Potassium Cobaltous Nitrite, 2KNO₂.Co(NO₂)₂

Potassium Cobaltous Nitrite, 2KNO₂.Co(NO₂)₂.xH₂O, is obtained by adding potassium nitrite to a neutral solution of cobalt chloride. It is precipitated as a yellow, hydrated mass, which is soluble in hot water to a red solution.

The cobalti-nitrites have been grouped by Rosenheim and Koppel into three classes or types, namely:

1. 3R₂O.Co₂)O₃.6N₂O₃ + xH₂O, hexanitrites or yellow cobalti-nitrites.
2. 2R₂O.Co₂)O₃.4N₂O₃ + xH₂O, tetranitrites or red cobalti-nitrites.
3. 2R₂O.Co₂)O₃. 3N₂O₃ + xH₂O, trinitrites.

As the authors point out, it does not appear to be generally possible to determine the constitution of these substances by ordinary physicochemical methods, for some of them are insoluble in water, others cannot be redissolved after crystallisation without decomposition, and yet others have not been prepared in a sufficiently pure state. There can be little doubt, however, that they are not simple double salts, but complex bodies, analogous to the cobalti-cyanides, which are discussed in a later section.

Type 1. 3R₂O.Co₂O₃.6N₂O₃ or R₃Co(NO₂)₆, Cobalti- hexanitrites or Yellow Cobalti-nitrites.