Spectrophotometric determination of traces of oxygen in water

Winkler's method of determining oxygen in water by titration of the iodine can not be applied with concentrations below 0.03 mg O₂/litre. We studied the spectrophotometric determination of the iodine-starch complex described by bairstow, francis, and wyatt and succeeded in improving its stability by the addition of potassium sulphate. The intensity of the blue colour is very sensitive to variation of temperature. Below 10° C this influence can be neglected. Upwards from 30° C the intensity rapidly decreases. Instead of performing the determination at 10° C we plotted the extinction coefficient of dilute solutions as a function of the iodine concentration for temperatures ranging from 10 to 30° C. With “the aid of this graph the iodine concentration can be found when the extinction coefficient and the temperature of a solution are determined.With this method we can determine oxygen in water in concentrations ranging from 0.005 to 0.4 mg O₂/litre. Duplicates (for water with a low content of oxygen) differ not more than 0.002 mg O2/litre.