Solvent Extraction-Molybdenum Blue Photometric Determination of Phosphorus in Molybdenum Products

Sample of metal molybdenum product was dissolved with HNO3-H2SO4 mixed acid, while sample of salt or oxide of molybdenum (i.e., molybdates, oxides and etc.,) was dissolved with aqua ammonia. In 0.5 mol·L-1 H2SO4 medium, phosphorus in the sample solution was oxidized to orthophosphate by H2O2, and upon addition of ammonium molybdate, phospho-molybdic acid was formed, which was extracted into organic phase with mixed solvent of n-butanol and CHCl3(1+3) and the organic phase was separated and made up to 25 mL with n-butanol. One drop of 100 g·L-1 SnCl2 solution was added to reduce the phospho-molybdic acid to molybdenum blue, and its absorbance was measured at the wavelength of 620 nm. Standard curve was prepared by using standard phosphorus solution and the linearity range within 0.48 mg·L-1 was used in this method. Samples of molybdenum powder and molybdenum oxide were analyzed by the proposed method, test for recovery was made by standard addition method, giving values of recovery in the range of 95.0%-103%, and RSD′s (n=11) in the range of 5.8%-11%.