Potentiometric measurement of chemical oxygen demand

Measurement of the COD of an aqueous solution involves firstly digestion of a test sample with dichromate solution, and secondly measurement of the dichromate remaining either by titration by ferrous ammonium sulfate (FAS) to a color indicator endpoint, or via colorimetric determination using a spectrophotometer. In this paper, a potentiometric end-point determination (using FAS titrant) is proposed as a third option for measuring the dichromate remaining. The potentiometric method is based on the observation that the titration end point corresponds to a region of minimum redox buffer. That is, the end point to the titration is sharp and easily identified. Furthermore the platinum-calomel electrode system used in this method responds quickly to potential changes within the (Fe 2+ /Fe 3+ ):(Cr 2 O 7 2. /Cr 3+ ) solution producing stable readings. These factors allow rapid and accurate COD measurement. Furthermore the method is based firmly on theoretical considerations allowing extension into a region of low COD. The method is applied to a broad spectrum of waste waters and dilutions of these, using both macro- and semi-macro techniques. Results are compared with data determined using colorimetric and color indicator end-point methods. It is shown that the potentiometric end-point titration (with FAS titrant) can be used with equal or better facility.