Cobalt Dithionate, CoS₂O₆

Cobalt Dithionate, CoS₂O₆, may be obtained in several states of hydration. The solution is formed by double decomposition of barium dithionate and cobalt sulphate solutions. On concentration by exposure at room temperature the clear liquid, freed by filtration from insoluble barium sulphate, deposits rose-coloured, triclinic crystals of the octahydrate, CoS₂O₆.8H₂O, which are excessively soluble in water, but which effloresce in dry air. Exposed to desiccation over sulphuric acid, 2 or 4 molecules of water are lost, whilst recrystallisation from absolute alcohol yields the hexahydrate, CoS₂O₆.6H₂O, as rose-coloured prisms. The salt may also be obtained by dissolving cobaltic hydroxide, resulting from the combined action of bromine and caustic soda upon a solution of a cobalt salt, in sulphurous acid. At first a brown colour is produced, due to the formation of cobaltic sulphate, but this rapidly gives place to a rose-red colour, which is permanent even on boiling. This is due to decomposition of the sulphate into the sulphite and dithionate respectively:

Co₂(SO₃)₃ = CoSO₃ + CoS₂O₆.

With concentrated aqueous ammonia the pentammoniate, CoS₂O₆.5NH₃, is obtained as small prisms.