Thiourea, CS(NH₂)₂

Thiocarbamide, Thiourea, CS(NH₂)₂. - Thiourea is formed by heating ammonium thiocyanate to 140° C., this salt undergoing an isomeric change analogous to that produced in ammonium cyanate by heat:

NH₄SCN ⇔ SC(NH₂)₂.

At the same time some guanidine thiocyanate, NH:C(NH₂)₂.HSCN, is produced, though a larger proportion of this latter substance is formed at 170°-180° C. The thiourea is freed from guanidine thiocyanate and unchanged ammonium thiocyanate by cold water, in which the thiourea is but sparingly soluble; it is then purified by recrystallisation from water.

A better method of preparation consists in heating carbon disulphide (6 grams) with ammonium carbonate (8 grams) in a sealed tube at 160° C., when the yield is quantitative.

Thiourea crystallises in rhombic prisms, and dissolves in eleven times its weight of cold water. When heated with water to 140° C. it reverts to ammonium thiocyanate. It forms salts analogous to those of urea; the most characteristic of these is the nitrate, CS(NH₂)₂.HNO₃.