Spectrophotometric determination of phosphate using sodium molybdate and its field application to the simultaneous measurement with ammonium in seawater.

Phosphate (PO) and ammonium (NH), two major components of the phosphorus and nitrogen cycles, respectively, play vital roles in aquatic ecosystems. Very few studies have reported methods for the simultaneous determination of PO and NH in seawater. Additionally, when ammonium molybdate solution was employed as a chromogenic reagent for the simultaneous determination of both PO and NH in a single analysis system, there was a possibility of cross-contamination. In this study, a spectrophotometric method for the simultaneous determination of PO and NH in seawater was established. To prevent interference from ammonium molybdate, sodium molybdate was used in the phosphomolybdenum blue reaction, yielding satisfactory results. The indophenol blue method was used for the determination of NH. The reagent concentrations for the determination of PO and NH were optimized, and the effect of salinity on the determination was investigated. The effect of salinity can be effectively ignored by setting the residence time for the reaction. The developed method was validated, with limit of detection observed to be 0.02 and 0.06 μM for PO and NH, respectively. The working range of the method was observed to be 15 μM for PO and 12 μM for NH, with the relative standard deviations (1 μM, n = 20) of 2.5 % and 1.9 %. The results obtained using the proposed method were in agreement with the reference method. Furthermore, the proposed method was successfully applied for the on-site monitoring of diurnal variation of PO and NH in surface seawater.