Redox titrations and limits of the concept

In a redox titration the end point is found by a colour change, just as in an acid-base titration.

Permanganate is its own indicator. The purple MnO₄⁻ is decolourised as it reacts with a reductant such as Fe²⁺ or C₂O₄²⁻; the first slight excess leaves a faint pink, visible at a concentration as low as 10⁻⁶ mol L⁻¹.

Dichromate gives no sharp colour change of its own, so an indicator such as diphenylamine is added. Just after the equivalence point it is oxidised to an intense blue.

Iodometric titration: Cu²⁺ oxidises iodide, 2Cu²⁺ + 4I⁻ → Cu₂I₂ + I₂, and the freed iodine is titrated with thiosulphate, I₂ + 2S₂O₃²⁻ → 2I⁻ + S₄O₆²⁻. Starch forms an intense blue with iodine, and the blue vanishes when the iodine is used up.

Iodine is only slightly soluble in water, but it stays in solution in the presence of KI as KI₃.

The electron-transfer picture is an idealisation. In covalent compounds it is more accurate to describe oxidation as a decrease in electron density around an atom and reduction as an increase.