A qualitative analysis mixture contains and ions. To separate them, excess potassium cyanide () is a — Qualitative and Quantitative Analysis Chemistry Question

💡 Solution & Explanation

Addition of excess $KCN$ forms soluble complexes for both: $[Cu(CN)_4]^{3-}$ and $[Cd(CN)_4]^{2-}$. The copper complex is highly stable ($K_d$ is very small), meaning the concentration of free $Cu^+$ is too low to exceed the $K_{sp}$ of copper sulphide when $H_2S$ is passed. Conversely, the cadmium complex is less stable, dissociating enough to provide a sufficient $Cd^{2+}$ concentration to exceed the $K_{sp}$ of $CdS$, forming a yellow precipitate.