Electrochemical and EQCM studies of growth and dissolution processes of deposits on gold electrodes: uranyl-hexacyanoferrate films

Electrochemical and electrochemical quartz crystal microbalance (EQCM) methods were used for the analysis of the growth and dissolution processes of sparingly soluble deposits. As a model of a sparingly soluble film, uranyl-hexacyanoferrate was selected. Additionally the rate of Prussian blue deposition was studied. We have shown that coupling of the EQCM technique with standard electrochemical methods may be useful in the study of mechanisms of electrodeposition phenomena when the solubility of the product of the electrode reaction is limited. The transformation of mass–time data into current–time dependences allows an analysis of the reaction in terms of electrocrystallization steps while other masking processes are superimposed during the polarization of the electrode. This approach may have general value and may be applied not only to hexacyanoferrate deposition but also to deposition of other products of the electrode reaction, which exhibit limited solubility. Analysis of growth of uranyl-hexacyanoferrate on the Au/quartz electrode under potentiostatic and cyclic voltammetry conditions, taken as an instance, reveals that the first step, limited by the growth of the crystallization nuclei, is clearly separated from the uniform growth, limited by diffusion. For thicker films, the slope of the frequency–charge dependence is dominated by the insertion/removal process of potassium ions, to and from the film. During the dissolution of the uranyl-hexacyanoferrate deposits, two clearly separated steps were observed. During the first, relatively fast oxidation of the deposit is observed which is accompanied by removal of potassium ions from the film. The second step corresponds to a chemical dissolution of the oxidized film. The rate coefficients corresponding to both steps were determined. Based on the gravimetric and electrochemical data, the molar mass of the deposited uranyl-hexacyanoferrate was estimated as equal to 690 ± 90 g/mol. From these and the EDX data, a chemical formula of the deposited uranyl-hexacyanoferrate was proposed as: K2(UO2)[Fe(CN)6].