Ammonium hydroxide is a precipitating agent in Group III of the qualitative analysis. It is a weak electrolyte and ionises in water as
The concentration of OH- ions is so large that not only the hydroxides of group III [Fe(OH)3 , Cr(OH)3 and Al(OH)3] but the hydroxides of higher groups also get precipitated.
The solubility products of various hydroxides are given below :
In order to precipitate only the hydroxides of group III, the OH- ions concentration must be lowered. This is achieved by the use of ammonium chloride before the addition of ammonium hydroxide. The increased concentration of NH4+ ions suppresses the ionisation of NH4OH due to common ion effect. Under this condition, the ionic products of hydroxides of group III alone exceed their solubility products and therefore, get precipitated.
