Q1(e). Iodometry and iodimetry

Answer in brief

Iodimetry is direct titration of a reductant with iodine; iodometry determines an oxidant indirectly through iodine liberated from iodide.

Key points

• In iodimetry, iodine is the oxidising titrant: I₂ + 2e⁻ → 2I⁻. A persistent starch-blue colour indicates the first excess of iodine.
• In iodometry, excess iodide reacts with the oxidising analyte to release iodine, which is then titrated with thiosulfate: I₂ + 2S₂O₃²⁻ → 2I⁻ + S₄O₆²⁻.
• The iodometric endpoint is disappearance of the starch-blue colour. Add starch near the endpoint when the iodine concentration has become low.