A detailed study of the influence of electrolyte activity on apparent formal potentials measured for self-assembled monolayers of ferrocenylhexanethiol at high coverages on gold electrodes is provided. Ion selective electrodes are used in a logical sequence of comparisons to eliminate liquid junction potentials and, ultimately, to provide a rigorous comparison of apparent formal potentials for the monolayer over a broad range of concentrations in aqueous solutions of HClO 4 and NaClO4. The results obtained in HClO4 and NaClO4 solutions are indistinguishable. Under the conditions examined it appears that protons and sodium ions have little influence on the apparent formal potentials. Hence it seems that protons and sodium ions have little influence on the overall interfacial potential distribution and, thus, little influence on the overall ionic structure near the monolayer/solution interface. The picture that emerges for the oxidized "monolayer" is one in which the high surface charge produced by the high surface coverage of ferroceniums is compensated by a highly compact layer of counterions.
Properties of relatively new polymeric materials are capitalized upon to address a relatively old problem in electrochemistry, the comparison of electrode potentials measured in different solvents. In particular, electrode potentials measured in acetonitrile and in water are compared at voltammetrically relevant concentrations. Activity coefficients are measured for silver perchlorate in acetonitrile using the isopiestic technique. These results are used to interpret the influence of silver perchlorate activity on apparent formal potentials measured in acetonitrile for an electrochemically polymerized redox polymer. Similar measurements are made on the same redox polymer in water, and in both solvents the cell potentials are Nernstian for a simple ionic stoichiometry. The cell potentials measured in acetonitrile are +0.244 V of those measured in water. This difference is consistent with the difference expected from estimates of the free energy of transfer of silver perchlorate. It is also consistent with predictions based on certain extrathermodynamic assumptions concerning the free energies of transfer of the single ions. The results reported here suggest that the free energy for transferring silver perchlorate from water to acetonitrile is -21.9 ( 0.9 kJ/mol and that the potential of the Ag|a(AgClO 4 ) CH3CN ) 1 couple is -0.227 ( 0.009 V of the Ag|a(AgClO 4 ) H2O ) 1 couple. The influence of electrolyte activity on formal potentials measured for redox couples dissolved in acetonitrile is also investigated. Our results demonstrate that internal reference couples such as ferrocene/ferrocenium should be used with caution. Over certain concentration ranges, ion pairing may play an important role in determining formal potentials measured for dissolved reference compounds. To provide a reliable set of values to which subsequent comparisons can be made in acetonitrile, we report apparent formal potentials as a function of electrolyte activity for several popular internal reference couples.In this paper we describe our efforts to measure activity coefficients in acetonitrile, to compare the potential for the Ag|X M AgClO 4 couple in water with that of the Ag|X M AgClO 4 couple in acetonitrile, and to measure formal potentials for a series of redox couples as a function of AgClO 4 activity in acetonitrile. These efforts were initiated due to dissatisfaction with our limited ability to interpret cell potentials in solvents other than water. Unfortunately, there is no completely unambiguous way of interpreting potentials in many nonaqueous solvents. Much of this ambiguity stems from the lack of suitable reference half-cells. Three major problems plague reference half-cells in nonaqueous solvents: (1) unknown liquid junction potentials, (2) questionable extrathermodynamic assumptions concerning internal reference couples, and (3) unknown activity coefficients for the reference couple. Our efforts to overcome these obstacles have enabled us to make a series of measurements in acetonitrile that are similar to the classical ...
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