Spectrophotometric determination of ammonia nitrogen in water by flow injection analysis based on NH3- o-phthalaldehyde -Na2SO3 reaction

The product of the NH₃- o-phthalaldehyde (OPA) -Na₂SO₃ reaction is rose red at pH more than 10.4, and its maximum absorption wavelength is 550 nm. Based on this, a novel spectrophotometric method with flow injection analysis has been established to determine ammonia nitrogen in water. Experimental parameters related to the flow injection method and the reaction were optimized throughout the experiments based on univariate experimental design. The length of coil is optimized as 1.6 m for storing up standard or sample solution. The optimal value is 8.20 mL/min for loading flow rate of coloring solution. The OPA concentration, sulfite concentration and reaction pH are chosen as 1.06 g/L, 0.050 g/L and 10.80, respectively. The reaction temperature and stop flow time affects the performances of the method. The linearity range and detection limit of the proposed method are 0.100–0.700 mmol/L and 0.007 mmol/L at the reaction temperature of 55 °C and stop flow time of 340 s, respectively. The sample throughput is more than 8 h⁻¹. Under the optimal experimental conditions, the recovery is 100.4%, 95.2%, 101.7% and 92.4% for the lake water, river water, groundwater and sewage, respectively. Two lake water samples were analyzed using both the proposed method and indophenol blue method, the results show no significant difference between these two methods. In comparison with other spectrophotometric determination method of ammonia nitrogen, the main merits of the proposed method are simplicity, reliability, reproducibility and high sample throughput.