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

- In complexometric titrations, masking agents are substances that selectively bind to certain metal ions to prevent them from interfering with the titration of other metal ions.
- Potassium cyanide (KCN) is commonly used as a masking agent for Iron (II) ions because it forms a very stable complex with Fe\(^{2+}\), effectively "masking" it.
- This prevents Fe\(^{2+}\) from reacting with the titrant, allowing accurate determination of other metal ions in the solution.
- Other options like thioglycerol, triethanolamine, and ammonium fluoride have masking properties for different ions but are not typically used for Fe\(^{2+}\).

Answer 2

The question asks which masking agent is used to mask iron(II) ions in complexometric titration. Masking agents work by forming a stronger, more stable complex with an interfering metal ion than that metal would form with EDTA, so let's check the metal each reagent is best known for masking.

1. KCN: Cyanide forms very stable complexes with transition metal ions such as Fe²⁺, Cu²⁺, Co²⁺, Ni²⁺, Zn²⁺ and Cd²⁺ by binding through the carbon of the cyanide group. The resulting hexacyanoferrate(II) complex is stable enough to remove Fe²⁺ from competing with EDTA, which is exactly what masking Fe²⁺ requires.
2. Thioglycerol: This sulfur-containing reagent is generally used to mask ions such as Bi³⁺ and Sb³⁺ through soft sulfur donor bonding, not iron.
3. Triethanolamine: This reagent is the standard masking agent for Al³⁺, Fe³⁺ and Mn²⁺, forming a complex through its amine nitrogen and hydroxyl oxygens. It targets the ferric, not the ferrous, oxidation state of iron.
4. Ammonium fluoride: Fluoride is best known for masking Al³⁺, Ti⁴⁺ and rare earth ions by forming stable fluoro-complexes, not Fe²⁺.

Matching each reagent to the metal ion it is classically used for shows that only cyanide is specifically associated with masking iron in its +2 oxidation state.

Therefore, the correct answer is KCN.