Comparative Validation of Vanadium(III) Chloride Reduction and Standard Methods for Nitrate Quantification in Organic-Rich Tropical Waters

Natural organic matter (NOM) can interfere with the determination of nitrate (NO3−) in tropical waters. This study compared three standard methods (APHA 4500-NO3− B, E, and J) with an optimized vanadium(III) chloride (VCl3) reduction method coupled with the Griess reaction. The VCl3 reaction was optimized at 1.0 M HCl in the VCl3 reagent, 60 °C, and 45 min. At dissolved organic carbon (DOC) above 3.0 mg C/L, direct ultraviolet (UV) spectrophotometry (Method B) overestimated nitrate, and its 275 nm correction exceeded the 10% rejection criterion of the method. Spike recovery with enzymatic reduction (Method J) decreased to 54.1 ± 4.2% at 20.0 mg C/L DOC, whereas cadmium (Method E) and VCl3 reduction gave 98.5 ± 2.2% and 101.2 ± 1.8%, respectively. For the VCl3 method, the limit of quantification (LOQ) was 0.010 mg N-NO3−/L (relative standard deviation, RSD, 3.8%; recovery 98.2% at this level), repeatability was 1.0–2.2% RSD, and inter-analyst intermediate precision was 2.2–3.8% RSD (cadmium reduction: 4.5–6.2%). In 12 natural water samples, the VCl3 and cadmium results differed by less than 2%. The VCl3 method avoids packed columns and cadmium waste.