The gas liberated on treating a mixture of two salts with dil. turns lime water milky and turbidity disappears with the passage of excess of gas. The aqueous solution of mixture gives white crystalline ppt. with NaCl solution. The filtrate gives a black precipitate, when is passed into it. The aqueous solution of mixture on heating gives reddish brown gas and when treated with ammonium hydroxide and excess of disodium hydrogen phosphate gives a white crystalline precipitate. The mixture contains