Sodium Cobalti-cyanide, Na₃Co(CN)₆

Sodium Cobalti-cyanide, Na₃Co(CN)₆.4H₂O, is obtained by neutralising the free acid with sodium carbonate, excess of either reagent being avoided. On concentration the salt crystallises in transparent, colourless needles which readily dissolve in water, but not in alcohol. At 100° C. the salt becomes anhydrous, at higher temperatures it melts and decomposes.

Bismuth, cadmium, cuprous, manganese, mercurous, and zinc cobalti-cyanides have likewise been isolated.

The rare earth metals yield heavy, microcrystalline precipitates of cobalti-cyanides of general formula M₂(CoC₆N₆)₂.9H₂O.

They are but sparingly soluble in water. In dilute hydrochloric acid their solubilities vary somewhat, the yttrium salt being some three times as soluble as that of erbium, a fact that enables the two metals to be separated in this way.