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# For the percipitation of cations of group four in the qualitative analysis, the medium is made alkaline before passing $H_2S$ gas. The purpose of alkaline Solution is

(a) to supress the ionisation of $H_2S$

(b) to increase the ionisation of $H_2S$

(c) to increase the ionisation of metal salt

(d) to decrease the ionisation of metal salt.

Answer: to increase the ionisation of $H_2S$

Dissociation of $H_2S$
$H_2S \rightleftharpoons 2H^+ + S^{-2}$

$NH_4OH \longrightarrow NH_4^- + OH^-$
$H^+$ combines with $OH^-$ to form water and thus the first equilibrium shifts in the forward direction to increase the ionisation of $H_2S$