Chemical oxygen demand determination in environmental waters by mixed-acid digestion and single sweep polarography Original Research Article

A new variant of the method for chemical oxygen demand (COD; 20–1100 mg O2 l−1) determination in environmental waters is described. The method is based on determination of excess Cr(VI) in a mixed-acid (H3PO4–H2SO4) digestion solution by single sweep polarography instead of titrimetry; COD is measured indirectly. The reflux time is shortened to 15 min by mixed-acid digestion. The effects of mixed-acid concentration, reflux time, amounts of dichromate, supporting electrolyte, and interfering substances on COD determination are discussed. Preliminary application are made to waste waters and sewage. The results obtained are in good agreement with those given by the standard reflux method with a relative error<1.5%.