PRECONCENTRATION OF TRACE COBALT ON SOLVENT-SOLUBLE MEMBRANE-FILTER USING NITROSO-R AND ITS APPLICATION FOR GRAPHITE-FURNACE ATOMIC-ABSORPTION SPECTROMETRY

A simple and highly effective pre-concentration method for a graphite furnace atomic absorption spectrometry of trace cobalt which involves the filtration of an ion pair of cobalt Nitroso R and a quarternary am monium ion on an organic solvent soluble membrane filter followed by t he redissolution using small volume of dimethylformamide is described. The optimum pre-concentration conditions were examined. The highest r ecovery was obtained by using benzyldimethyl (tetradecyl) ammonium for counter ion. The mechanism of the collection of the ion pair on the m embrane filter was also discussed. The recommended procedure is as fol lows. To 1 dm3 of sample containing 0 approximately 1 mug of cobalt(II ), 1 cm3 of 0.02 mol.dm-3 nitroso R solution and phosphate buffer, pH 8, are added, and after 3 min, 15 cm3 benzyldimethyl (tetradecyl) ammo nium chloride solution is added. After 1 min, the solution is filtered by membrane filter (cellulose nitrate, diameter ; 47 mm, pore size ; 1 mum). The membrane filter is dissolved in 2 cm3 or 5 cm3 of DMF. 20 mm3 of the solution is transfered to the graphite furnace of AAS and c obalt is determined at lambda = 240. 7 nm. The concentration factor (C F) was achieved 200 approximately 500. When CF was 200, RSD was 2.9% ( n = 5) and the sensitivity (= 2 x SD) was 10 ppt and when CF was 500, RSD was 6.0% (n = 5) and the sensitivity was 5 ppt. An application of the method for cobalt in river water was also descsribed.