Determination of Parts per Billion Phosphate in Natural Waters Using X-Ray Fluorescence Spectrometry

The importance of excess phosphate in environmental waters has been of much concern and discussion. The determination of orthophosphate or total phosphate converted to orthophosphate is frequently performed by environmental monitoring laboratories. A standard method for the determination of orthophosphoric acid involves the formation of 12-molybdophosphoric acid (12-MPA) in a strongly acidic solution. Reduction of the 12-MPA with ascorbic acid or stannous chloride forms the heteropolyblue complex which is measured spectrophotometrically. Because germanium, silicon, and arsenic also form heteropolyacids with molybdate anion in acidic solution, a number of techniques have been developed which allow the separation of these acids by extracting them into an organic solvent with subsequent analysis by the spectrophotometric method or atomic absorption determination of molybdenum (1-6). The extraction step not only allows separation of the 12-MPA from contaminants but also preconcentrates the analyte in the organic phase which increases the sensitivity of the analysis.