Talk:Osmotic pressure

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molarity, NOT molality[edit]

if R = 0.08206 L atm / mol K as it says, it could not possibly be molality, as that would not agree with the dimensions used here. also, according to Chemistry, The Central Science by Brown LeMay and Bursten, osmotic pressure = (n/V)RT= MRT I took the liberty of fixing it. please do not revert it without bringing evidence as to why you would do so. —Preceding unsigned comment added by (talk) 05:44, 14 April 2010 (UTC)

Which value of R?[edit]

This article says R = 8.314 but in all the osmotic pressure calculations I have done at uni we have used 0.08206 L atm mol K. Does anyone object to changing this? --Mdhowe 04:34, 18 October 2006 (UTC)

Yes. I think we should stick to SI units. Maybe 8.314472 L·kPa·K-1·mol-1. --kupirijo (talk) 22:22, 9 November 2008 (UTC)

I have no objection, that is the most commonly used value of R for osmotic pressure and it also uses the simplest dimensions. —Preceding unsigned comment added by (talk) 06:50, 14 April 2010 (UTC)

Turgor vs. osmotic pressure[edit]

I know this phenomenon as Osmotic pressure. I thought everybody did?

I had learned this phenomenon as "osmotic pressure". A quick google test gives ~100,000 results for "turgor" and ~175,000 for "osmotic pressure". I had imagined "turgor" would get more hits because it is one word, but it seems "osmotic pressure" is the correct term. In lieu of this information, I suggest this page be moved to "osmotic pressure" with appropriate redirects. -- Bubbachuck 04:07, 9 August 2005 (UTC)

Turgid and flacid plant parts

I am going to add a short description of turgid and flacid plant parts with a link to the article stoma. It seems like a good idea to me. Thoughts? 17:11, 19 October 2005 (UTC)lotusduck

What about turgour and erectile body parts. Does this not deseve a mention?--Light current 00:44, 11 January 2006 (UTC)

Osmotic pressure[edit]

I understand osmotic pressure as follows:

Solution in semipermeable membrane (permeable only to solvent) exposed to pure solvent. The osmotic pressure is the physical pressure applied to the solution required to prevent movement of pure solvent into the solution.

Another, less accurate, way to define it is the pressure a solution exerts through a semipermeable membrane to draw more solvent into itself.

I have never heard of "turgor pressure" and many erectile tissues in animals rely on blood flow rather than osmosis.

The definition of osmotic pressure given above ("...the physical pressure applied to the solution required to prevent movement of pure solvent into the solution") is a much better definition that the one actually given in the article ("...the hydrostatic pressure produced by a solution in a space divided by a semipermeable membrane due to a differential in the concentrations of solute"). In fact, the hydrostatic pressure is not "produced" by a solution, but rather must be applied by some outside entity, such as the rigid walls of a container resisting volume changes, or the force of gravity acting on a column of solution. I'll make the change unless anyone has any objections.Thewookie55 19:14, 24 September 2007 (UTC)

Turgor-Osmotic Pressure[edit]

Tugor and osmotic pressure are different things. Osmotic pressure is the pressure applied from the outside of the cell that pushes the water in. We could say it is the tendancy to take water. ( and ( Turgor pressure is from inside of the cell done by the water inside. ( Klesk

  • Turgor and Osmotic pressure should definitely be separated. Not only they are somewhat different things, but "turgor" also has some different meanings. American Heritage Dictionary:

tur·gor (tûr'gər, -gôr')


  1. The state of being turgid.
  2. Biology. The normal fullness or tension produced by the fluid content of blood vessels, capillaries, and plant or animal cells.

Separation of Topics[edit]

I have separated out the osmosis and turgor pressure information as osmotic pressure is the cause of turgor pressure, but they are not the same thing. Please feel free to add to either article. pschemp | talk 03:22, 9 July 2006 (UTC)

Psi not pi...[edit]

I think the symbol's wrong here: π = iMRT.

(I don't know how to change it, so I thought I'd just comment —The preceding unsigned comment was added by (talk) 14:18, 3 March 2007 (UTC).

The symbol I see in most references is a π. What is your source on the other symbol? --Pekaje 22:26, 29 July 2007 (UTC)
Uppercase pi does seem to be the standard symbol used in university-level textbooks, yes. Not to be confused with the lowercase pi used to denote the irrational constant. - Alltat (talk) 19:38, 14 March 2012 (UTC)

The correct symbol is π according to Physiology (Berne and Levy, 5th Ed) and Physiology (Costanzo, 3rd Ed). You might be thinking of Φ (Phi) which is the Osmotic co-efficient, and can be added to the equation in the form π = iMRTΦ. However, Osmotic pressure and Osmotic coefficient are not the same thing. Tim Set Match (talk) 15:25, 5 January 2008 (UTC)

I agree with the first post, it should be Psi, due to the fact Psi is a symbol, in this example symbolising the Osmotic potential. Pi is a constant which I believe isn't correct. e.g. ψ =iMRT. 12:47, 19 September 2008 —Preceding unsigned comment added by (talk)

Please take a look at any textbook for physical chemistry and/or biology that deals with the subject of osmosis. You'll find that Π (like pressure) is the overall agreed symbol for osmotic pressure. The number (not constant) π does not have anything to do with it. --Drahkrub (talk) 20:38, 19 September 2008 (UTC)

Osmotic Pressure & Medicine[edit]

Osmotic pressure seems to be important in medicine, there's a lot of talk about it in topics like traumatic brain injury.I'm not knowledgeable enough to write about it yet,but if I do the research would osmotic pressure as a medical topic be better as a separate article? Thanks216.40.21.8 13:05, 29 July 2007 (UTC)


A solute doesn't 'want' to do anything. —Preceding unsigned comment added by (talk) 14:34, 15 September 2008 (UTC)


How does one calculate the amount of energy needed to remove a given volume of solvent from a solution? Notice that the concentration of the solution changes as the volume does, and that pressure*volume[=]energy. Thoughts?RSido (talk) 01:49, 13 May 2009 (UTC)


Does somebody have a derivation of the Morse equation from first principles? (talk) 14:10, 14 May 2009 (UTC)

Its simply the ideal gas equation and it can be derived from its partition function as given by Sackur–Tetrode equation. The rigorous derivation of the osmotic equation is more complex due to the presence of the solvent. The derivation can be found in Hill's book on statistical mechanics. But the final result is the same. Bubblerock2 (talk) 15:53, 28 May 2012 (UTC)

Which PH level is the highest.[edit]

Blood, vinegar, Alkaline detergent or lemon juice.Bold text —Preceding unsigned comment added by (talk) 20:13, 22 March 2010 (UTC)


I came to this article to confirm that the units of osmotic pressure is Osm or mOsm. This was not included on this page, but Googling, I found many instances where it is used. Perhaps this would be of use to other readers. Wakablogger2 (talk) 21:22, 12 August 2010 (UTC)

The unit for osmotic pressure is Pascal. Occasionally you'll see atm, torr, bar, psi, etc., but the "proper" unit is Pascal. - Alltat (talk) 19:38, 14 March 2012 (UTC)

Value/units of R[edit]

On 04:10, 10 January 2013‎ user: changed the value of R from (8.3145 J K-1 mol-1) to (08205746 L atm K-1 mol-1). These are different forms of the same unit, and are both correct. However, in this context, I believe (it's been too long since second semester physics, sorry), that a pressure-based form (as opposed to the Joules form previously in the article) is appropriate, as the usual unit for osmotic pressure is, in fact pressure, and usually Pascals. But as the usual unit is Pascals, I'd think the kPa form (8.3145 L kPa K−1 mol−1) form of R would be preferred, not the atm form as it reads now. Note that the numerical value of the kPa form is the same as the Joules form.

I see this has been slightly discussed here before (not to mention the multiple decades since I've had to work one of these) so before changing it to (8.3145 L kPa K−1 mol−1), I thought I'd ask. Rwessel (talk) 07:30, 10 January 2013 (UTC)

T'was me who made that edit, and at the time I didn't realize L*kPa = J. I agree completely with you, anybody else have any input as to which constant would be more appropriate? Seems to me like the SI units would. Would you consider using the units J/(k*mol) incorrect in this situation though? (talk) 19:12, 15 April 2013 (UTC)

Partial pressure?[edit]

Dear Osmosis Experts: A friend who knows more physics than I do, but who is not a Wiki editor, pointed out what is to him a mis-wording in "This process will stop once the pressures of the water and sugar water on both sides of the membrane become equal." His suggested replacement: "The process stops when the partial pressure of water on the sugar-water side equals the pressure of the pure water on the other side." His point, I think, is that it is not equality of total pressure between the two liquids, but rather equality of water to water, disregarding the additional pressure due to the sugar. I am reluctant to edit the change myself, because I am not an osmoticist. Would one of you experts please make this change if you see it as valid? Two additional notes: (1) In the sentence before that one, how about replacing "the two solutions" with "the two liquids," (including the comma), as pure water is technically not a "solution", right? (2) My friend introduces the phrase "partial pressure"; Wikipedia has an article on it, but it seems to deal only with gases. Would a link to partial pressure be appropriate? Kotabatubara (talk) 19:01, 10 November 2015 (UTC)

I'm not sure I'm qualified to make this change, but I don't think the proposed revision is adequate either. If several-decades-old chem class memories are correct, there are two factors in play in this scenario (the "U" shaped tube) - osmotic pressure from the dilute solution side (pure water would just be *really* dilute) to the concentrated solution side, and *back* pressure from the rising water level on the concentrate side (notably the *partial pressure* of the water in the solution on that side). It's when those are equal that osmosis stops. And I don't think that either the original or the suggested replacement expressed that clearly. Rwessel (talk) 06:16, 11 November 2015 (UTC)
The idea of partial pressure is wrong. It's an actual hydrostatic pressure, as measured by a manometer. Petergans (talk) 08:10, 11 November 2015 (UTC)

The Van't Hoff Equation Is Wrong[edit]

The final Van't Hoff Equation in this article is incorrect. Currently it reads:


however if this were the case, an infinitely dilute solution would have an infinite osmotic pressure which is nonsense. The correct form of the Van't Hoff Equation is:


I don't have the time right now to check the derivation and determine what part is wrong (I expect that it is just between the integral and the first simplified equation with the activity coefficient) so I will just put this here until I have time to come back and check. If someone can either verify that this is wrong, or else tell me why I am wrong about this that would be great. Otherwise I'll change it when I get a chance.

the second formula is definitelt wrong. I need time to clarify the situation. Petergans (talk) 08:19, 19 April 2016 (UTC)
Acoording to Atkins & de Paula, Physical Chemistry, 8th. edition, p.154 Π=[B]RT, [B] = molar concentration of solute. The derivation is on p.155. I will change the text accordingly. Petergans (talk) 07:15, 21 April 2016 (UTC)