Continuous or Accumulation Determination of Nitrite and Nitrate in Seawater by Using Second-derivative Spectrophotometer for Gas Phase

Abstract

A conversion of nitrate to nitrite, reduction of nitrite to nitrogen monoxide, gas-liquid separation and a detection of nitrogen monoxide with second-derivative spectrophotmeter are used for continuous or accumulation determination of nitrite and nitrate in seawater. The sample is pumped at 4 ml/min through a converter containing cadmium-copper powder which converts nitrate to nitrite. The effluent is mixed with reducing agent (1 ml/min)(0.13 M sodium iodide-13 M phosphoric acid), which reduces nitrite to nitrogen monoxide at 40-50°C, and the mixture is fed through a gas-liquid separator (pore size 1 gm, microporous Teflon tube, 2 mm o. d., 1 mm i. d.50 cm length). Nitrogen monoxide evolved from a gas-liquid separato r is determined by either continuous system or accumulation system. In the continuous system the nitrogen monoxide evolved is purged by nitrogen (150 ml/min) into a heated optical cell (light pass length 25 cm, cell volume 80 ml, 150°C), where the second-derivative absorbance at 214.0 nm is recorded. Nitrite is determined directly while nitrate is determined indirectly and the sample throughput is 5/h. The method has detection limits of 1×10^-6 M nitrite and nitrate and has been applied to chlorine water. In the accumulation system nitrogen monoxide accumulated (20min feeding time) in the gas layer of separator is introduced by nitrogen into the evacuated optical cell of which operation conditions are same as that of continuous system. The method has detection limits of 2×10^-8 M of nitrite and nitrate and has been applied to seawater. The sample throughput is 2/h. The conversion efficiency of nitrate to nitrite with cadmium-copper column and the reactivation were evaluated by accumulation system.