ja073946i_si_002.pdf (275.47 kB)
Absolute Standard Hydrogen Electrode Potential Measured by Reduction of Aqueous Nanodrops in the Gas Phase
journal contribution
posted on 2008-03-19, 00:00 authored by William A. Donald, Ryan D. Leib, Jeremy T. O'Brien, Matthew F. Bush, Evan R. WilliamsIn solution, half-cell potentials are measured relative to those of other half cells, thereby
establishing a ladder of thermochemical values that are referenced to the standard hydrogen electrode
(SHE), which is arbitrarily assigned a value of exactly 0 V. Although there has been considerable interest
in, and efforts toward, establishing an absolute electrochemical half-cell potential in solution, there is no
general consensus regarding the best approach to obtain this value. Here, ion-electron recombination
energies resulting from electron capture by gas-phase nanodrops containing individual [M(NH3)6]3+, M =
Ru, Co, Os, Cr, and Ir, and Cu2+ ions are obtained from the number of water molecules that are lost from
the reduced precursors. These experimental data combined with nanodrop solvation energies estimated
from Born theory and solution-phase entropies estimated from limited experimental data provide absolute
reduction energies for these redox couples in bulk aqueous solution. A key advantage of this approach is
that solvent effects well past two solvent shells, that are difficult to model accurately, are included in these
experimental measurements. By evaluating these data relative to known solution-phase reduction potentials,
an absolute value for the SHE of 4.2 ± 0.4 V versus a free electron is obtained. Although not achieved
here, the uncertainty of this method could potentially be reduced to below 0.1 V, making this an attractive
method for establishing an absolute electrochemical scale that bridges solution and gas-phase redox
chemistry.