The coulometric determination of chemical oxygen demand

The chemical oxygen demand (COD) of solutions containing various organic compounds is calculated from the net faradaic charge (Q_net) estimated for the total electrolytic oxidation of Cr^III generated during oxidative degradation of the organic compounds in acidic media containing excess Cr^VI. Values of Q_net for conversion of Cr^III to Cr^VI are estimated from the linearized chronoamperometric data plotted as In {i_{tat, t}} vs. t. This procedure is preferred over determinations of Q_net from the total integrals of i_tot over the entire electrolysis period because of large errors that can result from uncertainty in the background current (i_bkg) for t → ∞. The proposed coulometric 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 consecutive determinations of COD. Average COD values for potassium acid phthalate 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. Statistical tests indicate no significant difference in the COD values determined for these compounds using the coulometric and EPA methods.