An automated spectrophotometric method for the direct determination of nitrite and nitrate in seawater: Nitrite removal with sulfamic acid before nitrate reduction using the vanadium reduction method

Abstract

A reverse flow injection method was established for simultaneous spectrophotometric determination of NO2− and NO3− in seawater. Vanadium chloride, a nontoxic and soluble reductant, was used to replace highly toxic cadmium for the reduction of NO3− to NO2−. Sulfamic acid was used in the automated spectrophotometric method, to remove NO2− interference before NO3− reduction was performed. The concentrations of NO2− and NO3− were directly quantified using their respective calibration curves. The experimental parameters for both NO2− and NO3− determination were optimized with a univariate experimental design process. The salinity effect was investigated, indicating that measurement correction was not necessary. Method detection limits of the proposed method for NO2− and NO3− detection were 0.09 µM and 0.18 µM, with linearity up to 12 µM and 70 µM, respectively. Recovery from seawater samples varied from 95.0% to 105.0% for NO2− and 95.0% to 104.3% for NO3−. The relative standard deviations of spiked samples with different concentrations were all below 1.2% (n = 7). The accuracy of the method was validated by comparison with the copper-coated cadmium column reduction method, showing that the results were satisfactory. The proposed method was then successfully applied to NO2− and NO3− concentration determination in seawater samples. The proposed method exhibited no significant interference from salinity, with easier operation and calculation methods, making it highly suitable for routine analysis and potentially for underway monitoring.