Dichromates

Dichromates are chemical compounds that contain the dichromate anion, Cr2O7^2−. The most common dichromate salts are potassium dichromate (K2Cr2O7), sodium dichromate (Na2Cr2O7), and ammonium dichromate ((NH4)2Cr2O7). They are typically used as strong oxidizing agents in laboratory and industrial processes and, in solution, Cr2O7^2− is encountered together with related species in a pH-dependent chromium(VI) system.

In aqueous solution, dichromate equilibrates with chromate and other chromium(VI) species depending on acidity. Under strongly acidic conditions, the dichromate ion is prevalent and acts as a powerful oxidant. Redox reactions convert Cr(VI) to Cr(III); for example, dichromate reduction in acid proceeds as Cr2O7^2− + 14 H+ + 6 e− → 2 Cr3+ + 7 H2O, often accompanied by a color change from orange to green. The Cr(VI) state is toxic, and its reactivity relies on acid-base and redox chemistry.

Applications of dichromates include oxidation of organic substrates, particularly in laboratory settings. They are used in reactions such as oxidation of primary alcohols to carboxylic acids and secondary alcohols to ketones under acidic conditions. They have also historical applications in photography, leather and wood processing, and various analytical procedures, though many uses have declined due to safety and environmental concerns.

Safety and environmental considerations are central to handling dichromates. Cr(VI) compounds are toxic and carcinogenic; exposure can affect skin, eyes, and respiratory health. They are strong oxidizers and can react violently with reducing agents. Waste containing dichromates must be treated to reduce Cr(VI) to Cr(III) before disposal, and releases are subject to environmental regulations to limit chromium(VI) contamination.