The molybdovanadophcsphoric acid method for the spectrophotometric determination of phosphorus has been extensively reviewed. The optimum concentrations of acid, vanadium (V), and molybdenum (VI) were determined by factorial experiment. The optimum color development occurs in solutions which are O.I4M in acid, 0.02 to 0.06M molybdenum (VI) and 1.0 to 4.OmM vanadium (V). The optimum range is 3 to 20 ppm P2O5 for 1cm cells. Dichromate is the only serious interference, but can be eliminated by volatilization of the chromium as chromyl chloride.
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