Chromic acid

Molecular chromic acid

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 ⇌ [HCrO₄]⁻ + H⁺

The pK_a for the equilibrium is not well characterized. Reported values vary between about −0.8 to 1.6. The value at zero ionic strength is difficult to determine because half dissociation only occurs in very acidic solution, at about pH 0, 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]⁻ ⇌ [Cr₂O₇]²⁻ + H₂O      log K_D = 2.05.

Furthermore, the dichromate can be protonated:

[HCr₂O₇]⁻ ⇌ [Cr₂O₇]²⁻ + H⁺      pK = 1.8

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 ⇌ 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 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

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⁺ ⇌ H₂Cr₂O₇ ⇌ H₂CrO₄ + CrO₃

It is probably present in chromic acid cleaning mixtures along with the mixed chromosulfuric acid H₂CrSO₇.

Uses

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. 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.

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.

It was used in hair dye in the 1940s, under the name Melereon.

As a bleach in black and white photographic reversal processing.

Reactions

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

Chromic acid in aqueous sulfuric acid and acetone is known as the Jones reagent, which will oxidize primary and secondary alcohols to carboxylic acids and ketones respectively, while rarely affecting unsaturated bonds.

Pyridinium chlorochromate is generated from chromium trioxide and pyridinium chloride. This reagent converts primary alcohols to the corresponding aldehydes (R–CHO).

Collins reagent is an adduct of chromium trioxide and pyridine used for diverse oxidations.

Chromyl chloride, CrO₂Cl₂ is a well-defined molecular compound that is generated from chromic acid.

Illustrative transformations

Oxidation of methylbenzenes to benzoic acids.

Oxidative scission of indene to homophthalic acid.

Oxidation of secondary alcohol to ketone (cyclooctanone) and nortricyclanone.

Use in qualitative organic analysis

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.

Alternative reagents

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). 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

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.

Chromium trioxide and chromic acids are strong oxidisers and may react violently if mixed with easily oxidisable organic substances. Fires or explosions may result.

Chromic acid burns are treated with a dilute Sodium thiosulfate solution.