Chromic acid

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Chromic acid
Structural formulae of dichromic acid (left) and chromic acid (right)
Identifiers
CAS number 7738-94-5 YesY
PubChem 24425
ChemSpider 22834 YesY
EC number 231-801-5
ChEBI CHEBI:33143 YesY
Gmelin Reference 25982
Jmol-3D images Image 1
Image 2
Properties
Molecular formula H2CrO4
Molar mass 118.01 g mol−1
Appearance Red crystals
Density 1.201 g cm−3
Melting point 197 °C (387 °F; 470 K)
Boiling point 250 °C (482 °F; 523 K) (decomposes)
Solubility in water 1666.6 g/L
Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
 YesY (verify) (what is: YesY/N?)
Infobox references

The term chromic acid is usually used for a mixture made by adding concentrated sulfuric acid to a dichromate, which may contain a variety of compounds, including solid chromium trioxide. This kind of chromic acid may be used as a cleaning mixture for glass. Chromic acid may also refer to the molecular species, H2CrO4 of which the trioxide is the anhydride. Chromic acid features chromium in an oxidation state of +6 (or VI). It is a strong and corrosive oxidising agent.

Molecular chromic acid[edit]

Partial predominance diagram for chromate

Molecular chromic acid, H2CrO4, has much in common with sulfuric acid, H2SO4. Both are classified as strong acids, though only the first proton is lost easily.

H2CrO4 is in equilibrium with [HCrO4] + H+

The pKa for the equilibrium is not well characterized. Reported values vary between about −0.8 to 1.6.[1] The value at zero ionic strength is difficult to determine because half dissociation only occurs in very acidic solution, at about pH zero, that is, with an acid concentration of about 1 mol dm−3. A further complication is that the ion [HCrO4] has a marked tendency to dimerize, with the loss of a water molecule, to form the dichromate ion, [Cr2O7]2−:

2 [HCrO4] is in equilibrium with [Cr2O7]2− + H2O, log KD = 2.05.

Furthermore, the dichromate can be protonated:

[HCr2O7] is in equilibrium with [Cr2O7]2− + H+, pK = 1.8[2]

The pK value for this reaction shows that it can be ignored at pH > 4.

Loss of the second proton occurs in the pH range 4–8, making the ion [HCrO4] a weak acid.

Molecular chromic acid could in principle be made by adding chromium trioxide to water (cf. manufacture of sulfuric acid).

CrO3 + H2O is in equilibrium with H2CrO4

but in practice the reverse reaction occurs when molecular chromic acid is dehydrated. This is what happens when concentrated sulfuric acid is added to a dichromate solution. At first the colour changes from orange (dichromate) to red (chromic acid) and then deep red crystals of chromium trioxide precipitate from the mixture, without further colour change. The colours are due to LMCT charge transfer transitions.

Chromium trioxide is the anhydride of molecular chromic acid. It is a Lewis acid and can react with a Lewis base, such as pyridine in a non-aqueous medium such as dichloromethane (Collins reagent).

Dichromic acid[edit]

Dichromic acid, H2Cr2O7, (structure illustrated top right) is the fully protonated form of the dichromate ion and also can be seen as the product of adding chromium trioxide to molecular chromic acid.

[Cr2O7]2− + 2H+ is in equilibrium with H2Cr2O7 is in equilibrium with H2CrO4 + CrO3

It is probably present in chromic acid cleaning mixtures along with the mixed chromic-sulfuric acid H2CrSO7.[citation needed]

Uses[edit]

Chromic acid is an intermediate in chromium plating, and is also used in ceramic glazes, and colored glass. Because a solution of chromic acid in sulfuric acid (also known as a sulfochromic mixture or chromosulfuric acid) is a powerful oxidizing agent, it can be used to clean laboratory glassware, particularly of otherwise insoluble organic residues. This application has declined due to environmental concerns.[3] Furthermore the acid leaves trace amounts of paramagnetic chromic ions — Cr(III) — that can interfere with certain applications, such as NMR spectroscopy. This is especially the case for NMR tubes.[4]

Chromic acid was widely used in the instrument repair industry, due to its ability to "brighten" raw brass. A chromic acid dip leaves behind a bright yellow patina on the brass. Due to growing health and environmental concerns, many have discontinued use of this chemical in their repair shops.

Reactions[edit]

Chromic acid is capable of oxidizing many kinds of organic compounds and many variations on this reagent have been developed:

Illustrative transformations[edit]

Use in qualitative organic analysis[edit]

In organic chemistry, dilute solutions of chromic acid can be used to oxidize primary or secondary alcohols to the corresponding aldehydes and ketones. Tertiary alcohol groups are unaffected. Because of the oxidation is signaled by a color change from orange to a blue-green, chromic acid is used as a qualitative analytical test for the presence of primary or secondary alcohols.[5]

Alternative reagents[edit]

In oxidations of alcohols or aldehydes into carboxylic acids, chromic acid is one of several reagents, including several that are catalytic. For example nickel(II) salts catalyze oxidations by bleach (hypochlorite).[10] Aldehydes are relatively easily oxidised to carboxylic acids, and mild oxidising agents are sufficient. Silver(I) compounds have been used for this purpose. Each oxidant offers advantages and disadvantages. Instead of using chemical oxidants, electrochemical oxidation is often possible.

Safety[edit]

Hexavalent chromium compounds (including chromium trioxide, chromic acids, chromates, chlorochromates) are toxic and carcinogenic. For this reason, chromic acid oxidation is not used on an industrial scale except in the aerospace industry.

Notes[edit]

  1. ^ IUPAC SC-Database A comprehensive database of published data on equilibrium constants of metal complexes and ligands
  2. ^ Brito, F.; Ascanioa, J.; Mateoa, S.; Hernándeza, C.; Araujoa, L.; Gili, P.; Martín-Zarzab, P.; Domínguez, S.; Mederos, A. (1997). "Equilibria of chromate(VI) species in acid medium and ab initio studies of these species". Polyhedron 16 (21): 3835–3846. doi:10.1016/S0277-5387(97)00128-9. 
  3. ^ J. M. McCormick (2006-06-30). "Cleaning Glassware". Truman State University. 
  4. ^ "NMR-010: Proper Cleaning Procedures for NMR Sample Tubes". Wilmad. Retrieved 2008-06-27. 
  5. ^ a b c Freeman, F. "Chromic Acid" Encyclopedia of Reagents for Organic Synthesis (2001) John Wiley & Sons, doi:10.1002/047084289X.rc164
  6. ^ Kamm O.; Matthews, A. O. (1941), "p-Nitrobenzoic Acid", Org. Synth. ; Coll. Vol. 1: 392 
  7. ^ Grummitt, O.; Egan, R.; Buck, A., "Homophthalic Acid and Anhydride", Org. Synth. ; Coll. Vol. 3: 449 (1955 
  8. ^ Eisenbraun, E. J., "Cyclooctanone", Org. Synth. ; Coll. Vol. 5: 310 (1973 
  9. ^ Meinwald, J.; Crandall, J.; Hymans W. E., "Nortricyclanone", Org. Synth. ; Coll. Vol. 5: 866 
  10. ^ J. M. Grill, J. W. Ogle, S. A. Miller (2006). "An Efficient and Practical System for the Catalytic Oxidation of Alcohols, Aldehydes, and α,β-Unsaturated Carboxylic Acids". J. Org. Chem. 71 (25): 9291–9296. doi:10.1021/jo0612574. PMID 17137354. 

References[edit]

External links[edit]