Optimization of a salinity-interference-free indophenol method for the determination of ammonium in natural waters using o-phenylphenol.

The indophenol blue (IPB) method based on Berthelot's reaction is one of the most widely used methods for the determination of ammonium in natural waters. This study comprehensively optimized the kinetics of the IPB reaction under different reagent concentrations, temperature and salinity. The normally used toxic and odorous phenol was replaced by the less toxic, stable flaky crystalline compound, o-phenylphenol. With the application of nitroprusside as the catalyst, the reaction can be finished within 20min at room temperature and the formed color compound is stable for 24h. Under the optimized conditions, the method shows high reproducibility (relative standard deviations of 0.64-1.71%, n = 11), highly linear calibration up to 100μM (R = 0.9995, n = 165, 17 days) and a low detection limit of 0.2μM. This method was successfully applied to measure ammonium in estuarine and coastal surface water (n = 63). The results showed insignificant differences with the results obtained using both the standard AutoAnalyzer method and a fluorometric o-phthaldialdehyde method at the 95% confidence level. Compared with previous studies, this method shows the advantages of relatively fast reaction, low toxicity and easy reagent preparation. It is salinity-interference-free and robust (no temperature control is required, reagents can be stored up to 10 days), and suitable for routine analysis under harsh field conditions.