Spectrophotometric determination of nitrate in small volume of seawater samples using a simple resorcinol method

A simple resorcinol method to determine nitrate (NO₃^-) in seawater using a microplate reader with a 48-well plate was established. The method involved the nitration of resorcinol in sulfuric acid to form a pink product that was detected at 505 nm. Reagent concentrations were optimized, and the effect of salinity on NO₃^- determination was investigated. The detection limit of this method was 0.8 µM, while the upper limit of the linear range was 100 µM. The recoveries ranged from 91.5 to 109.7% for spiked seawater samples with different salinities. The proposed method was compared with two reference methods, and the results revealed a good correlation. Compared to conventional methods that require the preparation of reactants, the proposed method used aqueous solutions as reagents for the reaction, which was simpler and more convenient. Compared to the methods that used organic solvents for the direct determination of NO₃^-, the proposed method was suitable for estuarine and coastal water samples with large salinity variations. All results indicated that the proposed method can satisfy the requirements of laboratory analysis and demonstrate high application potential for use in field determination.