When H₂S gas is passed through an acidified aqueous solution containing multiple metal ions, only those sulfides which are insoluble in acidic medium will precipitate. The solubility of sulfides depends on their solubility product (K_sp) values. In acidic conditions, the concentration of S^2- ions is low due to the common ion effect and suppression of H₂S dissociation.

The dissociation of H₂S in water is:

In acidic medium, high [H⁺] shifts equilibrium left, reducing [S^2-]. Only sulfides with very low K_sp (very insoluble) can precipitate under these conditions.

Comparing the K_sp values:

• MnS: ~10^-15 (high, soluble in acid)
• NiS: ~10^-27 (moderate, may not precipitate in strong acid)
• CuS: ~10^-36 (very low, precipitates even in acid)
• HgS: ~10^-54 (extremely low, always precipitates)

Thus, in acidified solution, only CuS and HgS have sufficiently low solubility products to precipitate when H₂S is passed. MnS and NiS remain in solution due to higher solubility.

Related Topics

Qualitative Analysis: This is part of systematic qualitative analysis for cations. Group II cations (Cu²⁺, Hg²⁺, etc.) are precipitated as sulfides in acidic medium, while Group IV (Mn²⁺, Ni²⁺) are precipitated as sulfides in basic medium.

Solubility Product (K_sp): The equilibrium constant for a solid dissolving in solution. Lower K_sp means lower solubility. Acid concentration affects sulfide precipitation by controlling [S^2-].

Key Formulae

For a sulfide MS, the dissolution is:

K_sp = [M²⁺][S^2-]

Ionic product IP = [M²⁺][S^2-]. Precipitation occurs when IP > K_sp.