Cr2O72

Cr2O7^2−, commonly called dichromate, is the dianionic form of chromium in the +6 oxidation state. In aqueous solution its speciation depends on pH: dichromate predominates under strongly acidic conditions, while the chromate ion CrO4^2− dominates at higher pH. Dichromate salts include potassium dichromate (K2Cr2O7) and sodium dichromate (Na2Cr2O7). The anion consists of two tetrahedral CrO4 units joined by a shared oxygen bridge, giving a Cr2O7 core with overall D4h-like symmetry in a simplified view.

Preparation and interconversion between species occur in acid-base equilibria. In water, acidifying chromate solutions yields dichromate through the equilibrium 2 CrO4^2− + 2 H+ ⇌ Cr2O7^2− + H2O; the reverse reaction occurs as the solution becomes less acidic. Dichromate solutions are commonly encountered through the dissolution of dichromate salts, which are widely used as oxidizing reagents.

Reactions and applications: Cr2O7^2− is a strong oxidizing agent, particularly in acidic media, and is reduced to Cr3+ under redox conditions, e.g., Cr2O7^2− + 14 H+ + 6 e− → 2 Cr3+ + 7 H2O. In practice, dichromate has historical and ongoing use in laboratory oxidation reactions, qualitative analysis for reducing agents, and certain analytical titrations. It also serves as a reagent in various organic and inorganic transformations, often via its oxidizing power in acidic environments.

Safety: Hexavalent chromium compounds are toxic and carcinogenic. Handling requires appropriate protective equipment, good ventilation, and careful waste disposal in accordance with regulations. Cr(VI) waste is subject to specialized treatment to prevent environmental release.