重铬酸钾Potassium dichromate

重铬酸根 Cr₂O₇²⁻ 由两个 CrO₄ 四面体共用一个氧原子构成，Cr–O–Cr 键角约 126°。Cr(VI) 没有 d 电子，橙色来自氧到铬的电荷转移吸收。溶液中存在平衡 2CrO₄²⁻（黄）+ 2H⁺ ⇌ Cr₂O₇²⁻（橙）+ H₂O，加酸变橙，加碱变黄。

The dichromate ion Cr₂O₇²⁻ consists of two CrO₄ tetrahedra sharing one oxygen atom, with a Cr–O–Cr angle of about 126°. Cr(VI) has no d electrons; the orange colour comes from oxygen-to-chromium charge-transfer absorption. In solution 2CrO₄²⁻ (yellow) + 2H⁺ ⇌ Cr₂O₇²⁻ (orange) + H₂O, so acid turns the solution orange and alkali turns it yellow.

在酸性溶液中：Cr₂O₇²⁻ + 14H⁺ + 6e⁻ → 2Cr³⁺ + 7H₂O，E° = +1.33 V，颜色由橙变绿。在有机化学中，酸化重铬酸钾把伯醇氧化为醛（边加热边蒸出），继续回流则氧化为羧酸；仲醇被氧化为酮；叔醇不被氧化，溶液保持橙色，可据此区分三类醇。

In acid solution: Cr₂O₇²⁻ + 14H⁺ + 6e⁻ → 2Cr³⁺ + 7H₂O, E° = +1.33 V, with the colour changing from orange to green. In organic chemistry, acidified potassium dichromate oxidises primary alcohols to aldehydes (distilled off as they form) and, under reflux, on to carboxylic acids; secondary alcohols give ketones; tertiary alcohols are not oxidised and the solution stays orange, which distinguishes the three classes.

重铬酸钾易于提纯、不吸湿、溶液稳定，可直接配制标准溶液，用于测定 Fe²⁺ 等还原性物质：Cr₂O₇²⁻ + 6Fe²⁺ + 14H⁺ → 2Cr³⁺ + 6Fe³⁺ + 7H₂O。环境监测中的化学需氧量（COD）也用重铬酸钾法测定。早期的呼气酒精检测仪利用乙醇使重铬酸钾由橙变绿。

Potassium dichromate is easy to purify, not hygroscopic and gives stable solutions, so it can be weighed directly as a primary standard for titrating reducing agents such as Fe²⁺: Cr₂O₇²⁻ + 6Fe²⁺ + 14H⁺ → 2Cr³⁺ + 6Fe³⁺ + 7H₂O. Chemical oxygen demand (COD) in environmental monitoring is also measured with dichromate. Early breath-alcohol testers relied on ethanol turning dichromate from orange to green.

六价铬化合物有毒，是已确认的人类致癌物，并会引起皮肤过敏和腐蚀。重铬酸钾是强氧化剂，与可燃物接触有着火危险。使用时应戴手套和护目镜，含铬废液须单独回收处理。

Chromium(VI) compounds are toxic, confirmed human carcinogens and cause skin sensitisation and burns. Potassium dichromate is a strong oxidiser and can ignite combustible material. Wear gloves and eye protection, and collect chromium waste separately for treatment.