Influence of Nitrite on Chemical Oxygen Demand Determined by Potassium Permanganate at 100^oC and on Dissolved Oxygen Dtermination by Azide Modification

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A marked elevation in COD of the water treated by activated sludge was observed, which suggested the existence of nitrite-N in high concentration. In such a case, it was impossible to determine DO concentration with normal addition of alkali-iodide-azide reagent. In order to find an alternative method which could avoid this difficulty, the effect of nitrite on COD value determined by potassium permanganate at 100℃ and on dissolved oxygen determination by azide modification was investigated. Chemical oxygen demand of 1mg/l nitrite-N solution was 1.1mg/l that was equal to theoretical oxygen demand for oxidation of nitrite-N to nitrate-N. The above fact suggests that the increase of COD at the presence of nitrite-N has to be taken into consideration in determining COD. With an addition of only 1mι of alkali-iodide-azide reagent, the part exceeding over 2.15mg of nitrite-N (about 15 to 20mg/l nitrite-N) were found still liberating iodine continuously and preventing analysis of DO value. It is then found necessary to analyze concentration of nitrite-N before DO determination and to add adequate amount of alkali-iodide-azide reagent, if samples contain more than 15 to 20mg/l nitrite-N.
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