Second-derivative spectrophotometric determination of nitrite and nitrate at 10–8M concentrations

Abstract

In the conversion of nitrate into nitrite and the reduction of nitrite to nitrogen monoxide, gas-liquid separation, accumulation of the nitrogen monoxide produced and second-derivative spectrophotometry are used. The sample is pumped at 4 ml min^–1 through a converter containing cadmium-copper powder, which converts nitrate into nitrite. The effluent is mixed with reducing agent (1 ml min^–1)(0.13 M sodium iodide-13 M phosphoric acid), which reduces nitrite to nitrogen monoxide, and the mixture is fed through a gas-liquid separator (pore size 1 µm, microporous PTFE tube, 90 °C). The feeding time is 20 min. Nitrogen monoxide accumulated in the separator is purged by nitrogen into an evacuated and heated optical cell (light path length 25 cm, cell volume 80 ml, 150 °C), where the second-derivative absorbance at 214.0 nm is recorded. The method has detection limits of 2 × 10^–8M of nitrite and nitrate and has been applied to seawater.