Iodometric microdetermination of phosphorus in organic compounds by use of An amplification reaction

Abstract

A high chemical amplification method is described for the iodometric microdetermination of phosphorus in organic compounds. This method depends on combustion by the oxygen flask, absorption of the combustion products in distilled water and conversion to orthophosphoric acid with bromine water. The phosphoric acid produced is then allowed to react with sodium molybdate in acid medium to form phosphomolybdic acid which is extracted into a mixture of diethyl ether and pentanol (5:1, V/V), and back extracted with sodium hydroxide. The molybdate thus produced (12 atoms molybdenum (VI) for each original phosphorus atom) is reduced with zinc in acetic acid medium to the quinquevalent molybdenum, which is then oxidised with potassium periodate in the pH 3.4-3.9 range, into the hexavalent form. In the same pH range, the excess periodate is masked with ammonium molybdate and the released iodate is titrated with sodium thiosulphate. This method affords 36-fold amplification as each phosphorus atom liberates 36 equivalent of iodine.