Automated total oxidized nitrogen method using vanadium as reductant with correlation to cadmium and hydrazine reductant methods in sea, natural, and waste waters

Importance of the topic

The accurate determination of total oxidized nitrogen (TON; nitrate + nitrite) is central to environmental monitoring, wastewater management, drinking water quality control and marine nutrient studies. Robust TON methods that are compatible with a wide range of sample matrices — including seawater with high salinity — and that minimize hazardous waste (e.g., cadmium-containing columns) are desirable for routine laboratory operations and regulatory compliance.

Objectives and study overview

This application study evaluated an automated discrete-analysis TON method based on vanadium(III) chloride reduction followed by the Griess colorimetric reaction, and compared its performance to two established reduction approaches: cadmium (Cd) column reduction (FOSS FIAstar 5000) and hydrazine reduction (Thermo Scientific Aquakem 250). The comparison covered multiple water types (waste, natural, low-concentration natural, brackish, household, swimming pool and high-salinity ocean samples) and assessed linearity, correlation, recovery, reduction efficiency and operational suitability for routine discrete analyzers.

Methodology

Principle:

• Nitrate in the sample is reduced with vanadium chloride to nitrite at ambient or mildly elevated temperature, then reacted with Griess reagents (sulphanilamide and N-(1-naphthyl)ethylenediamine) to form a pink azo dye measured at 540 nm.

Study design and samples:

• Sample types: 93 brackish samples (2.2–157 μg/L), 128 natural water samples (two subsets: 24 at 110–4100 μg/L and 104 at 0.5–320 μg/L), 11 household (0.022–4.11 mg/L), 20 waste water (1.2–13.7 mg/L), 2 swimming pool samples (~1.5 mg/L) and 4 ocean seawater samples used for spike/recovery tests.
• Controls and calibration: multi-point calibrations using nitrate standards; routine water-based QC controls and a certified reference (VKI RW1, 100 μg/L) were analyzed.
• Comparisons: Vanadium method results (typically duplicate measurements) were correlated to single or replicate results from hydrazine or Cd-reduction methods depending on sample type.

Used instrumentation

• Thermo Scientific Aquakem 250 discrete analyzer — main platform for vanadium and hydrazine applications.
• Thermo Scientific Gallery discrete analyzer — used for seawater application testing.
• FOSS Tecator FIAstar 5000 system with prepacked Cd reduction column — used for reference Cd-reduction analyses.

Results and discussion

Linearity and calibration:

• Calibration curves for vanadium applications displayed excellent linearity across tested ranges (R2 typically ≥0.998 up to 1.000), supporting quantitative use over both low μg/L and mg/L ranges.

Correlation with reference methods:

• Brackish water: TON Vanadium versus TON Cd — slope ≈1.012, R2 = 0.998; recoveries 62–122% across 2.2–157 μg/L.
• Natural water (high range 110–4100 μg/L): Vanadium versus Hydrazine — slope ≈0.973, R2 = 0.994; recoveries 71–115%.
• Low-concentration natural water: Vanadium versus Hydrazine and Cd — slopes ≈0.978 and 1.009, R2 ≈0.974–0.993; recoveries generally within ~73–121% for comparisons to Cd and 89–115% to Hydrazine.
• Household water (0.022–4.11 mg/L): Vanadium versus Hydrazine — slope ~0.987, R2 ≈1.000; recoveries 98–141% (noting elevated recoveries for a few very-low-level samples where a high-volume application is recommended).
• Waste water (1.2–13.7 mg/L): Vanadium versus Hydrazine — slope ~0.985, R2 = 0.996; recoveries 94–118%.

Seawater spike/recovery and reduction efficiency:

• Four ocean samples spiked with 0, 40, 80 μg/L NO3-N or NO2-N returned spike recoveries of ~92–103% using the vanadium seawater application, demonstrating robust performance in high-salinity matrices.
• Reduction efficiency assessed with 100 μg/L NO3-N/NO2-N standards indicated ~105% (consistent conversion and measurement performance).

Operational observations:

• The vanadium method performed well across diverse matrices and concentration ranges using discrete analyzers with minimized reagent consumption compared with flow-based Cd column systems.
• Unlike hydrazine reduction, the vanadium approach is compatible with seawater. Compared to Cd-column methods, vanadium avoids handling and disposal of cadmium and eliminates column fouling issues caused by oily samples.
• For very low TON samples, method variants with higher sample volume (NO32-VV200-type application) improved recovery and sensitivity.

Benefits and practical applications

• Multipurpose method: single reagent system adaptable to fresh, brackish and marine waters as well as wastewater and drinking water matrices.
• Safety and waste: eliminates use of a cadmium column and associated toxic waste stream; uses fresh reagent additions in discrete analysis.
• Compatibility with automation: directly implementable on commercial discrete analyzers (Aquakem, Gallery) enabling high-throughput routine monitoring and QA/QC workflows.
• Analytical performance: demonstrated high linearity, strong correlations to reference methods and acceptable recoveries across realistic concentration ranges.

Future trends and potential uses

• Wider adoption as a standard laboratory method for TON where seawater compatibility and reduced hazardous waste are priorities; potential for national method acceptance and inclusion in standards.
• Integration with online or semi-continuous monitoring platforms using miniaturized reagent delivery and optical detection to support coastal and wastewater real-time surveillance.
• Further method harmonization with enzymatic and chemiluminescent approaches to expand dynamic range and selectivity where required.
• Optimization of low-level applications (higher sample volumes, improved blanks) to reduce occasional elevated recovery at trace concentrations.

Conclusion

The vanadium(III) chloride reduction followed by Griess colorimetry, implemented on discrete analyzers, is a robust, versatile and safer alternative to hydrazine and cadmium reduction methods for TON analysis. It provides excellent linearity and close agreement with established methods across multiple matrices, including high-salinity seawater, while reducing toxic waste and operational limitations associated with Cd columns. The method is well suited for routine environmental, municipal and industrial water laboratories using automated discrete photometry.

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