氧化还原滴定Redox titration

滴定过程中体系电位按能斯特方程变化，在化学计量点附近发生突跃，突跃大小取决于两电对条件电位之差（一般需大于 0.3–0.4 V）。终点指示有三种：自身指示剂（KMnO₄ 本身紫红色，过量一滴即显浅红）、专属指示剂（淀粉遇 I₂ 变蓝）和氧化还原指示剂（如二苯胺磺酸钠、邻二氮菲-亚铁），也可用电位法确定终点。

During the titration the potential follows the Nernst equation and jumps near the equivalence point; the size of the jump depends on the difference in formal potentials of the two couples, which usually needs to exceed 0.3–0.4 V. End points are shown by self-indication (purple KMnO₄, whose first excess drop turns the solution pale pink), specific indicators (starch, blue with I₂) or redox indicators such as sodium diphenylamine sulfonate and ferroin, or found potentiometrically.

高锰酸钾法：酸性条件下 MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O，用草酸钠标定，测 Fe²⁺、H₂O₂、草酸和水的化学需氧量；重铬酸钾法：K₂Cr₂O₇ 可直接作基准物质、溶液稳定，用于测铁和 COD；碘量法：直接碘量法用 I₂ 标准溶液测维生素 C、SO₂ 等强还原剂，间接碘量法使氧化剂与过量 KI 反应析出 I₂，再用 Na₂S₂O₃ 滴定，用于测 Cu²⁺、溶解氧（温克勒法）、漂白粉有效氯等。

Permanganometry uses MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O in acid, standardised against sodium oxalate, to determine Fe²⁺, H₂O₂, oxalate and the COD of water. Dichromatometry uses K₂Cr₂O₇, a primary standard with stable solutions, for iron and COD. In direct iodometry standard I₂ titrates strong reductants such as vitamin C and SO₂; in indirect iodometry an oxidant liberates I₂ from excess KI, which is titrated with Na₂S₂O₃, to determine Cu²⁺, dissolved oxygen (the Winkler method) and available chlorine in bleach.

1787 年贝托莱用靛蓝溶液的褪色测定氯水浓度。1846 年马格里特用高锰酸钾滴定 Fe²⁺。1853 年本生系统发展了碘量法。1888 年温克勒建立了测定水中溶解氧的碘量法，至今仍是标准方法。

In 1787 Berthollet measured chlorine water by its bleaching of indigo solution. Marguerite titrated Fe²⁺ with permanganate in 1846, Bunsen developed iodometry systematically in 1853, and in 1888 Winkler devised the iodometric determination of dissolved oxygen, still a standard method.