Potassium dichromate

Potassium dichromate is the inorganic compound with the formula K₂Cr₂O₇. An orange solid, it is used in diverse laboratory and industrial applications. As with all hexavalent chromium compounds, it is chronically harmful to health. It is a crystalline ionic solid with a very bright, red-orange color. The salt is popular in laboratories because it is not deliquescent, in contrast to the more industrially relevant salt sodium dichromate.

Potassium dichromate is usually prepared by the reaction of sodium dichromate and potassium chloride. Alternatively, it can be also obtained from potassium chromate by roasting chromite ore with potassium hydroxide:

The solid crystallizes as two polymorphs. These salts are soluble in water, and the dissolution process it ionizes, releasing Cr₂O2−7. Thus in aqueous solution, it is the dichromate ion that matters in terms of chemical reactions and environmental impact. This anion is a corner-shared bitetrahedron, resembling pyrophosphate.

Potassium dichromate is an oxidising agent in organic chemistry. It is milder and more selective than potassium permanganate. It is used to oxidize alcohols. It converts primary alcohols into aldehydes and, under more forcing conditions, into carboxylic acids. In contrast, potassium permanganate tends to give carboxylic acids as the sole products. Secondary alcohols are converted into ketones. For example, menthone may be prepared by oxidation of menthol when treated with acidified solution of dichromate. Tertiary alcohols cannot be oxidized.

When heated strongly, it decomposes with the evolution of oxygen.^{[citation needed]}

When an alkali is added to an orange-red solution containing dichromate ions, a yellow solution is obtained due to the formation of chromate ions (CrO2−4). For example, potassium chromate is produced industrially using potassium carbonate:

Treatment with cold sulfuric acid gives red crystals of chromic anhydride (chromium trioxide, CrO₃):^{[citation needed]}

On heating with concentrated acid, oxygen is evolved:^{[citation needed]}