Question

What is the role of KCN in the test of Cd²⁺ ion in the presence of Cu²⁺ ion?

Answer 1

In the presence of both Cu²⁺ (copper ion) and Cd²⁺ (cadmium ion), KCN (potassium cyanide) plays a crucial role in selectively detecting Cd²⁺ by removing Cu²⁺ from the solution and preventing interference in the test.

1. Formation of Cyanide Complex with Cu²⁺:

• KCN dissociates in water to form cyanide ions (CN⁻).
• Cu²⁺ is a transition metal ion that can form strong coordination complexes with cyanide ions. When KCN is added to the solution, the cyanide ions react with Cu²⁺ to form a very stable complex, [Cu(CN)₄]²⁻.
• This complex formation effectively removes Cu²⁺ from the solution, as it is now part of the stable [Cu(CN)₄]²⁻ complex and does not participate in further reactions or precipitation.

2. Prevention of Precipitation of Cu²⁺:

• Normally, Cu²⁺ could react with sulfide ions (e.g., from Na₂S) to form a black precipitate of CuS.
• However, because Cu²⁺ is now bound in the stable [Cu(CN)₄]²⁻ complex, it is no longer free to react with sulfides, allowing the Cd²⁺ ions to be the only metal ion present for precipitation.

3. Selective Detection of Cd²⁺:

• With Cu²⁺ effectively removed by KCN, the test for Cd²⁺ can proceed without interference. When a sulfide reagent like Na₂S is added, Cd²⁺ forms a yellow precipitate of cadmium sulfide (CdS), confirming the presence of Cd²⁺ in the solution.