The preparation of copper(II) hexacyanoferrate (CuHCF) films on the surface of gold electrodes as well as their characterization in solutions of various alkali metal and NH4+ cations and in the presence of thallium(I) are described. The electrochemical quartz crystal microbalance and cyclic voltammetric techniques were used. In 0.50 M lithium nitrate, even at submillimolar concentration of TI(I), the formal potential of CuHCF was shifted to more positive values. At higher Tl(I) concentrations, the formal potential of the CuHCF redox reaction changed linearly with the logarithm of TI(I) concentration (in the 0.50 M solution of lithium or another alkali metal nitrate). From such dependencies, selectivity coefficients K-TI/m were calculated, and they show that the CuHCF film on the gold electrode interacts preferentially with TI(I). High affinity of TI(I) to copper hexacyanoferrate, that was observed in the presence of alkali metal cations, was explained by relatively strong donor-acceptor interactions of TI(I) ions with nitrogen in CN groups of the CuHCF film. It was also shown for simple M-4[Fe(CN)(6)] metal ferrocyanate salts (where M = Li+, Na+, K+, Rb+, Cs+ and Tl+) that there is a preferential interaction of Tl+ with CN group consistent with formation of a TI-NC-Fe bridge. (c) 2008 Elsevier Ltd. All rights reserved.