Voltammetric behaviour of thallium ( III ) at a solid heterogeneous carbon electrode using ion-pair formation

Thallium in its trivalent state forms very stable [TIX4]– complexes with halides in corresponding acidic media. Tetrahalogenothallates (III) are able to produce ion pairs with suitable positively charged counter-ions in aqueous solutions. These ion pairs, each possessing distinct lipophilic character, could be accumulated directly onto the surface of carbon paste electrodes without applying a potential (open circuit conditions). The nature of the electrode material and of the counter-ion respectively, influences the adsorptive behaviour of the ion pair significantly. Best results were obtained by using a new type of heterogeneous carbon electrode employing a solid binder (phenanthrene) instead of the commonly used pasting liquid. With respect to the counter-ions, tetraphenylphosphonium and tetrabutylammonium proved to be the best. Parameters influencing the optimum conditions for the analytical procedure (pH, counter-ion concentration, etc.) were investigated. The relationship between current signal and concentration of thallium, and its dependence on the accumulation time were studied. The detection limit was found to be ⩽ 10 µg kg–1, signals were reproducible within an sr of 7.7% for a thallium concentration of 1 mg kg–1. The interference of other ionic species was also evaluated. The method was employed for the determination of thallium in lead- and cadmiumcontaining zinc alloys.