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(a) This method describes a procedure for determining the quantity of oxygen which an industrial waste water will consume from a dichromate solution under specified conditions. (b) This method for chemical oxygen demand is recommended as a supplement to the biochemical oxygen demand (BOD) test, but not as a substitute for it (Note 1). 1.—In spite of weaknesses inherent in a biological assay, the biochemical oxygen demand (BOD) is the only test which indicates directly the quantity of oxygen that will be utilized by natural agencies in stabilizing or ganic matter. However, the test for chemical oxygen demand (COD) may be used in evaluating the treatment and control of industrial waste water, and may be correlated with the BOD. Since these tests, COD and BOD, are not uniformly effective in oxidizing organic chemical compounds, there is no inherent constant relationship between the results of these tests for all industrial waste waters. If the basic composition of an industrial waste water continues to be relatively uniform, a fairly constant correlation may be expected between these paramenters. (c) This method can be applied to individual concentrated industrial wastes and to the combined effluents from a chemical plant. Ferrous iron, nitrites, sulfites, sulfides, halides, etc., are oxidized as well as organic constituents. Chlorides are quantitatively oxidized by this method, with 1 ppm of chloride as Cl− exerting the equivalent of 0l226 ppm chemical oxygen demand. Therefore, small amounts of organic matter cannot be reliably determined in the presence of a high chloride content, such as would be found in sea water. Furthermore, low results may be obtained under certain conditions, since not all organic compounds are completely oxidized (see Section 10).