With Na 2 S, along with 2nd group cations, some cations of higher groups i.e., III rd and IV th groups may be precipitated because the higher concentration of S 2– ions is obtained according to following reaction.

Na 2 S 2 Na + + S 2–

To precipitate II nd group cations lower concentration of S 2– ions is required (because of low K sp of II nd group sulphides) and this can be obtained easily by H 2 S in presence of dilute HCl.

H 2 S 2 H + + S 2– ; HCl H + + Cl –

Due to common ion effect the ionisation of H 2 S is suppressed and the concentration of S 2– ions obtained is just sufficient to precipitate the cations of II nd group.