A simple solution to the problem of selective detection of phosphate and arsenate by the molybdenum blue method.

We report a simple and fast method for the quantification of both phosphate and arsenate in water using the molybdenum blue method. The method does not require established pre-treatment procedures for physically separating arsenate and phosphate or reducing the arsenate to arsenite. In our method, the heteropolymolybdate ions in the solution are precipitated and collected on a membrane that is transparent in both the visible and infrared regions of the spectrum. The phosphate is determined by recording a visible spectrum in transmission mode through the membrane. This membrane is then air-dried, and then an infrared spectrum is recorded through the membrane in transmission mode. The concentration of arsenate is then determined from the intensity of an As-O band positioned at 879 cm in the infrared spectrum. Using this method, a detection limit of 0.86 μg L phosphate and 13.9 μg L arsenate in water was achieved. Matrix spikes on environmental samples gave a 108% recovery of arsenate and a 105% recovery of phosphate over a dynamic range of 25-250 μg L of arsenate and phosphate in the sample.