H2S gas when passed through a solution of cations containing HCl precipitates the cations of second group of qualitative analysis but not those belonging to the fourth group. It is because
The cation of group II are precipitated as their sulphides. Solubility product of sulphide of group II radicals are very low. Therefore, even with low conc. of S2− ions, the ionic product exceeds the value of their solubility product and the radical of group II gets precipitated. The low conc. of S2− ions is obtained by passing H2S gas through the solution of the salt in the presence of dil. HCl which suppresses degree of ionisation of H2S by common ion effect.
common 10n Note : Solubility product of group IV radicals are quite high. It is necessary to suppress the conc. of S2− ions, otherwise radical of group IV will also get precipitated along with group II radicals.