Effect of thiosulfate on 5-day biochemical oxygen demand

In iodometric methods the amount of sodium thiosulfate required to transform all the iodine to iodide is related to the initial DO concentration in the sample. However, if thiosulfate or any other reduced sulfur compound is initially present, it reacts with the iodine and produces a negative interference (a result lower than the actual). In this study, DO was measured by the membrane electrode technique (method 421 F), which involves an oxygen probe that responds only to the amount of oxygen in the sample, thereby eliminating any interference caused by the reaction of other compounds with iodine.