Isoperthiocyanic Acid, C₂S₃(NH)₂

Isoperthiocyanic Acid, C2S3(NH)2, as stated above, is formed by the action of acid of moderate strength - e.g. 40 per cent, hydrochloric acid - on ammonium thiocyanate. It can be crystallised from water in golden-yellow needles, and is reconverted by potassium cyanide into thiocyanate: C2S3N2H2 + KCN = CNSK + 2CNSH. That it is probably an iso-acid containing imide groups is shown by its reduction by nascent hydrogen to thiourea and carbon disulphide: C2S3(NH)2 + 2H = CS(NH2)2 + CS2. Salts of the isomeric normal perthiocyanic acid, C2N2S(SH)2, are formed by the action of alkalis on the iso-acid, but the free acid is unstable. Normal and iso-dithiocyanic acid, and respectively, are also known; as well as trithiocyanic acid, (CNSH)3, which is formed by the interaction of cyanuric chloride and alkali sulphide.