Q1(f). Iodometry versus iodimetry

Answer in brief

Iodimetry directly titrates a reducing agent with iodine. Iodometry determines an oxidising agent through iodine liberated from excess iodide.

Key points

• Iodimetry: standard iodine dissolved in KI is the titrant; a reducing analyte consumes I₂/I₃⁻, e.g. I₂ + SO₃²⁻ + H₂O → 2I⁻ + SO₄²⁻ + 2H⁺.
• The first persistent excess of iodine gives a blue starch-iodine end point in direct iodimetry.
• Iodometry: add excess KI to the oxidising analyte; for example, 2Cu²⁺ + 4I⁻ → 2CuI(s) + I₂.
• Titrate the liberated iodine with standard thiosulfate: I₂ + 2S₂O₃²⁻ → 2I⁻ + S₄O₆²⁻.
• In iodometry add starch near the pale-yellow stage; the end point is disappearance of blue.