Evaluation of chemical oxygen demand (COD) based on coulometric determination of electrochemical oxygen demand (EOD) using a surface oxidized copper electrode Original Research Article

Chemical oxygen demand (COD) was estimated by exhaustive electrolysis using a home-made cell with a copper working electrode in 0.1 M NaOH. The net Faradaic charge, corresponding to the number of electrons consumed in electrolyzing (oxidizing) the organic species, was correlated to the COD evaluated by the conventional methods using dichromate or permanganate. The time taken for a single measurement was about 30 min, much less than the 2–4 h required in the conventional methods. Known components of the secondary effluents from several waste water treatment plants in Japan (l-glutamate, glycine, d-glucose, phenol, lignin, tannic acid, hemicellulose, and humic acid) were chosen as standard samples, and each showed a linear response proportional to the concentration of the analytes. For real water samples from rivers and lakes, a linear relationship was observed between this method and both of the conventional methods between about 1 and 10 mg ml−1 COD, the range allowed by the Environmental Quality Standard of Japan. An artificial water sample was prepared and a calibration graph resembling the results of the real samples was obtained. NMR spectra of d-glucose, l-glutamate, glycine, phenol, quinone, hydroquinone, lignin, tannic acid, hemicellulose, and humic acid were obtained before and after electrolysis, and qualitative changes are discussed. Fluorescence measurements were also performed before and after the electrolysis of the polymeric compounds used in this experiment, allowing the oxidation ratios to be estimated.