Interfering Effect of Silicate in the Quantification of Phosphate by the Molybdenum-blue Method

Abstract

Phosphorus (P) in aqueous solutions is often measured as reactive orthophosphate by the molybdenum-blue method. The main drawback of this method is the presence of interfering ions, most importantly silicate (SiO₄) when concurrently present with phosphate (PO₄). Here, we address how SiO₄ interferes in the determination of PO₄ with various SiO₄:PO₄ ratios. Experimental data showed that increasing the aqueous phase ratios of SiO₄:PO₄ results in the overestimation of aqueous PO₄. For example, a SiO₄:PO₄ aqueous ratio of 1500 can overestimate 10 μM PO₄ up to ~250%. However, this interfering effect of SiO₄ becomes negligible with increasing aqueous phase PO₄ concentrations with similar SiO₄:PO₄ ratios. For the higher concentrations of PO₄ (e.g., 25, 35, and 50 μM), increasing the aqueous phase of SiO₄:PO₄ ratios results in about 20% (maximum) overestimation of PO₄ concentrations. Analysis of spectral features (UV-Vis) reveals that SiO₄ also reacts with the analytical reagents and forms a molybdenum-blue complex with maximum absorbance at 812 nm beside the representative peak for PO₄ at 890 nm. Accounting for the absorbance at 890 nm for various SiO₄ concentrations in the absence of PO₄, we estimate the concentrations of 10 μM PO₄ at various SiO₄:PO₄ ratios, which are in good agreement with the measured concentrations. The feasibility of this spectrophotometric method for the quantification of aqueous PO₄ with various SiO₄:PO₄ ratios has also been addressed by considering the US National Water Information System database. Taken together, this study highlights the interfering effects of aqueous SiO₄ in the spectrophotometric quantification of PO₄ in natural waters.