Flow-injection photometric determination of sub-nanogram amounts of cobalt by its catalysis of the oxidative coupling of 3-methyl-2-benzothiazolinone hydrazone with N,N-dimethylaniline in a micellar medium

Abstract

A catalytic flow-injection photometric method was developed for the determination of levels of cobalt as low as 10 −10 mol 1 −1. The method is based on the catalytic action of cobalt(II) on the oxidative coupling of 3-methyl-2-benzothiazolinone hydrazone with N,N-dimethylaniline to form a colored dye ( λ max = 590 nm) in the presence of hydrogen peroxide. The extremely activating effect was obtained in the presence of 1,2-dihydroxybenzene-3,5-disulfonate (Tiron) and sodium hydrogencarbonate as activators. Furthermore, the sensitivity of the method was enhanced by adding sodium dodecylsulfate as a surfactant. Linear calibration graphs were obtained over the range 0.04–0.2 ng ml −1 and 0.1–0.5 ng ml −1 cobalt(II). respectively, at a sampling rate of 30 per hour. The detection limit (signal/noise = 3) was 5 pg ml −1 and the relative standard deviations for 0.1 and 0.04 ng ml −1 cobalt ( n = 10) were 1.0 and 2.3%, respectively. The method was applied to the determination of cobalt in pepperbush.