Determination of low chemical oxygen demand values in water by the dichromate semi-micro method

For nearly 40 years,'.' the dichromate open reflux method for the determination of the chemical oxygen demand (COD) has been and is still used in water analysis3 although it has several shortcomings: it requires large amounts of reagents and sample, it is space demanding, tedious, time consuming, and, in particular, its sensitivity (about 5 mg 1-1 when using 0.0417 M K2Cr207),3 is too low for application to waters with low organic matter contents such as drinking, ground and nonpolluted surface waters. To reduce the consumption of chemicals and save space, alternative methods of C O D determination on a semi-micro scale, with a comparable sensitivity, have been introduced within the past 20 years.&-" The digestion conditions are largely identical with those used in the original method; i.e., 50% v/v (63.6% m/m) HZS04 in the digestion mixture and heating at 150 "C for 2 h. The boiling under a reflux condenser, however, is usually replaced by heating in thermostated blocks, and the quantification is often based on the spectrophotometric determination of Cr3+ ions4ys-7or on the amperometric titration of the unconsumed dichromate.8 Efforts have been made to reduce the time of the original procedure10-1'; however, for many compounds (e.g., aliphatic acids)" shortening the digestion period leads to a decrease in oxidation efficiency, unless the concentration of H2S04 and the temperature are increased. A solution to this problem, in many respects successful, has been offered by Wagner and Ruck,l3 who devised a rapid method applicable to the determination of C O D at levels encountered in drinking waters, i.e., in the range of about 1-10 mg 1-1. By increasing the concentration of H2S04to 83% m/m and the temperature to 170 "C, they were able to reduce the time of digestion to 45 min. Using perfectly purified reagents, they attained detection limits below 1 mg 1-1. The increased digestion temperature, however, brought about problems associated with the masking of chloride because at temperatures above 150 "C, H g S 0 4 alone failed to mask chloride effectively. This problem was circumvented by removing chloride as HCI in vapour form prior to the digestion,13,14 a procedure that is, however, tedious and time consuming. This paper describes an approach to the determination of low C O D values on a semi-micro scale, with digestion conditions optimised t o attain comparable efficiency to that of the standard method,3 and with a considerably reduced digestion time. The interfering effect of chloride is limited by the combined masking effects of HgS04 and Cr3+ ions.15 This semi-micro method is compared with the open reflux method using samples of various waters and chemical compounds. In
the open reflux method, which is used as a reference method, the digestion conditions of the standard method3 were maintained, however, the manner in which the sample and reagent were dispensed, was modified, in order to achieve improved accuracy and ensure comparable precision, so that both methods could be compared.
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A semi-micro modification of the dichromate method for the determination of low concentrations (1-35 m g 1-1) of the chemical oxygen demand is described. It is based on the experimentally established dependencies of the oxidation efficiency on the concentrations of H2SO4, K2Cr207and Ag2S04, and o n the temperature and time of heating. The procedure involves increasing the concentration of H2SO4 and heating in closed test-tubes i n a heating block at 170 "C for 40 min and the spectrophotometric determination of dichromate at 455 nm. The interfering effect of chloride, u p to a concentration of 100 m g 1-1, is masked by using a combination of HgS04 and KCr(S04)2. The modification has been tested on 30 samples of ground, tap, precipitation and surface waters and on 16 chemical compounds, and gives results comparable to those from a standard reflux method. Keywords: Chemical oxygen demand; sealed test-tube digestion; chloride suppression; chromium(111) addition
Josef Hejzlar and JiFi KopaCek Section of Hydrobiolog y, Institute of Landscape Ecology, Czechoslovak Academ y of Sciences, Na sadkach 7, 370 05 ceske Budgjovice, Czechoslovakia
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Chemicals All chemicals used were of analytical-reagent grade, supplied by Lachema (Brno, Czechoslovakia). The digestion solutions and sulphuric acid reagents were prepared from 96 o r 92% m/m sulphuric acid pre-treated by filtration and heated to boiling-point in an all-glass apparatus equipped with an air cooler. Sulphuric acid, 98.3% d m , was obtained by distillation of the 96% m/m acid in an all-glass apparatus. All filtrations were carried out through a sintered glass funnel [type S1, 0.094.15-mm porosity (Jena Glass, Jena, GDR)]. Water for sample dilution and for the preparation of standard and reagent solutions was obtained from distilled water by re-distilling with the addition of K M n 0 4 and H2S04. Reagents for the Semi-micro Method