REDOX titration

Meaning: A redox titration finds the unknown concentration of an oxidizing or reducing agent by adding a standard solution of the opposite kind. The equivalence point is reached when moles of electrons lost equal moles of electrons gained. Permanganate is the classic titrant: purple MnO₄⁻ is reduced to nearly colorless Mn²⁺ in acid, so the first persistent faint pink marks the endpoint. Alternatively, an electrode can track the potential, which jumps sharply near equivalence.

Equation: neOx × MOx × VOx = neRed × MRed × VRed, where ne is electrons transferred per formula unit (dimensionless), M is molarity (mol/L), and V is volume (L).

On the MCAT: A passage titrates Fe²⁺ with KMnO₄ and gives the volume of titrant used. You balance the half-reactions, find the mole ratio, and calculate the unknown concentration. Questions may also ask which species is oxidized or what color change signals the endpoint.

Common confusion: Students assume a 1:1 ratio, as in a monoprotic acid-base titration. In acid, MnO₄⁻ gains 5 electrons (Mn goes from +7 to +2) while each Fe²⁺ loses 1, so 1 mol MnO₄⁻ reacts with 5 mol Fe²⁺. Count electrons, not formula units.