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 molybdenumblue 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.
Diaz, J.M. and E.D. Ingall (2010). Fluorometric quantification of natural inorganic polyphosphate. Environmental Science and Technology, 44(12): 4665-4671.
Edmunds, W.M. and P.L. Smedley (2013). Fluoride in natural waters. In: Essentials of Medical Geology (pp. 311-336).
Estela, J.M. and V. Cerdà (2005). Flow analysis techniques for phosphorus: An overview. Talanta, 66(2 SPEC. ISS.), pp. 307-331.
Filippelli, G.M. and W.M. Street (2002). The global phosphorus cycle. Reviews in Mineralogy and Geochemistry, 48(1): 391-425.
Galceran, M.T. (2005). Ion chromatographic separations of phosphorus species: A review. Talanta, 66: 376-410.
Galhardo, C.X. and J.C. Masini (2000). Spectrophotometric determination of phosphate and silicate by sequential injection using molybdenum blue chemistry. Analytica Chimica Acta, 417(2): 191-20.
Haberer, J.L. and J.A. Brandes (2003). A high sensitivity, low volume HPLC method to determine soluble reactive phosphate in freshwater and saltwater. Marine Chemistry, 82: 185-196.
Iler, R.K. (1979). The chemistry of silica: solubility, polymerization, colloid and surface properties, and biochemistry. John Wiley & Sons Inc.
Murphy, J. and J.P. Riley (1962). A modified single solution method for the determination of phosphate in natural waters. Analytica Chimica Acta, 27: 31-36.
Nagul, E.A., McKelvie, I.D., Worsfold, P. and S.D. Kolev (2015). The molybdenum blue reaction for the determination of orthophosphate revisited: Opening the black box. Analytica Chimica Acta, 890: 60-82.
NWIS (2017). The National Water Information System: Web Interface. National Water Quality Monitoring Council, USA. https://www.waterqualitydata.us/
Ringuet, S., Sassano, L. and Z.I. Johnson (2011). A suite of microplate reader-based colorimetric methods to quantify ammonium, nitrate, orthophosphate and silicate concentrations for aquatic nutrient monitoring. Journal of Environmental Monitoring, 13(2): 370-376.
Sabur, M.A. (2019). Interactions of Phosphate and Silicate with Iron oxides in Freshwater Environments. University of Waterloo.
Schindler, D.W., Hecky, R.E., Findlay, D.L., Stainton, M.P., Parker, B.R., Paterson, M.J., Beaty, K.G., Lyng, M. and S.E.M. Kasian (2008). Eutrophication of lakes cannot be controlled by reducing nitrogen input: Results of a 37-year whole-ecosystem experiment. Proceedings of the National Academy of Sciences of the United States of America, 105(32): 11254-11258.
Struyf, E., Smis, A., Van Damme, S., Meire, P. and D.J. Conley (2009). The global biogeochemical silicon cycle. Silicon, 1(4): 207-213.
Winkler, D., Banke, S. and P. Kurz (2020). Fluorimetric detection of phosphates in water using a disassembly approach: A Comparison of Fe III-, Zn II-, Mn II- and Mn III-salen Complexes. Journal of Inorganic and General Chemistry, Scheme, 1: 933-939.
Woods, J. and M. Mellon (1941). Molybdenum blue reaction: A spectrophotometric study. Industrial & Engineering Chemistry Analytical Edition, 13(11): 760-764.
Worsfold, P., McKelvie, I. and P. Monbet (2016). Determination of phosphorus in natural waters: A historical review. Analytica Chimica Acta, 918: 8-20.
Xie, C., Xu, J., Tang, J., Baig, S.A. and X. Xu (2013). Comparison of phosphorus determination methods by ion chromatography and molybdenum blue methods. Communications in Soil Science and Plant Analysis, 44(17): 2535-2545.
Yang, J., Bai, J., Liu, M., Chen, Y., Wang, S. and Q. Yang (2018). Determination of phosphorus in soil by ICP-OES using an improved standard addition method. Journal of Analytical Methods in Chemistry, 2018(4): 1-8.
Zhang, J.Z., Fischer, C.J. and P.B. Ortner (1999). Optimization of performance and minimization of silicate interference in continuous flow phosphate analysis. Talanta, 49(2): 293-304.
Zhu, X. and J. Ma (2020). Recent advances in the determination of phosphate in environmental water samples: Insights from practical perspectives. TRAC Trends in Analytical Chemistry, 127: 115908.