**4.1.1 Composition of the electrolytes**

The chemicals used for the study of the hexaammineruthenium (III)/(II) redox reaction are: [Ru(NH3)6]Cl3 (Sigma-Aldrich, 98%), Na2HPO4 · 2H2O and NaH2PO4 ·12H2O (both ProLabo A.R.). Solutions are made with once-distilled and deionized water. A 0.1 M phosphate buffer pH 7 solution is used as the supporting electrolyte and the concentration of the electroactive species hexaammineruthenium (III) is 0.001 M. The following chemicals are used for the ferri/ferrocyanide reaction: K4[Fe(CN)6] ·3H2O, K3[Fe(CN)6] and KCl (all Merck P.A.). The supporting electrolyte is a 1 M KCl solution and the concentrations of the electroactive components ferri/ferrocyanide are 0.005 M. In that way, a negligible migration flux, constant activity and diffusion coefficients of the electroactive species, a low electrolyte resistance and a uniform current distribution are assumed for both electrochemical systems.
