Graphite Furnace Atomic Absorption Spectrometric Determination of Ultra-Trace Levels of Cobalt after Solvent Extraction Using W/O/W Emulsions as Liquid Surfactant Membranes

Abstract

A high-enrichment method by liquid-liquid extraction using liquid surfactant membranes was developed for the determination of cobalt at pg cm^-3 levels by graphite furnace atomic absorption spectrometry (GF-AAS). Two cubic centimeters of w/o [water in oil, in this case dilute hydrochloric acid in kerosene (1:1) containing 2-ethylhexyl hydrogen 2-ethylhexylphosphonate (PC-88A)] emulsion droplets coated with sorbitan monooleate are dispersed in a resulting volume of 1000cm³ of an aqueous sample solution (pH 4.7-5.8) containing less than 1mg of cobalt(II). The analyte is extracted into the organic phase to form a complex with PC-88A and successively back-extracted into the inner acidic aqueous phase. After the w/o emulsion phase is separated from the outer aqueous phase, the emulsions are demulsified into the organic and inner aqueous phases. The cobalt content is then determined by GF-AAS with an injection volume of 10mm³ of the inner aqueous phase. By this procedure, a concentration factor of 550 was achieved for cobalt(II). As a result, the detection limit of cobalt was 10pg cm^-3, and the calibration graph was linear up to 1mg cm^-3 of cobalt(II). The relative standard deviation for ten replicate measurements of 50pg cm^-3 cobalt(II) was 7.1%. The results of an analysis of simulated nuclear reactor coolant water are given.