CHEMIJA. 2007. Vol. 18. No. 2. P. 7–11 © Lietuvos mokslų akademija, 2007 © Lietuvos mokslų akademijos leidykla, 2007 Electroreduction of silver sulphite complexes In order to investigate the kinetics of silver sulfite complexes, electroreduction rotation disk electrode (RDE) and chronopotentiometry methods were used. In a series of isopotential solu- tions, the reaction order (0.67) was calculated, and the values of silver complexes diffusion coef- ficients (3.36·10 –6 – 5.54·10 –6 , cm 2 s –1 ) and silver complexity degrees (2.31–2.67) were found. An equation estimating not only metal complexes, but also the impact of free ligand ions on the diffusion overvoltage change vs time has been derived. Using this equation the limi- tations of diffusion were successfully eliminated and charge transfer overvoltage values were determined. Key words: silver, sulfite, electroreduction, kinetics Aušra Valiūnienė*, Alicija Andruškevič, Gintarė Viselgienė, Gintaras Baltrūnas Department of Physical Chemistry, Vilnius University, Naugarduko 24, LT-03225 Vilnius, Lithuania INTRODUCTION ere have been only several investigations carried out in the field of silver sulfite complex chemistry and electrochemistry. In pub- lications related to this subject, applicable questions are predomi- nantly analyzed [1–3]. Silver sulfite complexes are stable in low alkaline solutions [4–5], whereas the silver plating process related to the electroreduction of these complexes is quite perspective. For instance, in study [4], a proposed electrolyte made on the ground of NaAgSO 3 at room temperature without solution mixing ensures the working densities 2–3 mAcm –2 of the cathodic current. An intense mixing provides the possibility to increase them to 50 mAcm –2 . In the same study it was emphasized that sulfite electrolyte is char- acterized by an excellent throwing power, whereas the surfaces coated with galvanic silver are small crystalline and easily polar- ized. A similar conclusion is educed in the study [5]. Furthermore, excellent adhesion of silver coating with copper and brass substrate has been taken into consideration. is allowed refusing pre-silver plating or amalgamating procedures. Recently, the throwing power of electrolyte has become particularly important, since it exactly determines one or another application process in microtechnique. In one of the few publications [6] it has been reported that the employment of the rotating disk electrode (RDE), limiting cur- rent densities of the electroreduction of complexes in solutions, prepared with 0.025 M free sulfite ions and 0.925 M NaNO 3, were measured. e complex concentrations in the analysed solutions were the following: 10 –4 , 2·10 –4 and 5·10 –4 M. In the study it was stated that the limiting current densities depended linearly on the square root of RDE rotation speed. e diffusion coefficient of Ag(SO 3 ) complexes calculated from the experimental data [6], D = 5.6·10 –6 cm 2 s –1 , was approximately three times less then the diffusion coefficient of free (hydratate) silver ions deter- mined in the same study [6] – D = 1.55·10 –5 cm 2 s –1 . e interaction between the silver surface and sulfite ions was investigated by large throwing power of X-ray photoelec- tronic spectroscopy. It was determined [7] that during silver di- oxide adsorption on silver monocrystal(100) surface, the coating of the chemisorbed sulfite formed on the surface. e electro- chemically roughened silver surface stimulated by Raman scat- tering (SERS) showed [8] that according to the adsorptional be- havior of oxyanion composed with silver, could be divided into two categories: sulfate and dithionate that adsorb on silver sur- face only electrostatically with weak SER spectrum, and sulfite and tiosulfate clearly chemisorbe on the silver surface causing significant changes in the spectra. An obvious resemblance of the adsorbed sulfite ion SER spectrum to normal Raman spec- trum determined to solid silver sulfite allowed the authors [8] to make an assumption that the coating of Ag 2 SO 3 developed on the silver surface. Our provisional electroreduction kinetic study of silver sulfite complexes suggested that in the region of the equilibrium potential the slowest stage was silver electrocrystallization. is highly complicated the research of the charge transfer stage. We had to refuse the methods of the rotating disk electrode and electrochemical impedance. In order to investigate the kinetics of silver sulfite complex electroreduction, the chronopotenti- ometry method was used, because this method gave us the pos- sibility to perform the measurements at relatively big values of electrode polarization, i. e. when the potential of the investigated electrode was rather far from equilibrium. EXPERIMENTAL e solutions were prepared based on distilled water applying Pa qualification Na 2 SO 3 and Ag 2 SO 4. e electrolytes were pre- pared freshly before every new experimental series relating to quite rapid sulphite ion oxidation by air oxygen. All experiments were performed at a temperature of 20 °C. * Corresponding author. E-mail: ausra.valiuniene@chf.vu.lt