Simple Alternative Approach to the Molybdenum Blue Method for Quantification and Speciation of Phosphate and Arsenate in Freshwater

The Molybdenum Blue Method (MBM) is the standard visible spectroscopic approach for the detection of phosphate in water. A major drawback of the MBM is that arsenate also reacts with the molybdate ion to produce the same peak in the visible spectrum. We describe an alternative infrared (IR) spectroscopic-based approach to the MBM that enables both speciation and quantification of phosphate and arsenate spectroscopically from their respective bands at 960 and 785 cm^-1. In our approach, phosphate and arsenate are precipitated using excess AgNO₃, followed by passing the suspension of Ag₃PO₄ and Ag₃AsO₄ particles through an infrared transparent membrane. The membrane is dried, and an infrared spectrum is recorded in the transmission mode through the membrane. Controlling the particle size to be in the range of 200-300 nm is the key to establishing detection limits that meet the EPA requirement. Our calibration plots for phosphate and arsenate gave a slope of 0.0203 ± 0.001 and 0.0192 ± 0.0007 cm²/μg, respectively, with excellent linearity (<i>R</i>² = 0.991). Limit of detection (LOD) values, based on 10× standard deviations using the 50 ppb phosphate and 25 ppb arsenate, are 6 and 9 ppb, respectively, which are lower than the required limits set by the EPA.