Determination of Nitrate in Seawater by Vanadium Trichloride Reduction-Spectrophotometry

To solve the problems of complex operation, time-consuming, high toxicity, and difficulty in adapting to rapid determination of large quantities of samples in the determination of nitrate in seawater by copper cadmium column reduction method, a method mentioned by the title was proposed. 5 mL of sample (seawater sample diluted with water to salinity not greater than 7) and 1 mL of reducing agent (a mixed solution of 10 g · L⁻¹ vanadium trichloride solution, 40 g · L⁻¹ sulfonamide solution, and 1.0 g · L⁻¹ aphthalene ethylenediamine hydrochloride solution at volume ratio of 5∶1∶1) were taken in 10 mL-tube, and the mixture was heated in water bath at 45 ℃ for 60 min, and then removed to cool for 5 min in ice-water mixture. The absorbance difference of the system was determined at 543 nm. It was shown that the nitrate reduction rate of this method was above 95%; and when the sample salinity was not above than 7, the influence of salt effect on determination was eliminated, and this method could be used for determination of nitrate in fresh water and seawater. Linear relationship between mass concentration of nitrate and its corresponding absorbance difference was kept within 300.0 μg · L⁻¹, with detection limit of 1.0 μg · L⁻¹. Tests for accuracy and precision were made on the 50.0 and 200.0 μg · L⁻¹ spiked sample solution, respectively, giving results that the determined values were basically consistent with the known values, with RSDs (n=10) of the determined values less than 0.50%. This method and HY/T 147.1－2013 were used to determine actual seawater samples. The results of two methods were consistent, and there was no significant difference between two methods. This method reduced the environmental pollution caused by reagents, and avoided the preparation of copper cadmium column and column passing operations, which could be used for rapid and batch determination of nitrate in seawater.