ed. p.359) first writes the H3O+ equation and then says In simplified notation and gives the H+ equation, while Petrucci et al. (8th ed. p.672) and only gives the H3O+ equation.
In summary both versions are valid and widely used. So I think that to be of maximum help to uninitiated readers, this section should include both versions and make clear that chemists treat them as equivalent. Dirac66 (talk) 21:29, 7 August 2015 (UTC)
I fear that we would be perpetuating a text-book error. Kw refers to an equilibrium, so its value is independent of any non-equilibrium process. Thermodynamically, nothing is gained by putting another water molecule on both sides of the equilibrium expression. In reality the hydronium ion is further aquated. Why not go all the way and use H9O4+? I prefer the common convention that, in solution, H+ refers the the solvated hydrogen ion. This convention applies to other aquo-ions like K+ whose hydration structure in aqueous solution is not known in detail.Petergans (talk) 07:36, 8 August 2015 (UTC)
Wikipedia depends on sources, so before deciding that textbooks are in error we need to find other sources. I have now checked Housecroft and Sharpe's Inorganic Chemistry (2nd ed p.163) which also writes only the equation with H3O+, but does add the explanation that In aqueous solution, protons are solvated and so it is more correct to write H3O+(aq) than H+(aq). Even this is oversimplified because the oxonium ion is further hydrated and species such as H5O2+, H7O3+ and H9O4+ are also present. So we can use this as a source (rather than general chemistry books) and refer to H3O+ as well as the larger cations. I do not object to also mentioning that H+(aq) is used as a common abbreviation for the whole set, but I do object to completely suppressing mention of H3O+ in this section. Dirac66 (talk) 16:30, 8 August 2015 (UTC)
I also consider the point about H+ + H2O → H3O+ being a nonequilibrium to be irrelevant because all modern chemists understand that H+(aq) not a bare proton but an abbreviation for H3O+(+ H5O2+ + ...). So what we are calling H+ is actually the same as H3O+ (except for the added H5O2+ etc.) and there is no conversion to consider. The real question is which formula is most helpful to the reader: H3O+ is closer to the truth than H+ taken literally (as a bare proton), but H+ is used by chemists as an abbreviation for the complete mixture of all the hydrated cations so it can be considered exact provided the reader understands it. That is why it is essential to explain the meaning of H+(aq) at least. Also since many authors do use H3O+ as an approximate description, it is important to mention it as well although yes, we can explain that it is incomplete. Dirac66 (talk) 17:31, 8 August 2015 (UTC)
We are here dealing with the conflict between thermodynamic concepts and molecular structure - neither has much to say to the other. Kw is a thermodynamic quantity and its value embraces the state of solvation of the hydrogen and hydroxide ions. I wish we could keep thermodynamics and molecular structure separate from each other, but it is a fact than they are scrambled together in many sources. In WP it behoves us to maintain the distinction, citing separate sources for the separate aspects. BTW, the same issue arises with metal complexes as in a reaction such as M+L=ML - the state of aquation of the metal ion is ignored when talking about the stability constant. Petergans (talk) 16:39, 9 August 2015 (UTC)

Reaction equilibrium formula error[edit]

In reference to Acid-base reaction#Acid-base equilibrium: In the following text I believe the Ka1 and Ka2 constants are on the wrong side of the first two equations, leading to another error where their ratio is reciprocated in the last equation.

The equilibrium constant for this reaction can be derived from the acid dissociation constants of adenine and the hydrogen phosphate ion.

[AH] = Ka1[A−][H+]

[H2PO4−] = Ka2[HPO42−][H+]

The notation [x] signifies "concentration of x". When these two equations are combined by eliminating the hydrogen ion concentration, an expression for the equilibrium constant, K. is obtained.

[AH][HPO42−] = K[A−][H2PO4−], K=Ka1/Ka2

The above formula contradicts the formula contained on Acid dissociation constant ...not being an expert I did not edit the entry. — Preceding unsigned comment added by DGElder (talkcontribs) 20:21, 22 January 2016

The above was posted at WT:Be bold. I moved it to here, and formatted, and will ask for help at WT:WikiProject Chemistry#Acid-base reaction. Johnuniq (talk) 21:52, 22 January 2016 (UTC)
There is no error. The expression
[AH] = Ka[A][H+]
is a definition of Ka Petergans (talk) 11:52, 23 January 2016 (UTC)
Um, yes, there is an error in this definition. The usual form of the definition is Ka = [A][H+] / [AH]. If we isolate [AH], we have [AH] = [A][H+] / Ka. Or equivalently Ka[AH] = [A][H+]. The last form is what DGElder seems to have had in mind in saying that the Ka are on the wrong side.
However the error is not in this article, but in Acid-base reaction#Acid-base equilibrium. I will check that article and make the necessary changes. Dirac66 (talk) 01:53, 24 January 2016 (UTC)

I hope you are right...

There is potential confusion relating to association and dissociation constants. My suggestion is always to write the definitions explicitly. For a monobasic acid

[HA] = K[H+][A-], (dissociatoon)
[H+][A-] = K[HA], , (association)

There is neither right nor wrong with these definitions. Both are equally valid, so it is important always to state explicitly which definition is being used. In the case of an article entitled dissociation constants the statement is in the title. Please note: the dissociation definition is preferred by organic chemists, but the association definition is used in general-purpose computer programs for equilibrium constant determinations (and hence publications) This is because association constants have always been used for metal complexes. For consistency stability constants are defined as association constants for both metal complexes and the ligands. Note also that pK for dissociation of a monobasic acid is numerically equal to the logarithm of the association constant.Petergans (talk) 10:32, 24 January 2016 (UTC)

I agree that both dissociation and association constants are used in chemistry, so that it is necessary to make clear which is meant in each context. The discussion above refers specifically to Acid-base reaction#Acid-base equilibrium, where the text refers to acid dissociation constants of adenine and phosphate ions. No metal complexes are involved.
In other contexts of course, association constants may be more appropriate. But not here. Dirac66 (talk) 16:35, 24 January 2016 (UTC)


Erlenmeyer flasks with different pH values (1 - 14) and an indicator. The different pH values can be caused by acids with the same concentration but different acid dissociation constants.

I have removed this pretty picture because it shows neither the pH values nor which indicator is responsible for the colour. If I knew which indicators were used I would be able to insert it in the appropriate article, which is not this one. Petergans (talk) 09:08, 27 January 2016 (UTC)

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Basicity and acidity constants in infoboxes for metal hydroxides ?[edit]

The Wikipedia articles on several metal hydroxides have infoboxes with pKa and pKb values and no explanation. For example the infobox on calcium hydroxide says Acidity pKa 12.4 and Basicity pKb 2.37. That infobox has links for acidity and basicity which point to this article, which defines both Ka and Kb with reference to the usual Bronsted deprotonation and protonation reactions. However examples in chemistry books are usually organic, e.g. carboxylic acids and their anions, or amines and their cations. As a chemist I have difficulty believing that the values for Ca(OH)2 really refer to the formation of Ca(O)OH- and Ca(OH2)(OH)+ (??). I have found one website [here] which gives a similar value (2.43) for pKb with the explanation that it is for the Arrhenius-type dissociation of one OH-, in this case to give Ca(OH)+.

So we seem to have infobox values in other Wikipedia articles with a link to an explanation in this article which does not apply. What to do? Is this alternative Arrhenius-type definition of Kb for bases such as hydroxides widely accepted? (I have not seen it elsewhere.) If so, should we mention it in the Basicity section of this article as an alternative to the usual Bronsted definition of Kb for appropriate solutes. If not, should we delete all the Basicity values from the metal hydroxide infoboxes because they do not conform to the correct (Bronsted) definition of Kb? And what about the acidity value for Ca(OH)2 etc. Dirac66 (talk) 22:14, 17 December 2016 (UTC)

The species Ca(O)OH- could occur in a very basic solvent such as liquid ammonia,
Ca(OH)2 + NH3 ⇌Ca(O)(OH)- + NH4+
but Ca(OH)2 only behaves as a base in aqueous solution. The two pK values are related to each other by pKa + pKb = pKW. When a value for one of the two constants is determined and reported in a publication there is uncertainty regarding the value of the other one unless the value of pKW, which depends on solvent, temperature and ionic strength, is specified; one value cannot be converted to the other willy nilly. (n.b.12.4 + 2.37 = 14.77 !) Petergans (talk) 10:03, 19 December 2016 (UTC)
Hm. I had assumed that the pKa and pKb in the Ca(OH)2 infobox were both intended to describe Ca(OH)2. Whereas the relation pKa + pKb = pKW which you cite applies of course to an acid-base conjugate pair, and not to the pKa and pKb of a single species. I don't know which of us (if either) is interpreting the numbers correctly, and the ChemBuddy website whose URL I cited above claims that the pKb describes an Arrhenius-type dissociation of OH-. So now we have three interpretations - clearly we need the source of the numbers so we can verify which is correct. Dirac66 (talk) 19:58, 19 December 2016 (UTC)
I have searched the literature using SC-database. The equilibrium
Ca2+ + OH- ⇌ Ca(OH)+
has a reported logKa values at 25C in the range 0.96 - 1.51 (17 publications) and logKb between 10.3 and 13.36 (3 publications). The log beta value for
Ca2+ + 2OH- ⇌ Ca(OH)2
has been estimated from solubility data at 3.14 (1 publication); this value is reasonable but has not been confirmed experimentally. Petergans (talk) 12:35, 20 December 2016 (UTC)
OK, these Ka and Kb values tell us that some authors do use this notation for this non-Bronsted equilibrium. Can you find in the SC-database or elsewhere a clear statement of what reactions have these equilibrium constants? I could start guessing but it seems preferable to have a source. You have written that logKa and logKb refer to the same equilibrium which I do not understand.
As for the beta value, does this refer to the β12 at Stability constants of complexes#Stepwise and cumulative constants? If so, does [ML2] refer to solid Ca(OH)2, in which case it seems β12 should be very simply related to solubility and I don't see why experimental confirmation is a problem? Or does it refer to dissolved neutral Ca(OH)2, which might be harder to measure? Dirac66 (talk) 20:40, 20 December 2016 (UTC)

The following convention is generally understood, but should be stated explicitly in publications. For the general equilibrium (no electrical charges specified)

pA + qB + rH ⇌ ApBqHr

the equilibrium constant is defined, in terms of the equilibrium concentrations, [A], etc (more rigorously, activities {A}...) by

[ApBqHr] = βpqr[A]p[B]q[H]r

The only caveat is that this defines an association constant, with product on the left.For dissociation constants reagents and product swap positions. pKa is an acid dissociation constant, whereas pKb is a base association constant. The two concepts can be applied to the same protonation/deprotonation process, in which case the sum equates to pKw, as stated above.Petergans (talk) 10:53, 22 December 2016 (UTC)

Yes, this definition is clear as long as the species in question are identified and well defined, which should be done in the article on each chemical species if there is any doubt. For protonation/deprotonation the species are usually clear to chemists, but perhaps not always to all Wikipedia readers.
And the relation pKa + pKb = pKw is true provided pKa and pKb refer to a single acid-base conjugate pair, but then of course the two values do not describe the acidity and basicity of the same species as the infobox format might suggest if no notes are added. For example, the infobox for acetic acid says "Acidity pKa 4.76" and "Basicity pKb 9.24 (basicity of acetate ion)". The parenthetical note is there to help the reader who might otherwise imagine that 9.24 is the pKb of acetic acid !?? Similarly, the infobox for Bicarbonate says "Acidity (pKa) 10.3 (Conjugate acid of carbonate)" and "Basicity (pKb) 7.7 (Conjugate base of carbonic acid)". Here the reader might wonder why pKa + pKb is 18 and not 14, so the notes clarify that they do not refer to the same conjugate pair.
For Ca(OH)2, pKa + pKb = 14.77 which is closer to 14. I assumed above that the values nevertheless refer to two different conjugate pairs (three species) as for HCO3-, whereas I think you assumed that they do refer to a single pair (species) and that the difference from 14 is due to some other effect: temperature? non-ideality? different experimental results? I don't know which is correct and I think we need a source which specifies the reactions involved. Then we could put a note in the infobox, as for acetic acid and bicarbonate ion. Dirac66 (talk) 19:27, 22 December 2016 (UTC)
For the alkali metal hydroxides (MOH) which are simpler, one idea is a note in the infobox after the pKb values saying (Dissociation of OH-). However if the equilibrium is assumed to be MOH(s) = M+(aq) + OH-(aq), then (in ideal solution) Kb = Ksp = [M+][OH-] = s2. Unfortunately this fits the infobox data very poorly. For LiOH s = 128 g/L = 5.33 mol/L leading to Kb = 28.44 and pKb = -1.45, but the reported basicity is pKb = -0.36. Since pKb is a logarithmic scale, the error in Kb is greater than a factor of 10. Can this large an error be due to non-ideality, or have I used a wrong reaction for basicity? For NaOH the disagreement is worse; the solubility leads to pKb = -2.89 compared to the reported pKb =+0.2. And for CsOH the solubility leads to pKb = -2.60 and the reported pKb =-1.76. (For KOH and RbOH there are no pKb values in the infoboxes, although there is an acidity pKa 13.5 for KOH. If that means pKb = 14 - 13.5 = 0.5, it again disagrees with the value from solubility which is -2.67.) Dirac66 (talk) 02:25, 27 December 2016 (UTC)

I understand the points you are making. The only way to avoid ambiguity is to attach a definition to the property whose value is being quoted. Maybe there should be a separate page which describes the alternative definitions for pKa etc. with a link to that page attached to each numerical value in an infobox. Petergans (talk) 11:13, 27 December 2016 (UTC)

Finally I have added a brief note with the alternative definition of Kb to this article. I will forget about the purported Ka values whose definition remains unclear to me, since as you say pKa + pKb does not seem to equal 14 as it should. Next I will change the infobox values and sources for the metal hydroxides. [Edit by Dirac66, 4 January 2017]

Metal ion hydrolysis[edit]

There are two complementary ways of defining a simple hydrolysis equilibrium

Mn+ + OH- ⇌ M(OH)(n-1)+ : [M(OH)] = K[M][OH]
Mn+ + H2O ⇌ M(OH)(n-1)+ + H+ : [M(OH)] = K*[M][H]-1

Both definitions (K and K star) are widely used in the literature. The vakues are related as follows

[M(OH)] =K[M][OH] = KKw[M][H]=1
K*=KKW; log K* ≈ log K - 14

In general

M + n(OH) ⇌ M(OH)n, logK* ≈ logK - 14n.

See also Metal_ions_in_aqueous_solution#Hydrolysis_of_aqua_ions Petergans (talk) 10:39, 7 January 2017 (UTC)

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