= Redox titration
A <redox titration> adds a measured <amount of substance> of one <reactant> to determine another through a known <redox> <chemical equation>. The <burette> reading becomes an amount through <concentration>; the balanced <chemical equation> supplies the amount ratio. A colour endpoint additionally needs a chemical explanation, as in the <permanganate-iron titration>.
The useful bridge is an <electron> inventory. If each sample entity supplies $z_A$ <electrons> and each <titrant> entity accepts $z_T$, matching the formal exchanged <amount of substance> of <electrons> gives
$$
z_A n_A=z_T n_T,\qquad n_A=\frac{z_T}{z_A}c_TV_{\mathrm{eq}}.
$$
Here $c_T$ is the <titrant> <molar concentration> and $V_{\mathrm{eq}}$ the <volume (thermodynamics)> attributable to the sample at the <equivalence point>. These are net <half-reaction> counts, not a claim that free <electrons> accumulate in the flask. The <redox> conversion must be sufficiently complete and selective for the stated <stoichiometry> to identify $n_A$.
This is why a small visible signal can reveal a much larger invisible inventory: the signal locates a stopping point, while <titrant standardization> and <stoichiometry> turn delivery into <amount of substance>. The <titration endpoint> estimates the <equivalence point>; repeatable colour alone establishes neither correct <titrant> <concentration> nor chemical selectivity. The <permanganate-iron endpoint and blank model> separates these assumptions.
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