KINETIC APPROACH TO THE MEASUREMENT OF CHEMICAL OXYGEN-DEMAND WITH AN AUTOMATED MICRO BATCH ANALYZER

An automated micro batch analyzer utilizes pressurized reservoirs of concentrated H2SO4, K2Cr2O7/HgSO4, and AgNO3 that are equipped with microprocessor-driven high-speed on/off valves. The sample (460 μL) is blown in to a glassfilled PTFE reaction cavity provided with a temperature programmable heater. Silica optical fibers continuously monitor the disappearance of dichromate in the reaction mixture in a total volume of =1.7 mL with mixing provided by magnetic stirring. The continuous monitoring approach not only provides the equilibrium absorbance but also provides information on the rate of oxidant consumption and thus on the nature of substances contributing to the chemical oxygen demand (COD). The system can also be configurated to study reaction rates as a function of temperature and to thus further characterize the oxidant consuming constituents. in the rapid testing mode, the instrument can process ~13 samples/h, using a 3-min measurement time at 140 °C with a limit of detection of approximately 10 ppm COD. All sample compounds studied undergo oxidation by dichromate in a rate process that is first order each with respect to the oxidant and the substrate. On the basis of this observation, observed temporal oxidation profiles may be extrapolated to estimate the oxidant consumption at equilibrium. © 1990, American Chemical Society. All rights reserved.