chromate

Chromate refers to the chromate ion, CrO4^2−, and to salts and esters of chromic acid. In water, chromate exists mainly as CrO4^2−; under acidic conditions, it converts to dichromate, Cr2O7^2−. The ion is tetrahedral with chromium in the +6 oxidation state. Many chromate salts are soluble, but some, such as lead chromate, PbCrO4, are sparingly soluble and have historical pigment use.

Preparation and occurrence: Chromate salts are commonly prepared by neutralizing chromic acid or chromium trioxide with bases (for example, NaOH or KOH) to give sodium or potassium chromate. Addition of acid to chromate solutions yields dichromate. Mineral chromates are rare; crocoite (lead chromate) is a natural example, while most chromates are synthetically produced for industrial use.

Uses: Chromates are used as pigments and corrosion inhibitors. Chrome yellow pigments historically referred to lead chromate; other chromates such as zinc chromate and strontium chromate are used as primers and protective coatings. They also find uses in glass, ceramic pigments, and certain electroplating processes.

Safety and environment: Chromium(VI) compounds, including chromates, are toxic and carcinogenic. They pose inhalation and dermal hazards and can cause environmental harm, particularly to aquatic life. Handling requires proper PPE and containment; waste is regulated as hazardous waste and disposed accordingly.