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

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.