Determination of Nitrate and Nitrite in Aqueous Sample Using Vanadium(III) Chloride as a Reductant

Nitrate and nitrite are active nitrogen species involved in the nitrogen cycle, which is one of the most complex and important biogeochemical cycles in the marine environment. Excessive nitrate and nitrite are dangerous to both human and aquatic organisms. Therefore, sensitive, accurate and convenient methods for determining nitrate and nitrite are highly significant and demanding. The most popular method is the spectrophotometry based on the reduction-Griess reaction. In this study, VCl₃ was used as a reductant instead of the well-known but toxic cadmium column because of its easier operation, lower cost and less toxicity. Both manual and automated methods were established. (1) The manual method was aim to measure nitrate plus nitrite using simple equipment in the routine analysis laboratories. The kinetics of the V(III)-Griess reaction under different reagent concentrations, temperatures and salinities were optimized. Under the optimized conditions, the quantitative analysis of freshwater samples could be completed in 150 s and saline samples took 11 min without salinity interference. The method showed high reproducibility (relative standard deviations of 0.96-1.14%, n=11, with different operators), linear calibration up to 20 μM (R²=0.9991, n=25, 5 days) and a detection limit of 0.15 μM. (2) Based on the manual optimization, an integrated syringe-pump-based environmental-water analyzer (iSEA) was applied for simultaneous analysis of nitrate and nitrite. The iSEA was equipped with a heating device and 1-cm and 3-cm Z shape flow cells. This fully automated analyzer had a detection limit of 0.14 μM for nitrate and 0.02 μM for nitrite. With automated dilution, linear calibration for nitrate was up to 400 μM (R² > 0.999). The relative standard deviation of 24-h measurement (n=288) of nitrate was 0.92% and that of nitrite was 1.4%. The sample throughput was 12 h^-1. These two methods were successfully applied to measure nitrate plus nitrite or nitrate and nitrite in samples and the reference solutions of different salinities (n=112). The results showed insignificant differences with reference methods. After several cruises tests, the analyzer combined with an on-line filtration system showed excellent spatial resolution for underway analysis of nitrate and nitrite in estuarine and coastal waters.