Redox titrations and identifying agents

In a redox titration, an oxidant is titrated against a reductant (or the other way round). The end point can be shown in three ways:

• Self-indicator: potassium permanganate, MnO₄⁻, is an intense purple, while its product Mn²⁺ is almost colourless. As long as there is Fe²⁺ or C₂O₄²⁻ left to react with, each drop loses its colour. The first lasting pink tinge shows that the reductant is used up. It is visible even at about 10⁻⁶ M.
• Redox indicator: dichromate, Cr₂O₇²⁻, has no strong colour change of its own. So an indicator such as diphenylamine is added; just after the equivalence point, the dichromate oxidises it to an intense blue.
• Iodometry: an oxidant (such as Cu²⁺) is allowed to free iodine from iodide ions (I⁻). The iodine is then titrated with thiosulphate: I₂ + 2S₂O₃²⁻ → 2I⁻ + S₄O₆²⁻. Starch gives a deep blue colour with iodine, so the end point is when the blue disappears.

To identify the agents in any reaction, assign oxidation numbers and watch which way they move:

• The species whose oxidation number falls is reduced, so it is the oxidant.
• The species whose oxidation number rises is oxidised, so it is the reductant.

Reducing power depends on the partner too. Methanal (HCHO) easily reduces Ag⁺ ions in Tollens' reagent to silver metal (the 'silver mirror' test), but does not reduce Cu²⁺ so easily. This shows Ag⁺ is a stronger oxidant than Cu²⁺.