Determination of phosphate and arsenate by a flotation-pairing ion exchange/ extraction method

Abstract

Flotation-pairing ion exchange-extraction and spectrophotometnc determination of trace amounts of phosphate and arsenate were examined. Ion association complexes of heteropoly acids and a bulky cationic dye floated on the interface between an aqueous and a less polor organic solvent phase. After the flotation, the heteropoly acids were exchanged for an anionic dye, which have a high molar absorptivity and the similar maximum absorption wavelength to the cationic dye. The sum of the absorbances of the cationic and the anionic dyes was measured. In this work, malachite green (MG) and 4/-hydroxy-3,5,3',5',2',6'-hexachlorofuchsone (C16BZ), a hexachloro derivative of benzaurin, were used as a cationic and an anionic dye. The recommended procedure for phosphate determination is as follows. To a 50-cm3 separatory funnel, take sample solution (up to 10 cm3); if arsenate coexists, add 0.1 cm3 of 2X10-2 M Na2S2O3 to reduce arsenate to arsenite. Then, add 1 cm3 of 10 M hydrochloric acid, 1 cm3 of 0.4 M (as Mo) molybdate, 0.5 cm3 of 5X10-4 M Malachite Green, and 2 cm3 of flotation solvent (Isobutyl methyl ketone +cyclohexane = 1 +8 v/v). After shaking for 5 min, discard the aqueous phase, and wash the organic phase 2 times with 5 cm3 of water. After 1 cm3 of 2X10-4 M anionic dye solution, 8 cm3 of 1,2-dichloroethane and 5 cm3 of borax buffer (pH 9) are added, shake 10 times by hands. After the phase separation, measure the absorbance of the organic phase at 620 nm. As arsenate reacts under the same conditions as phosphate, arsenate can be determined by subtracting the amounts of phosphate from the total amounts of phosphate and arsenate which were obtained in the absence of Na2S203. The apparent molar absorptivity of phosphate or arsenate was 5.7X105 dm3 mol -1 cm -1, and the absorbance of the reagent blank was 0.005. Six times concentration was achieved when 30 cm3 of aqueous phase and 5 cm3 of extraction solvent were used. Bulky anionic ions, such as iodide, perchlorate and lauryl sulfate, did not interfere with the determination of phosphate and arsenate. This method was applied to the determination of phosphate in seawater and iron and steel samples. © 1993, The Japan Society for Analytical Chemistry. All rights reserved.