Influence Of Salinity On Chemical Oxygen Demand Measurement And A Primary Correction

Chemical oxygen demand (COD) is a commonly used indicator during marine water quality monitoring. COD is commonly used to evaluate comprehensive organic pollution of marine water, as well as in investigations of marine ecological environment quality assessment and marine eutrophication. However, in low-salinity areas of estuaries and offshore waters, measurements of COD and the alkaline permanganate index (CODMn) are both affected by chloride ions (Cl-); therefore, it cannot accurately reflect organic pollution level in such areas and measured COD values do not correspond to those of alkaline CODmh-This has resulted in a lack of data for contrastive analysis of terrestrial pollutant input and present marine pollutant quantity, as well as distorted evaluations of the environmental self-purification capacity of estuaries and offshore waters. This also explains why it is difficult to evaluate organic pollution in offshore waters comprehensively and accurately and to develop pollution control measures aimed at diverse organic pollution sources.This paper analyzed the influence of salinity on COD measurement by investigating the oxidation efficiency of the COD measuring method at diverse salinities. The oxidation efficiencies of COD and alkaline CODMn measuring methods were compared at different salinities. The relationship between the measured values of COD in marine water and those of alkaline CODMn in fresh water was revealed, and the influence of Cl- was quantified. We also proposed a formula for correcting measured COD values that generated corrected values comparable to measured alkaline CODMn values.The main conclusions are as follows:(1) The COD measuring method was stable at salinities of 25 to 35, but the measured COD values were low at salinities below 25. The organics concentration also affected COD measurement, and its influence was more obvious at low salinities. Additionally, the oxidation efficiency declined as the Cl- concentration decreased at salinities of 0 to 25. Cl- only influenced the process of alkaline KMnO4 oxidization of reducing substances, and had no effect on the subsequent titration during measurement of COD.(2) The measured value of the alkaline CODMn remained basically unchanged at salinities of 0 to 5, indicating that the alkaline CODMn measuring method was stable and not influenced by Cl" at these salinities. The measured value of alkaline CODMn increased significantly as salinity rose from 5 to 35. The oxidation efficiency of the alkaline CODMn measuring method was generally higher than that of COD for the organics concentrations evaluated in this paper. The oxidation efficiency of the alkaline CODMn measuring method at salinities of 0 to 5 was close to that of COD at 25 to 35 when glucose was set as the substrate.(3) A primary correction for COD values measured in low-salinity areas of estuaries and offshore waters were proposed. The corrected COD values corresponded directly with values determined within a salinity range of 25 to 35, as well as those obtained for alkaline CODMn values of 0 to 5.