Relationships between Measured Potential and Concentrations of Redox Centers in Polymer Networks

by Fritschfaules, I.; Faulkner, L. R.

We have shown that a nernstian relationship exists between the concentrations, C, of ferri- (O) and ferrocyanide (R) and the applied potential in thin films of methyl-quaternized poly(4-vinylpyridine) (QPVP) on electrodes in aqueous KNO3 and potassium p-toluenesulfonate (KOTs) electrolytes. Plots of E - E-degrees' versus in (C(O)/C(R)) closely follow expected nernstian behavior (ideally, slope = 25.5 mV and intercept = 0 mV at 22-degrees-C). For the KNO, system, the observed slope was 26.4 +/- 0.3 mV and the intercept was 3.6 +/- 0.14 mV. For the KOTs system, the slope was 29.3 +/- 0.3 mV and the intercept was 7.8 +/- 0.11 mV. The nearly nernstian behavior is somewhat surprising, considering the complex dynamics and thermodynamics of QPVP/Fe(CN)6(3-/4-), where (a) both forms of the redox couple partition to different extents between film and solution, (b) the mass transport in the film is oxidation-state dependent, and (c) the full-widths at half maximum of peaks in cyclic voltammetry are greater than the nernstian response of 89.7 mV. Calibration curves in the form of C(O)C(O,max) or C(R)/C(O,max) versus E - E-degrees' allow us to convert the potential profiles in our companion paper to concentration profiles. Here C(O,max) is the maximum (equilibrium) concentration of O at E - E-degrees' = +300 mV. Likewise, C(R,max) is the equilibrium concentration of R at a strongly reducing potential (E - E-degrees' = -300 mV). Concentrations of O and R were determined by preconditioning the film for 1 h at a given potential, followed with long-pulse-width (20 000-32 000 ms) chronocoulometry (LPCC) featuring a step to either -300 mV or +300 mV vs E-degrees', respectively. Experiments were carried out at 50-mu-m X 2-mm electrodes with the experimental arrangement described in the companion paper. The ratio of C(O,max)/C(R,max) is 0.44 for KNO3 and 0.37 for KOTs electrotytes, suggesting that the partitioning Is almost independent of the identity of the electrolyte.

Journal
Analytical Chemistry
Volume
64
Issue
10
Year
1992
Start Page
1127-1131
URL
https://dx.doi.org/10.1021/ac00034a010
ISBN/ISSN
1520-6882; 0003-2700
DOI
10.1021/ac00034a010