Question

In complexometric titration, the masking agent used to mask Iron (II) ion

• A. KCN
• B. Thio glycerol
• C. Tri ethanol amine
• D. Ammonium Fluoride

Hint:

KCN is an effective masking agent for Iron (II) ions in complexometric titrations due to stable complex formation.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Masking is used in complexometric titration when a sample has more than one metal ion but only one is meant to be titrated. The interfering ion is tied up in a stable complex so it no longer reacts with EDTA.

Step 2: Key Formula or Approach:
A good masking agent must form a complex with the interfering ion that has a higher stability (formation) constant than the EDTA complex of that same ion, so the ion effectively disappears from the titration.

Step 3: Detailed Explanation:
Cyanide ion binds Fe²⁺ through its carbon lone pair, forming the hexacyanoferrate(II) ion, \( [\text{Fe}(\text{CN})_6]^{4-} \), which has a very large stability constant. Because this complex is so much more stable than the Fe-EDTA complex, adding KCN effectively locks Fe²⁺ away, letting EDTA react only with the other metal ions present. Triethanolamine, thioglycerol and fluoride each work the same way in principle, but their strongest affinities lie with Fe³⁺, Bi³⁺/Sb³⁺, and Al³⁺/Ti⁴⁺ respectively, not with Fe²⁺.

Step 4: Final Answer:
KCN is the masking agent used to mask iron(II) ions in complexometric titration.