Chromic acid generally refers to a collection of compounds generated by the acidification of solutions containing chromate and dichromate anions or the dissolving of chromium trioxide in sulfuric acid. Often the species are assigned the formulas H2CrO4 and H2Cr2O7. The anhydride of these "chromic acids" is chromium trioxide, also called chromium(VI) oxide; industrially, this compound is sometimes sold as "chromic acid."
Regardless of its exact formula, chromic acid features chromium in an oxidation state of +6 (or VI), often referred to as hexavalent chromium. Chromium, like most transition metals, can exist in a number of oxidation states. The +6 oxidation state is the highest known; the +3 oxidation state is the most stable, and the +2 oxidation state is also well known, although it is a strong reducing agent. In its reactions chromic acid is reduced in redox reactions to the purple [Cr(H2O)6]3+ ion, or other Cr(III) species, which usually have a green colour.
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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) is a powerful oxidizing agent, it can be used to clean laboratory glassware. This application has declined due to environmental concerns. Furthermore the acid leaves residues that can interfere with certain applications, such as NMR spectroscopy. Chromic acid has also been widely used in the band 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.
Chromic acid is capable of oxidizing many kinds of organic compounds and many variations on this reagent have been developed:
In organic chemistry, dilute solutions of hexavalent chromium 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.[1]
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.[6] 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.
Hexavalent chromium compounds are toxic and carcinogenic. For this reason, chromic acid oxidation is not used on an industrial scale.