Modification and validation of an analytical method for the simple determination of nitrate in seawater by reduction to nitrite with zinc powder

As copper-coated cadmium granules, which are recommended for use in the standard method for analyzing nitrate, are environmentally unfriendly and forbidden in some countries, an alternative method is required. To enhance the method for determining low nitrate concentration in seawater, the optimal conditions for reducing nitrate to nitrite using zinc powder were assessed in this study. The modified zinc method was optimal at an 0.8:1 (v/v) ratio of buffer solution (pH 8.5) (4 ml) to water sample (5 ml) with 50 mg of zinc powder and a standing time of 30 min. The nitrite was reacted with sulfanilamide and N-(1-Naphthyl)-ethylenediamine dihydrochloride after nitrate reduction, and spectrophotometrically quantified at 543 nm. This modified method was found to have a linear dynamic range of 0.1–7.0 μM, a low detection limit (0.1 μM), good intra-day and inter-day reproducibility (<5% relative standard deviation), high-efficiency reduction (>95%), and excellent accuracy in analysis of certified reference material (95% relative accuracy). The results obtained using the modified method agreed well with those obtained using the conventional cadmium column method. Overall, this alternative green chemistry method is a simplified procedure for determining nitrate in seawater with a high efficiency.