Chromic acid

Partial predominance diagram for chromate

Molecular chromic acid, H₂CrO₄, has much in common with sulfuric acid, H₂SO₄. Only sulfuric acid can be classified as part of the 7 strong acids list. Due to the laws pertinent to the concept of "first order ionization energy", the first proton is lost most easily. It behaves extremely similarly to sulfuric acid deprotonation. Since the process of polyvalent acid-base titrations have more than one proton (especially when the acid is starting substance and the base is the titrant), protons can only leave an acid one at a time. Hence the first step is as follows:

H₂CrO₄ ⇌ [HCrO₄]⁻ + H⁺

The pK_a for the equilibrium is not well characterized. Reported values vary between about −0.8 to 1.6.[3] 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[4]

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, H₂Cr₂O₇ 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. Dichromic acid will behave the same exact way when reacting with a primary or secondary alcohol. The caveat to this statement is that a secondary alcohol will be oxidized no further than a ketone, whereas a primary alcohol will be oxidized to a aldehyde for the first step of the mechanism and then oxidized again to a carboxylic acid, contingent on no significant steric hindrance impeding this reaction.

Dichromic acid undergoes the following reaction:

[Cr₂O₇]²⁻ + 2H⁺ ⇌ H₂Cr₂O₇ ⇌ H₂CrO₄ + CrO₃

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

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.[5] Furthermore, the acid leaves trace amounts of paramagnetic chromic ions (Cr³⁺) that can interfere with certain applications, such as NMR spectroscopy. This is especially the case for NMR tubes.[6] Piranha solution can be used for the same task, without leaving metallic residues behind.

Chromic acid was widely used in the musical 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.[7]

It is used as a bleach in black and white photographic reversal processing.[8]

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.[9]
• Pyridinium chlorochromate is generated from chromium trioxide and pyridinium chloride. This reagent converts primary alcohols to the corresponding aldehydes (R–CHO).[9]
• 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.

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.[15]

• Piranha solution – Oxidizing acid mixture containing sulfuric acid and hydrogen peroxide able to dissolve elemental carbon

• Alcohols from Carbonyl Compounds: Oxidation-Reduction and Organometallic Compounds (PDF)

• International Chemical Safety Card 1194
• NIOSH Pocket Guide to Chemical Hazards. "#0138". National Institute for Occupational Safety and Health (NIOSH).
• IARC Monograph "Chromium and Chromium compounds"