Study of the complexation chemistry and speciation of thallium for on-line preconcentration with immobilised quinolin-8-ol

The effects of various buffering reagents and pH conditions were investigated by flame atomic absorption spectrometry to optimise complexation of T1³⁺ with quinolin-8-ol (8-Q), immobilised on controlled-pore glass beads in a 5-cm column. As Tl³⁺ is a softer acid than the other trivalent cations of the Group III elements, the effects of the buffers are different from those observed previously for Al³⁺, Ga³⁺ and In³⁺. A mixed buffer of 0.1 mol l^–1 acetate and 0.1 mol l^–1 ammonium chloride at pH 10 proved most successful, although 0.1 mol l^–1 maleate was also satisfactory over a pH range of 4–10. As thallium normally exists as Tl⁺ in solution, an oxidation method was developed to convert the ions to Tl³⁺, which is more efficiently complexed by 8-Q. Addition of 1–10 l of bromine per 100 ml of sample was sufficient to oxidise Tl⁺ without heating. Excess bromine was removed by addition of phenol. With a flow-rate of 6 ml min^–1, the detection limit of Tl³⁺ is 3 ng ml^–1, for a 3-miri preconcentration time. The enrichment factor under these conditions is 55 and the characteristic concentration is 2 ng ml^–1. The major ions in sea water did not interfere with Tl³⁺ preconcentration and the tolerable limits of Fe³⁺, Cu²⁺ and Al³⁺ are high enough to permit analysis of river and sea waters. The method was applied successfully to the determination of thallium in potassium-enriched table salt. It was also shown that the concentrations of Tl⁺ and Tl³⁺ in a solution can be derived using the described procedure, allowing speciation of inorganic thallium.