Group II cations are precipitated by passing $\mathrm{H}{}_2\mathrm{S}$ into the solution made about $0.3\mathrm{M}$ in $\mathrm{HCl}$. The role of the $\mathrm{HCl}$ is to

1. Aform soluble chloro-complexes of all group IV cations
2. Braise $[\mathrm{S}{}^{2-}]$ so that all sulphides precipitate
3. Cconvert $\mathrm{H}{}_2\mathrm{S}$ completely into $\mathrm{S}$
4. Dlower $[\mathrm{S}{}^{2-}]$ through the common ion effect of $\mathrm{H}{}^{+}$ on $\mathrm{H}{}_2\mathrm{S}$ ionisation, so only very insoluble sulphides precipitate