UPSC Combined Geo-Scientist, Mains · 2026 · Mains Paper I
Q1(e). Iodometry and iodimetry
Question
Distinguish between iodometry and iodimetry with examples.
Answer in brief
Iodimetry is a direct titration of a reducing agent with standard I₂. Iodometry is an indirect method: an oxidising agent liberates I₂ from excess I⁻, and the I₂ is titrated with standard Na₂S₂O₃.
Key points
- I₂ is volatile and I⁻ is air-oxidised in acid, so iodometric titrations use a covered flask and are done at once; starch is added late to avoid an I₂–starch complex that releases I₂ slowly.
The full model answer
Worked out step by step, as it should go in the answer booklet. The solution sheet follows it.
| Iodimetry | Iodometry | |
| Titrant | standard I₂ (from I₂/KI) | standard Na₂S₂O₃ |
| Analyte | reducing agent: As₂O₃, Sn²⁺, SO₃²⁻, ascorbic acid | oxidising agent: Cu²⁺, Cr₂O₇²⁻, IO₃⁻, ClO⁻ |
| Reaction | I₂ + 2e⁻ → 2I⁻ | 2I⁻ → I₂ + 2e⁻, then I₂ + 2S₂O₃²⁻ → 2I⁻ + S₄O₆²⁻ |
| Example | H₃AsO₃ + I₂ + H₂O → H₃AsO₄ + 2I⁻ + 2H⁺ (pH 8, NaHCO₃) | 2Cu²⁺ + 4I⁻ → 2CuI ↓ + I₂ |
| Starch added | at the start (blue = end point) | near the end (blue → colourless) |
| Medium | neutral or weakly alkaline | weakly acidic; excess I⁻ |
I₂ is volatile and I⁻ is air-oxidised in acid, so iodometric titrations use a covered flask and are done at once; starch is added late to avoid an I₂–starch complex that releases I₂ slowly.
Coversiodine methodsIodometry and iodimetry
All parts of Question 1
- 1(a) Zinc blende and wurtzite: radius-ratio rationale5 marks
- 1(b) Spin-forbidden bands in 4d and 5d complexes5 marks
- 1(c) Phosphate buffer at pH 7.405 marks
- 1(d) Onset of AgCl precipitation5 marks
- 1(e) Iodometry and iodimetry5 marks
- 1(f) Masking agents in the estimation of Ca²⁺5 marks
- 1(g) First-row metal in EAN-obeying organometallics5 marks
- 1(h) Radioactive displacement: ²¹⁰Pb and ²²⁶Ra5 marks
- 1(i) Electronic spectra of Eu³⁺ and Am³⁺ complexes5 marks
- 1(j) Urea required to precipitate Fe³⁺ as Fe(OH)₃5 marks