Potassium Cobalti-cyanide, K₃Co(CN)₆

Gmelin first prepared and and described Potassium Cobalti-cyanide, K₃Co(CN)₆. It is readily produced by the oxidation of the cobalto-cyanide by warming its solution in the presence of air. This reaction is interesting, for when the solution is rapidly oxidised by atmospheric oxygen, twice as much oxygen is absorbed as corresponds to the equation:

2K₄Co(CN)₆ + H₂O + О = 2K₃Co(CN)₆ + 2KOH, the excess of oxygen remaining, at the close of the reaction, as hydrogen peroxide.

If, however, the oxidation is allowed to proceed slowly, only a slight excess of oxygen is absorbed, and the amount of hydrogen peroxide resulting is proportionately low.

This is readily explained on the assumption that the reaction proceeds in two stages, in accordance with the theory of Engler, namely:

1. Direct oxidation of the cobalto-cyanide with a molecule of oxygen according to the scheme:
   2K₄Co(CN)₆ + 2H₂O + O₂ = 2K₃Co(CN)₆ + 2KOH + H₂O₂.
2. Indirect oxidation of the cobalto-cyanide with the hydrogen peroxide; thus:
   2K₄Co(CN)₆ + H₂O₂ = 2K₃Co(CN)₆ + 2KOH.

In the first case of rapid oxidation by atmospheric oxygen, the indirect reaction has little time to make itself felt. By slow atmospheric oxidation, however, the indirect reaction has a pronounced effect.

On boiling a solution of potassium cobalto-cyanide, hydrogen is evolved in amount equivalent to the oxygen required for the slow oxidation.

Potassium cobalti-cyanide may also be produced by the addition of dilute acetic acid or hydrogen chloride to a solution of the cobalto-cyanide in excess of potassium cyanide. The acid liberates hydrocyanic acid, which reacts as follows:

2K₄Co(CN)₆ + 2HCN = 2K₃Co(CN)₆ + 2KCN + H₂.

Upon evaporation the salt may be obtained in yellow, anhydrous crystals isomorphous with potassium ferricyanide. Density 1.906.

Finally, potassium cobalti-cyanide results when Fischer's salt (potassium cobalti-nitrite) is added in small portions to a warm, concentrated solution of potassium cyanide. Nitrogen or nitrous oxide is evolved and the solution becomes yellow. On cooling, the potassium cobalti-cyanide crystallises out in pale yellow needles.