Cr2O7

Cr2O7^2−, commonly known as the dichromate ion, is an oxoanionic form of chromium in the +6 oxidation state. In aqueous solution its predominance depends on pH: dichromate is favored under acidic conditions, while chromate (CrO4^2−) dominates in basic media. The two species are interconvertible through protonation and dehydration equilibria.

The dichromate ion consists of two CrO4 tetrahedra linked by a shared oxygen, giving Cr2O7^2− with chromium centers in distorted octahedral environments. The ion carries a 2− overall charge and occurs in salts such as potassium dichromate (K2Cr2O7) and sodium dichromate (Na2Cr2O7). These salts are typically orange-yellow in color.

Cr(VI) compounds, including Cr2O7^2−, are strong oxidizing agents, especially in acidic solutions. Reduction of dichromate proceeds to Cr3+: Cr2O7^2− + 14 H+ + 6 e− → 2 Cr3+ + 7 H2O. In basic media chromate is the prevalent species and exhibits different redox behavior. Dichromate salts are widely used in redox titrations and as oxidizing reagents in qualitative and quantitative inorganic analysis.

Preparation and occurrence: industrially, dichromate salts are produced by oxidation or condensation routes starting from chromate solutions, often involving acid steps that promote dimerization to Cr2O7^2−. These salts are common laboratory reagents and have been used in various industrial processes, though usage has declined in some areas due to safety concerns.

Safety: chromium(VI) compounds are toxic and carcinogenic. They are strong oxidizers and can cause irritation and more serious health effects upon exposure. Proper handling includes appropriate PPE, good ventilation, and careful waste disposal in accordance with hazardous-material regulations.