VOLTAMMETRIC BEHAVIOR OF THALLIUM(III) AT A SOLID HETEROGENEOUS CARBON ELECTRODE USING ION-PAIR FORMATION

Thallium in its trivalent state forms very stable [TIX(4)]-complexes w ith halides in corresponding acidic media. Tetrahalogenothallates(III) are able to produce ion pairs with suitable positively charged counte r-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-i on respectively, influences the adsorptive behaviour of the ion pair s ignificantly. Best results were obtained by using anew type of heterog eneous carbon electrode employing a solid binder (phenanthrene) instea d of the commonly used pasting liquid. With respect to the counterions , tetraphenylphosphonium and tetrabutylammonium proved to be the best. Parameters influencing the optimum conditions for the analytical proc edure (pH, counter-ion concentration, etc.) were investigated. The rel ationship between current signal and concentration of thallium, and it s dependence on the accumulation time were studied. The detection limi t was found to be less than or equal to 10 mu g kg(-1), signals were r eproducible within an s(r) of 7.7% for a thallium concentration of 1 m g kg(-1). The interference of other ionic species was also evaluated. The method was employed for the determination of thallium in lead- and cadmium-containing zinc alloys.