A fully automated method for the determination of chemical oxygen demand

Abstract

A novel approach to the determination of chemical oxygen demand based on the use of a porous catalytic silver electrode is described. The conditions used in the digestion step are essentially the same as for the standard method, but the amount of oxidant (dichromate or permanganate) consumed is determined by allowing the excess of oxidant to react with hydrogen peroxide to liberate oxygen, which is measured coulometrically by the electrochemical sensor. The effectiveness of the present automated method as a detector of organic pollution is assessed by studying the oxygen absorbed values of synthetic sample solutions of several pure organic compounds. Percentage oxidation values are calculated and compared with those obtained with standard methods. Various aspects of interferences and their removal are also discussed.THE importance of chemical methods for the determination of the oxygen demand of water and waste water can hardly be over-emphasised. Although chemical methods do not permit differentiation between biologically stable and unstable forms of organic matter, they are valuable as indicators of organic pollution, especially that caused by trade wastes. The latter often contain toxic substances and bactericides, which may render the results of a biochemical procedure meaningless. Further, chemical methods have the merits of being relatively simple and of giving quick results when compared with the biochemical method and are therefore of value in routine monitoring and control of water pollution, even when the sample effluent is free from bactericides.The commonest two chemical methods used, the dichromate value (C.O.D.) method and the permanganate value method, are well documented in the literature and have been incorporated in several standard and recommended methods for the analysis of effluents.1-5 These standard procedures usually consist of a digestion step in which the organic waste is oxidised followed by a titrimetric finish to determine the amount of oxidant consumed. Although these procedures are rapid compared with the biochemical oxygen demand (B.O.D.) test, they still require considerable time and manipulation. Consequently, increasing demands for such analyses could only be met by the introduction of automated procedures that provide increases in speed, economy and precision and also possibilities of continuous monitoring of the effluent stream. Conventional automated procedures differ from the standard procedures in that the determination step is based on the measurement of optical densities, either a t the wavelength maxima of the reagent or of its reduction product.6-s Fleet, Ho and Tenyglg have recently described automated methods for the determination of permanganate and dichromate based on their reaction with hydrogen peroxide in acidic media to liberate oxygen, which, in turn, is monitored with a new type of coulometric oxygen sensor. The reactions can be represented by the following equations-2Mn04-+ 5H,02 + 6Hf = 2Mn2+ + 8H20 + 5 0 , . . .' (1) 2Cr0, + H,O, = Cr20,.+ H20 chrorn ic per...

An Improved-Bagging Model for Water Chemical Oxygen Demand Measurements Using UV-Vis Spectroscopy

“…Chemical methods generally use strong oxidants, such as potassium permanganate and potassium dichromate, to oxidize water samples under strong acid conditions, and then calculate the COD in water by measuring the amount of oxidant consumed. Chemical methods have disadvantages such as secondary pollution and long measurement periods, which are not suitable for online and real-time measurements [3][4]. The wavelength range for measuring COD via spectroscopy is generally in the ultraviolet-visible interval.…”

Kinetic approach to the measurement of chemical oxygen demand with an automated micro batch analyzer

“…Numerous variations of the dichromate-based COD measurement method are in the literature . Semiautomated (32,33) and automated versions with (34)(35)(36)(37)(38)(39) and without (40)(41)(42) air segmentation as well as adaptation to a complex batch mixer (43) have been described. One commerical COD analyzer also utilizes batch mixing on a relatively large volumetric scale (44).…”