Study on the solid phase extraction of Co(II)-QADEAB chelate with C 18 disk and its application to the determination of trace cobalt

Abstract

A sensitive, selective and rapid method has been developed for the determination μg l −1 level of cobalt based on the rapid reaction of cobalt(II) with 2-(2-quinolylazo)-5-diethylaminobenzoic acid (QADEAB) and the solid phase extraction (SPE) of the colored chelate with Waters Porapak® Sep-Park C 18 disk. The QADEAB can react with Co(II) in the presence of pH 3.8 acetic acid–sodium acetate buffer solution and cetyl trimethylammonium bromide (CTMAB) medium to form a violet chelate of a molar ratio 1:2 (cobalt to QADEAB). This chelate can retained on Waters Porapak® Sep-Park C 18 disk quantitatively when they passed the disk as aqueous solution. After the enrichment finished, the retained chelate can be eluted from disk by 2.5 ml of ethanol (contain 5% acetic acid). In the measured solution, the molar absorptivity of the chelate is 1.58×10 5 l mol −1 cm −1at 635 nm, and Beer's law is obeyed in the range of 0.01–0.4 μg ml −1. The relative standard deviation for 11 replicate sample of 0.01 μg ml −1 level is 2.23%. The detection limit is 0.01 μg l −1 (in original samples). This method can be applied to the determination of μg l −1 level of cobalt in drinking water with satisfactory results.