Q1(g). pH dependence of cyanide masking

Answer in brief

An alkaline medium, typically about pH 10–11, favours CN⁻ and effective masking; acidic conditions protonate CN⁻ to HCN.

Key points

• Acid–base equilibrium: HCN ⇌ H⁺ + CN⁻, with pKa near 9.2.
• The fraction present as CN⁻ is αCN = Ka/(Ka + [H⁺]) = 1/[1 + 10^{pKa − pH}].
• Raising pH above pKa increases free CN⁻, which forms stable cyanido complexes with interfering metal ions.
• The effective formation constant is proportional to αCNⁿ, so masking weakens sharply on acidification.
• Use an alkaline pH compatible with the analytical system; acidification also liberates volatile, highly toxic HCN.