in environmental chemistry, the chemical oxygen demand (cod) test is commonly used to indirectly measure the amount of organic compounds in water. most applications of cod determine the amount of organic pollutants found in surface water (e.g. lakes and rivers) or wastewater, making cod a useful measure of water quality. it is expressed in milligrams per liter (mg/l) also referred to as ppm (parts per million), which indicates the mass of oxygen consumed per liter of solution. ==overview== the basis for the cod test is that nearly all organic compounds can be fully oxidized to carbon dioxide with a strong oxidizing agent under acidic conditions. the amount of oxygen required to oxidize an organic compound to carbon dioxide, ammonia, and water is given by: :\mbox{c}_n\mbox{h}_a\mbox{o}_b\mbox{n}_c + \left( n + \frac{a}{4} - \frac{b}{2} - \frac{3}{4}c \right)\mbox{o}_2 \rightarrow n\mbox{co}_2 + \left( \frac{a}{2} - \frac{3}{2}c \right)\mbox{h}_2\mbox{o} + c\mbox{nh}_3 this expression includes the oxygen demand caused by the oxidation of ammonia into nitrate. the process of ammonia being converted into nitrate is referred to as nitrification. the following is the correct equation for the oxidation of ammonia into nitrate. :\mbox{n}\mbox{h}_3 + 2\mbox{o}_2 \rightarrow \mbox{n}\mbox{o}_3^- + \mbox{h}_3\mbox{o}^+ it is applied after the oxidation due to nitrification if the oxygen demand from nitrification must be known. dichromate does not oxidize ammonia into nitrate, so this nitrification can be safely ignored in the standard chemical oxygen demand test. the international organization for standardization describes a standard method for measuring chemical oxygen demand in iso 6060 . ==history== for many years, the strong oxidizing agent potassium permanganate (kmno4) was used for measuring chemical oxygen demand. measurements were called oxygen consumed from permanganate, rather than the oxygen demand of organic substances. potassium permanganate's effectiveness at oxidizing organic compounds varied widely, and in many cases biochemical oxygen demand (bod) measurements were often much greater than results from cod measurements. this indicated that potassium permanganate was not able to effectively oxidize all organic compounds in water, rendering it a relatively poor oxidizing agent for determining cod. since then, other oxidizing agents such as ceric sulphate, potassium iodate, and potassium dichromate have been used to determine cod. of these, potassium dichromate (k2cr2o7) has been shown to be the most effective: it is relatively cheap, easy to purify, and
Anaerobic_digestion Aquatic_ecology Chemical_oceanography Environmental_chemistry Water_quality_indicators