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

Accurate quantification of nitrate (NO_3-) in tropical drinking water sources is routinely compromised by significant interference from natural organic matter. This study evaluated three regulatory standard methods (APHA 4500-NO_3- B, E, and J) against an optimised homogeneous vanadium (III) chloride (VCl₃) reduction assay. Optimal VCl3 operational parameters were established at 1.0 M HCl, 60°C incubation, and a 45 min reaction time. Results revealed major disparities in matrix tolerance across evaluated pathways. Direct UV Spectrophotometry (Method B) failed in samples with dissolved organic carbon levels exceeding 3.0 mg C/L due to non-linear spectral overlap. Enzymatic reduction (Method J) suffered severe humic acid inhibition, dropping recoveries to 54.1±4.2 % at 20.0 mg C/L DOC. Conversely, chemical reduction pathways (Cd and VCl3) maintained high selectivity and matrix tolerance (98.5%-101.2% recovery) by shifting spectral detection to = 540 nm. The optimised VCl3 method achieved a limit of quantification (LOQ of 0.010 mg N-NO3-/L, intra-day repeatability of 1.0%-2.2% RSD, and superior multi-analyst reproducibility (2.2%-3.8% RSD) compared to cadmium column reduction (4.5-6.2% RSD). Operating as a single-phase liquid system, the VCl₃ protocol eliminates packed-bed column maintenance, flow-rate dependence, and toxic cadmium waste generation, providing a robust, cost-effective, high-throughput solution for tropical water testing laboratories.