THE COULOMETRIC DETERMINATION OF CHEMICAL OXYGEN-DEMAND

The chemical oxygen demand (COD) of solutions containing various organ ic compounds is calculated from the net faradaic charge (Q(net)) estim ated for the total electrolytic oxidation of Cr-III generated during o xidative degradation of the organic compounds in acidic media containi ng excess Cr-VI. Values of Q(net) for conversion of Cr-III to Cr-VI ar e estimated from the linearized chronoamperometric data plotted as ln{ i(tot,t)} vs. t. This procedure is preferred over determinations of Q( net) from the total integrals of i(tot) over the entire electrolysis p eriod because of large errors that can result from uncertainty in the background current (i(bkg)) for t --> infinity. The proposed coulometr ic procedure offers the benefit that reagent solutions can be reused, thereby minimizing the need for disposal of wastes containing toxic Cr -VI. This procedure was applied in a single digest solution for consec utive determinations of COD. Average COD values for potassium acid pht halate and glucose were 103.8 % (s = 6.0, N = 10) and 100.2 % (s = 4.2 , N = 11), respectively, based on the theoretical degradation to CO2. In comparison for these same samples, an EPA approved method, based on colorimetric determination of Cr-III, gave COD values of 101.4 % (S = 1.4, N = 5) and 100.1 % (s = 1.4, N = 5) of the theoretical. Statisti cal tests indicate no significant difference in the COD values determi ned for these compounds using the coulometric and EPA methods.