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Updated: Jul 11, 2026

Evaluation of Oxidative Stress in Biological Samples Using the Thiobarbituric Acid Reactive Substances Assay
Published on: May 12, 2020
Incomplete oxidation of ethylenediaminetetraacetic acid in chemical oxygen demand analysis
James E Anderson1, Sherry A Mueller, Byung R Kim
1Systems Analyses and Environmental Sciences Department of Research and Advanced Engineering, Ford Motor Company, Dearborn, Michigan, USA. jander63@ford.com
Abstract:
Ethylenediaminetetraacetic acid (EDTA) was found to incompletely oxidize in chemical oxygen demand (COD) analysis, leading to incorrect COD values for water samples containing relatively large amounts of EDTA. The degree of oxidation depended on the oxidant used, its concentration, and the length of digestion. The COD concentrations measured using COD vials with a potassium dichromate concentration of 0.10 N (after dilution by sample and sulfuric acid) were near theoretical oxygen demand values. However, COD measured with dichromate concentrations of 0.010 N and 0.0022 N were 30 to 40% lower than theoretical oxygen demand values. Similarly, lower COD values were observed with manganic sulfate as oxidant at 0.011 N. Extended digestion yielded somewhat higher COD values, suggesting incomplete and slower oxidation of EDTA, as a result of lower oxidant concentrations. For wastewater in which EDTA is a large fraction of COD, accurate COD measurement may not be achieved with methods using dichromate concentrations less than 0.1 N.
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