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, 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 similar 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 pKa 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 KD = 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).
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.
It is used as a bleach in black and white photographic reversal processing