Salinity-Interference-Free, Automated, Spectrophotometric Vanadium Reduction Method for the Determination of Nitrite and Nitrate in Estuarine and Coastal Waters

Nitrite (NO2−) and nitrate (NO3−) are critical intermediates in aquatic nitrogen cycling, serving as key indicators for water quality assessment and biogeochemical processes. Although the copper-cadmium column reduction method has been widely used for seawater NO3−, its application is limited by cadmium toxicity, labor-intensive column maintenance, and technical complexity. The emerging vanadium(III) chloride (VCl3) reduction method offers a promising alternative but suffers from matrix interferences, necessitating salinity-specific calibration. An automated VCl3 reduction method is presented in this study that eliminates salinity interference through flow analysis technology with online auto-dilution capability. The method limits of detection were 0.005 μmol·L−1 (NO2−) and 0.18 μmol·L−1 (NO3−) and limits of quantification were 0.016 μmol·L−1 (NO2−) and 0.60 μmol·L−1 (NO3−) with linear dynamic ranges of 0–6 μmol·L−1 and 0–100 μmol·L−1, respectively. Spike recoveries ranged from 99.0–109.5% (NO2−) and 91.0–96.2% (NO3−) with precision <1.0% relative standard deviation (n = 12). Comparative validation against the conventional copper-cadmium column reduction method demonstrated excellent agreement (R2 > 0.99). Successful real-world applications confirmed the method’s robustness for seawater analysis, establishing its suitability for environmental monitoring applications.