Ferric ammonium ferrocyanide, FeNH₄[Fe(CN)₆]

Ferric ammonium ferrocyanide, FeNH₄[Fe(CN)₆], results when potassium ferrocyanide is heated with a solution of hydroxylamine hydrochloride. It is a deep blue insoluble powder. The hydrated salt FeNH₄[Fe(CN)₆].H₂O is known as Monthiers' blue, having been obtained by Monthier as the result of oxidising the white precipitate thrown down by the action of potassium ferrocyanide upon ammoniacal ferrous chloride solution. It may also be prepared by dissolving iron wire in a solution containing ammonia, ammonium chloride, and potassium ferrocyanide, and oxidising the green precipitate, thereby obtained, with hydrogen peroxide.

A compound of the same formula is obtained by oxidation in neutral solution of the compound obtained by reducing Prussian blue with hydrogen sulphide.

The corresponding potassium salt, FeK[Fe(CN)₆].H₂O, has already been described under the name of Williamson's violet. This when treated with excess of chlorine yields a green hydrated mass known as Prussian green. Its composition corresponds to the formula Fe(CN)₃, but its constitution is probably represented by (Fe[Fe(CN)₆])_n, i.e. ferric ferricyanide.