Phosphate (PO43-) and ammonium (NH4+), 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 PO43- and NH4+ in seawater. Additionally, when ammonium molybdate solution was employed as a chromogenic reagent for the simultaneous determination of both PO43- and NH4+ in a single analysis system, there was a possibility of cross-contamination. In this study, a spectrophotometric method for the simultaneous determination of PO43- and NH4+ 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 NH4+. The reagent concentrations for the determination of PO43- and NH4+ 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 PO43- and NH4+, respectively. The working range of the method was observed to be 15 μM for PO43- and 12 μM for NH4+, 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 PO43- and NH4+ in surface seawater.
Keywords: Ammonium; Field application; Phosphate; Simultaneous determination; Sodium molybdate; Spectrophotometric.
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