The chromium atoms are in oxidation state +6 in both, and the chromate and dichromate ions are fairly strong oxidizing agents. Chromium in the +6 (or VI) oxidation state is often referred to as hexavalent chromium.
In an aqueous solution, chromate and dichromate anions are in a chemical equilibrium.
2 CrO42− + 2 H3O+ . Cr2O72− + 3 H2O
Ammonium Dichromate
Ammonium dichromate is the inorganic compound with the formula (NH4)2Cr2O7. This salt is sometimes known as Vesuvian Fire, because of its use in the formation of tabletop "volcanoes". It has been used in pyrotechnics and in the early days of photography. The volcano demonstration involves igniting a pile of the salt, which initiates the following conversion:
(NH4)2Cr2O7(s) → Cr2O3(s) + N2(g) + 4H2O(g)
Potassium Dichromate
Potassium dichromate, K2Cr2O7, is a common inorganic chemical reagent, most commonly used as an oxidising agent in various laboratory and industrial applications. As with all hexavalent chromium compounds, it is potentially harmful to health and must be handled and disposed of appropriately. It is a crystalline ionic solid with a vivid red-orange colour.
Sodium Dichromate
Sodium dichromate is the chemical compound with the formula Na2Cr2O7. Usually, however, the salt is handled as its dihydrate Na2Cr2O7·2H2O. Its chemistry, appearance, and behaviour are very similar to those of the more widely encountered potassium dichromate. This chemical is around twenty times more soluble in water than the potassium salt (49 g/L at 0 °C) and its equivalent weight is also lower, which is often desirable.
