A solventless method for detecting trace level phosphate and arsenate in water using a transparent membrane and visible spectroscopy

The molybdenum blue method is the American Public Health Association (APHA) approved method for the detection and quantification of phosphate in water. The standard molybdenum blue method, APHA 4500 PE has a detection limit of 30 mu gL(-1) phosphate (10 mu gL(-1) phosphorus) in freshwater with a 5 cm cuvette. To further lower the detection limit to sub mu gL(-1) levels, we have developed a simple, fast, and solventless method for conversion of phosphate present in solution to a solid for quantification by Visible spectroscopy. The process converts the anionic heteropolymolybdate ions into a solid colloidal precipitate by charge neutralization with the cationic surfactant cetyltrimethylammonium bromide (CTAB), and the precipitate is then captured on a Visible transparent membrane. A Visible spectrum is then recorded in transmission mode through the membrane and the concentration of the phosphate is determined from the intensity of a band cantered at 700 nm. Using this method, the detection limit for phosphate in water is lowered to 0.64 mu gL(-1). The approach has also been extended to detect arsenate in water with a detection limit of 4.8 mu gL(-1) arsenate.. The method is also used to investigate real matrices with accuracy that matches the standard APHA method for detection of phosphate in water.