Ammonium sulphide ((NH4)2S). Ammonium sulphide is prepared by the action of hydrosulphuric acid upon ammonium hydroxide:
2NH4OH + H2S = (NH4)2S + 2H2O.
If the action is allowed to continue until no more hydrosulphuric acid is absorbed, the product is the acid sulphide, sometimes called the hydrosulphide:
NH4OH + H2S = NH4HS + H2O.
If equal amounts of ammonium hydroxide and ammonium acid sulphide are brought together, the normal sulphide is formed:
NH4OH + NH4HS = (NH4)2S + H2O
It has been obtained in the solid state, but only with great difficulty. As used in the laboratory it is always in the form of a solution. It is much used in the process of chemical analysis because it is a soluble sulphide and easily prepared. On exposure to the air ammonium sulphide slowly decomposes, being converted into ammonia, water, and sulphur:
(NH4)2S + O = 2NH3 + H2O + S.
As fast as the sulphur is liberated it combines with the unchanged sulphide to form several different ammonium sulphides in which there are from two to five sulphur atoms in the molecule, thus: (NH4)2S2, (NH4)2S3, (NH4)2S5. These sulphides in turn decompose by further action of oxygen, so that the final products of the reaction are those given in the equation. A solution of these compounds is yellow and is sometimes called yellow ammonium sulphide.