High affinity of thallium ions to copper hexacyanoferrate films

High affinity of thallium(I) ions with respect to a copper hexacyanoferrate (CuHCF,) film was found. Interaction between the CuHCF film and thallium(I ) ions was investigated using the electrochemical quartz crystal microbalan ce and cyclic voltammetric techniques. In 0.5 M KNO3 solution, at submillim olar concentrations of TI(I) ions, the CuHCF film reversibly transforms, du ring electrochemical experiments, from the potassium into the thallium form . For slightly higher (several millimoles) Tl(I) concentrations, the electr ochemical and gravimetric responses prove that the CuHCF film behaves as th e thallium form only. Its formal potential (E-f(0)), calculated from the de pendence of the E-f(0) on [TI(I)], is for [TI(I)] = I M, 0.28 V more positi ve than that found for the potassium form. Experimental results obtained su ggest that in both cases, potassium(I) and thallium(I) ions, exchange in in terstitial positions takes place. Since the ionic radii and the hydration p arameters of both ions are similar, we concluded that this high affinity of thallium(I) ions with respect to the CuHCF film, 250-100 times higher than that of potassium(I) ion. results from chemical interactions. In consequen ce, the solubility product of a thallium analogue of copper hexacyanoferrat e is much Smaller than that of the potassium form. For different forms they are equal to (pK values) 37.8 (K2Cu3[Fe(CN)(6)](2)). 46.3 (Tl2Cu3[Fe(CN)(6 )](2)), and 17.2 (Cu-3[Fe(CN)(6)](2)). (C) 2001 The Electrochemical Society .