Fractionation and spectrophotometric determination of thallium with N,N ′-diphenylbenzamidine, brilliant green and cetylpyridinium chloride

Abstract

A spectrophotometric method for the determination of inorganic chemical species of thallium i.e.Tl(I), Tl(III) and total Tl at trace level in industrial waste waters, effluents and surface water of the city and neighbouring cities is described. The method is based on the extraction of chlorothallate(III) complex in the presence of cetylpyridinium chloride (CPC) at 0.5–4.5 M HCl with N,N’-diphenylbenzamidine (DPBA) in toluene and subsequent colour reaction of the extract with brilliant green (BG). The molar absorptivity of the Tl(III)-Cl−–CP+–DPBA–BG+ complex in toluene is 1.82x105 Lmol−1 cm−1 at 640 nm. The standard deviation and relative standard deviation of the method (n=10) were found to be 0.007 ppm Tl and 1.3% of the mean absorbance, respectively. The detection limit, defined as the concentration causing more absorbance than twice the standard deviation of 10 replicate measurements of the blank solutionis 5 μg L−1 Tl in water. For effluent samples the concentration ranges of Tl(I), Tl(III) and total Tl were found to be 24.5–49.0, 30.6–36.7 and 55.1–85.7 μg L−1, respectively. While for surface waters these ranges were 39.2–108.1, 19.6–58.2 and 58.8–166.3 μg L−1 respectively. The concentration of these chemical forms of thallium for the waste water samples were found to be 97.9, 49.0 and 146.9 μg L−1, respectively. These data were validated by comparison with standard methods and the AAS method.