Double potential step chronocoulometry: Part II. Measurement of the chemical reaction rate in an EC mechanism when both electrode reactant and product are adsorbed
✍ Scribed by Richard P. Van Duyne; Thomas H. Ridgway; C.N. Reilley
- Publisher
- Elsevier Science
- Year
- 1972
- Weight
- 751 KB
- Volume
- 34
- Category
- Article
- ISSN
- 0022-0728
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✦ Synopsis
In an accompanying report 1, we rederived the analytical solution to the EC kinetics problem for double potential step chronocoulometry, The numerical results of this derivation for the [Qb/Qf[ vs. x/kz working curve are now in agreement with those obtained from finite difference computer calculations. The analytical solution has been extended to include cases in which adsorption of reactant and product complicate measurement of the rate of the follow up chemical reaction. This extention of the theory was necessitated by our choice of the benzidine-rearrangement reaction as a "model" system for purposes of experimentally verifying the newly calculated EC working curve.
It is well known that hydrazobenzene chemically or electrochemically generated from azobenzene in acidic solutions undergoes the benzidine-rearrangement reaction to form benzidine and diphenyline 2. Previous electrochemical studies of this reaction 2 -6 have established: (i) the EC nature of the electrode reaction (i.e. benzidine and diphenyline are electro-inactive in the potential region used to generate hydrazobenzene and do not react chemically with the environment to form electroactive material) involving the overall consumption of 2e-; (ii) a reasonably consistent set of pseudo first-order rate constants for the rearrangement reaction spanning the range 10-2 S-1 to 10 2 S-1 ; (iii) that azobenzene adsorbs on mercury electrodes at potentials between the mercury oxidation limit and the onset of azobenzene reduction (,,~ 0.09 V vs. SCE); and (iv) that adsorption phenomena must be quantitatively accounted for in order to determine the rate of the chemical follow-up reaction using linear sweep voltammetric techniques, whereas chronoamperometrically determined rate constants are less susceptible to adsorption difficulties.
Almost the entire spectrum of electrochemical methodology has been brought to bear on the azobenzene system including classical polarography 7, cyclic voltammetry 6, various modes of chronopotentiometry and thin-layer electrochemistry 3:, double potential step chronoamperometry 2, and the voltammetric technique involving potential step generation with linear sweep reversal 5.
Conspicuous by its absence, chronocoulometry in either single or double potential step variants has not to our knowledge been applied to the azobenzene system. This is somewhat surprising considering that chronocoulometry has ~ reached "method of choice" stature for studies of electroactive species adsorption at potentials