Why is dilute specifically added before passing gas to precipitate Group II cations in the qualitati — Qualitative and Quantitative Analysis Chemistry Question

💡 Solution & Explanation

Due to the common ion effect from the $H^+$ ions provided by the strong acid $HCl$, the ionization of the weak acid $H_2S$ is suppressed. This results in a low concentration of $S^{2-}$ ions, which is only sufficient to exceed the low solubility product ($K_{sp}$) of Group II sulphides (like $CuS, HgS$), preventing the precipitation of higher $K_{sp}$ sulphides of Group IV.