化学需氧量Chemical oxygen demand

国标重铬酸钾法：在强酸性介质中，以硫酸银为催化剂、硫酸汞掩蔽氯离子，加入过量 K₂Cr₂O₇ 回流 2 小时氧化有机物；剩余的重铬酸钾以试亚铁灵为指示剂，用硫酸亚铁铵标准溶液返滴定，由消耗量计算 COD（记作 COD_Cr）。高锰酸钾法测得的“高锰酸盐指数”（COD_Mn）氧化能力较弱，用于较清洁的地表水和饮用水。

In the standard dichromate method, the sample is refluxed for two hours with excess K₂Cr₂O₇ in strong acid, with silver sulfate as catalyst and mercury(II) sulfate to mask chloride; the remaining dichromate is back-titrated with standard ferrous ammonium sulfate using ferroin indicator, and the consumption gives COD (written COD_Cr). The weaker permanganate method gives the permanganate index (COD_Mn), used for cleaner surface and drinking water.

19 世纪后期已用高锰酸钾评价水中有机物。1930 年代起重铬酸钾回流法逐渐取代高锰酸盐法，因其能氧化 95% 以上的大多数有机物。1950 年代引入硫酸银催化剂和汞盐掩蔽。现代还发展了快速消解分光光度法、在线 COD 分析仪，以及无汞、无铬的绿色测定方法。

Permanganate was used to assess organic matter in water in the late nineteenth century. From the 1930s the dichromate reflux method gradually replaced it, because it oxidises more than 95% of most organic compounds. Silver sulfate catalysis and mercury masking were added in the 1950s. Rapid-digestion spectrophotometric methods, online COD analysers and mercury- and chromium-free green methods followed.

COD 测定只需 2–3 小时，比生化需氧量（BOD₅，5 天）快得多，且能反映难生物降解的有机物。BOD₅/COD 比值常用于判断废水的可生化性：大于 0.3 通常适合生物处理。中国《污水综合排放标准》和《地表水环境质量标准》都把 COD 列为必测指标。

COD takes only two to three hours, far faster than five-day biochemical oxygen demand (BOD₅), and it also captures poorly biodegradable organics. The BOD₅/COD ratio indicates biodegradability; above about 0.3 a wastewater usually suits biological treatment. Chinese discharge and surface-water quality standards both list COD as a required parameter.