THE METHYL VIOLOGEN INCORPORATED ZEOLITE MODIFIED CARBON-PASTE ELECTRODE .2. ION-EXCHANGE AND ELECTRON-TRANSFER MECHANISM IN AQUEOUS-MEDIUM

An investigation of ion exchange between methyl viologen ions (MV++) and sodium ions (Na+) in zeolite Y has been completed in order to describe the electron transfer mechanism operative in a zeolite modified carbon paste electrode (ZMCPE). Preliminary work needed the characterization of the Linde Y-54 zeolite batch used and the search for equilibrium conditions. The ion exchange isotherm for the MV++ exchange for Na+ demonstrates the great selectivity for the large organic cation and a maximum capacity of 1.0 x 10(-3) mole MV++ per gram of zeolite, corresponding to an occupation of about 64% of the total exchanging sites. The use of zeolite containing different amounts of methyl viologen has emphasized that the selectivity is greatly influenced by the activity coefficients of the exchanging species within the zeolite framework. This property of the ion exchange equilibrium allowed to determine the electron transfer mechanism that is operative for the cathodic reduction of MV++ initially exchanged in a ZMCPE. In aqueous solution, MV++ is reduced after being exchanged for the electrolyte cation. Finally, a structural model of the electrode has been proposed. It is in good agreement with experimental results and takes into account all the electrochemical behaviour of the ZMCPE and the advocated electron transfer mechanism.