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UPSC Combined Geo-Scientist

Chemist

  • 3 descriptive papers
  • 200 marks each
  • 3 h per paper

Mains Paper I covers inorganic chemistry.

UPSC Combined Geo-Scientist, Mains · 2026 · Mains Paper I

Q1(f). Masking agents in the estimation of Ca²⁺

  • 5 marks
  • Topic: Complexometric titrations
  • Answered
Question

Give at least two examples of masking agents. How are they used in the estimation of Ca²⁺ ion, if the solution contains ions like Ni²⁺ or Ag⁺? Explain with suitable equations.

Answer in brief

Masking agents: cyanide, triethanolamine, fluoride, thiourea. CN⁻ converts Ni²⁺ and Ag⁺ into very stable cyano-complexes that do not react with EDTA, while Ca²⁺ is not complexed and is titrated with EDTA.

Key points

  • a reagent that forms a stable, inert complex with an interfering ion so that it cannot react with EDTA or the indicator. Examples: CN⁻ (masks Ni²⁺, Ag⁺, Zn²⁺, Cu²⁺, Co²⁺); triethanolamine (Al³⁺, Fe³⁺); F⁻ (Al³⁺, Fe³⁺);…
  • make the sample alkaline (KCN only in alkaline solution, never acid: HCN is released). Add KCN: Ni²⁺ + 4CN⁻ → [Ni(CN)₄]²⁻ (β₄≈30)⁺ + 2CN⁻ → [Ag(CN)₂]⁻ (β₂≈21)
  • Ca²⁺ does not bind CN⁻. Adjust to pH 12 (NaOH; Mg²⁺ precipitates as Mg(OH)₂) and titrate with standard EDTA using murexide, red → violet: Ca²⁺ + H₂Y²⁻ → CaY²⁻ + 2H⁺
  • K_f of [Ni(CN)₄]²⁻ (10³⁰) is far greater than that of NiY (4×10¹⁸), so EDTA cannot take Ni²⁺ back.

The full model answer

Worked out step by step, as it should go in the answer booklet. The solution sheet follows it.

  1. Masking agent: a reagent that forms a stable, inert complex with an interfering ion so that it cannot react with EDTA or the indicator. Examples: CN⁻ (masks Ni²⁺, Ag⁺, Zn²⁺, Cu²⁺, Co²⁺); triethanolamine (Al³⁺, Fe³⁺); F⁻ (Al³⁺, Fe³⁺); thiourea (Cu²⁺).
  2. Method: make the sample alkaline (KCN only in alkaline solution, never acid: HCN is released). Add KCN: Ni²⁺ + 4CN⁻ → [Ni(CN)₄]²⁻ (β₄≈30)⁺ + 2CN⁻ → [Ag(CN)₂]⁻ (β₂≈21)
  3. Ca²⁺ does not bind CN⁻. Adjust to pH 12 (NaOH; Mg²⁺ precipitates as Mg(OH)₂) and titrate with standard EDTA using murexide, red → violet: Ca²⁺ + H₂Y²⁻ → CaY²⁻ + 2H⁺
  4. Why it works: K_f of [Ni(CN)₄]²⁻ (10³⁰) is far greater than that of NiY (4×10¹⁸), so EDTA cannot take Ni²⁺ back.

hot, alkaline cyanide must be handled in a fume hood; the demasking of Zn²⁺ from [Zn(CN)₄]²⁻ with formaldehyde lets two metals be titrated in one flask.

Model answer to Q1(f), 2026 Mains Paper I: solution page 5
Solution page 5 of 26
Read the whole 2026 solution2026 question paper

CoversmaskingMasking agents in the estimation of Ca²⁺

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