Calcium Hyposulphite, CaS₂O₄

Calcium Hyposulphite, CaS₂O₄, may be obtained by gently warming calcium hydride with sulphur dioxide. The reaction is slow under ordinary pressure and can only be completed above 900 mm. By the electrolysis of a solution of calcium bisulphite under suitable conditions calcium hyposulphite crystallises out in silky needles, a current efficiency of 60.70 per cent, being obtained. The dry salt is unstable. The solution of the salt has strong reducing properties. With hydrochloric acid it becomes yellow and deposits sulphur.