Ammonium Dichromate, (NH₄)₂Cr₂O₇

A solution of chromium trioxide is divided into two equal portions; one is saturated with ammonia, and the two solutions are then mixed. Ammonium Dichromate, (NH₄)₂Cr₂O₇, separates, after evaporation, as large garnet-red needles.

An alternative method of preparation depends upon the interaction in aqueous solution of ammonium chloride and potassium dichromate. Ammonium dichromate is very soluble in water, and may be separated from ammonium chloride and potassium dichromate by precipitation with alcohol. The crystals are monoclinic (probably isomorphous with the monoclinic form of potassium dichromate), and have density 2.15. On heating, nitrogen is evolved (at a red heat, flame is produced), but the reaction is not so simple as that expressed by the usual equation:

(NH₄)₂Cr₂O₇ = Cr₂O₃ + 4H₂O + N₂,

since at the same time ammonia, oxides of nitrogen, and some oxygen are produced.

The double salts (NH₄)₆Cr₂O₇Cl₄.6Hg(CN)₂.4H₂O, (NH₄)₂Cr₂O₇.HgCl₂, and (NH₄)₂Cr₂O₇Cl₂(HgCl₂)₄.2H₂O have been prepared. Ammonium trichrornate, (NH₄)₂Cr₃O₁₀, and ammonium tetrachrornate, (NH₄)₂Cr₄O₁₃, are obtained similarly to the corresponding potassium salts. The former crystallises in the rhombic system, and has density 2.329; the latter has density 2.343, and is isomorphous with potassium tetrachromate.