Polypyrrole modified electrode prepared by electropolymerization of pyrrole in the presence of a complexing ligand, benzo-15-crown-5 (B15C5), was prepared and investigated as a La3+- selective electrode in acetonitrile. The potentiometric response of the electrode was linear within the La3+ concentration range 1 • 10_4 to 1 • 10_1 M with a Nernstian slope of 19.5 mV decade_1 in AN. The electrode was applied to study the complexation of the lanthanum (III) ion in acetonitrile with other basic solvent molecules (D) such as dimethyl sulfoxide, N,N-dimethylformamide, propylene carbonate, N,N,Diethylaniline and methanol. The successive complex formation constant (bi) and Gibbs energies of transfer (DGtr) of La3+ in AN in relation to such D were obtained.

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