They are precipitated from their alkaline solutions as cobalt sulphide by sulphuretted hydrogen, but this precipitation is prevented by the presence of citric and tartaric acids; similarly the presence of ammonium salts hinders their precipitation by caustic alkalis.
potassium sulphide dissolved in 3 gallons of water.
Cobalt chloride, CoC1 2, in the anhydrous state, is formed by burning the metal in chlorine or by heating the sulphide in a current of the same gas.
Iridium sulphide, IrS, is obtained when the metal is ignited in sulphur vapour.