Potassium Cobalti-nitrite, K₃Co(NO₂)₆

Potassium Cobalti-nitrite, K₃Co(NO₂)₆.xH₂O, is the most familiar example of this type. It is also known as cobalt yellow and Fischer's salt in honour of its discoverer. It is readily prepared by adding potassium nitrite to an aqueous solution of a soluble cobalt salt acidified with acetic acid. It results in a very pure condition when cobalt carbonate is suspended in an aqueous solution containing an equivalent amount of potassium carbonate or nitrite, and treated with nitrous fumes (resulting from the action of nitric acid upon arsenious oxide) until it has suffered complete decomposition. The amount of combined water varies from 0 to 4 molecules according to circumstances.

It is a bright yellow, crystalline powder, insoluble in water, and on account of its stability or "inertness" is used as an artists' pigment. It is a very pure compound when prepared as indicated above, for neither iron nor nickel yields a corresponding derivative; it is therefore valued for painting porcelain, because the blue colour produced on baking is correspondingly pure and beautiful. Furthermore, as has already been mentioned, the salt yields a valuable method of separating nickel from cobalt.