Removal of nitrite interference in the Winkler determination of dissolved oxygen in seawater

Nitrite is a well recognized interference in iodometry. However, while steps for its removal are part of the routine practice in the standard methods for the determination of dissolved oxygen in freshwater and wastewater when the iodometric Winkler method is used, they are not included in any of the widely used standard procedures for seawater analyses. In de-ionized reagent-grade water, artificial seawater and natural surface seawater, the concentration of apparent oxygen increased linearly with increasing concentrations of added nitrite, indicating that the presence of nitrite does lead to an over-estimation. The average ratio of increase in apparent oxygen to nitrite added was 0.4 ± 0.1 mol mol− 1. At the widely claimed accuracy and precision in the determination of dissolved oxygen in seawater of ± 0.1 to ± 0.3%, the interference of nitrite may be safely ignored only when the concentration of nitrite is below about 0.3 μM. At higher concentrations, such as those found at the nitrite maxima and in coastal waters, it should be taken into account. Since nitrite can be easily and readily removed by the addition of azide, such a step should be incorporated into the standard procedures for the determination of dissolved oxygen in seawater.

► Nitrite interferes in the Winkler determination of dissolved oxygen in seawater.
► Each mole of nitrite results in an apparent oxygen of 0.4 ± 0.1 mol.
► Nitrite interference may be safely ignored only when its concentration is < 0.3 μM.
► The interference of nitrite can be removed by the addition of azide