Detection of phosphate in the presence of arsenate

Ramesh Chandra D. A. V. College Jullundur, Punjab, India

I

Detection of Phosphate in the Presence of Arsenate

There are t ~ procedures't2 o for the detection of phosphate in the presence of arsenate. In the first,' both anions are precipitated aith magnesia mixture, arsenate reduced to arsenit,e by SO2 in acidic medium, and the arsenite eliminated by precipitation as As&. Phosphate is then detected in the centrifugate with ammonium molybdate. The procedure is laborious. In the s e c ~ n d ,ammonium ~ molybdatetartaric acid reagent is employed, as tartaric acid prevents the interference due to arsenate. This procedure is rapid. The procedure here described is based on the observation that magnesium ammonium arsenate dissolved in dilute HC1 gives a brown precipitate in the cold on the addition of sodium dithionite, and the centrifugate does not give a positive test for arsenate with ammonium molybdate and nitric acid, even on boiling for minutes. The brown . vrecivitate is tritarsenic sulfide, A s ~ S . ~ , Since ' phosphate remains unaffected, it can be detected with ammonium molybdate after the elimination of arsenat,e. ~ V O G E LA. , I . "A Text-Book of Macro and Seminlicro Qualitative Inoreanic Analvsis." 4th ed.. Lonemans. Qrrm and Go. Ltd., ond dog 1954, p. 479. Ibid., pp. 388,389.

408

/

Journal of Chemical Education

Procedure

Precipitate the two anions with magnesia mixture, folloming the directions given.' Detect amenate by the usual tests, and phosphate by the procedure described. Suspend 20 mg of the ppt in 0.5 ml of water and add one or two drops of 5 N HCI to dissolve the ppt. Add just one drop more, fallowed by 25-50 mg of sodium dithionite. Wait one minute, and centrifuge. Reject the residue and transfer the centrifugate into a semimicro boiling tube. Add 1 ml of eancentrate HNO*. Sulfur gets precipitated. Boil till sulfur in oxidized and the liquid becomes clear. Finally add 1 ml of ammonium molybdate m d heat to boiling. Yellow ppt or color shows the presence of phosphate. a M m ~ o a ,J. W., "A Comprehensive Treatise on Inorganic and Theoretical Chemistry," Longmans, Green and Co. Ltd., London, 1946, Vol. 10, p. 173. Ibid.. pp. 183, 184.