Electrode quasi-reversible

Success in protein voltammetry depends critically upon the electrode and how it is prepared and modified. The current response may stem from protein molecules free in solution and undergoing a reaction upon diffusing to the electrode surface, or it may stem from molecules that are already bound tightly (adsorbed) to the electrode. Quasi-reversible diffusion-controlled electrochemistry has been documented for a wide range of proteins, mostly the smaller variety (molecular mass <15kDa) that function as mobile electron carriers [3], Diffusion-controlled electrochemistry requires that the protein interacts with the electrode in a transient manner, that is, weakly, so that the electrode does not become blocked. Increasingly, however, attention has turned to electrodes that bind protein molecules tightly, so that the sample is studied as a stable monolayer that typically comprises less than a picomole [1]. 

Figure Bl.28.7. Schematic shape of steady-state voltaimnograms for reversible, quasi-reversible and irreversible electrode reactions.

Quasi-reversible systems, 32 Quaternary ammonium salts, 153 Quinliydrone electrode, 151... 

The cyclic voltammograms of these systems display quasi-reversible behavior, with AEv/v being increased because of slow electrochemical kinetics. Standard electrochemical rate constants, ( s,h)obs> were obtained from the cyclic voltammograms by matching them with digital simulations. This approach enabled the effects of IR drop (the spatial dependence of potential due to current flow through a resistive solution) to be included in the digital simulation by use of measured solution resistances. These experiments were performed with a non-isothermal cell, in which the reference electrode is maintained at a constant temperature... 

Conducted in 10% CH2Cl2-90% acetonitrile for compounds [54] and [56] and in acetonitrile [55] upon addition of 2 equiv of the respective cation supporting electrolyte, 0.10 mol dm-3 TBABF4. The potential of the reduction current peak r, reversible q, quasi-reversible s, single reduction peak without corresponding reoxidation peak ec, electron transfer followed by a chemical reaction ec, ad, electron transfer followed by a chemical reaction with insoluble product which adsorbs on to the electrode surface. Prewaves are in parentheses. 

The key parameters from a CV measurement include the wave shape, the peak potential(s), pa and pc, and, more importantly, their dependence on the scan rate. For reversible and many quasi-reversible systems, the average of pa and equals or closely approximates EV2. Forjudging the reversibility of an electrode reduction like reaction (A.l) at 25°C, the useful criteria are ... 

The reduction of cycloheptatriene was studied in aprotic solvents at a platinum electrode. A reversible wave at —2.5 V for the production of the radical anion was observed in ammonia containing 0.1 M KI. Quasi-reversible or irreversible reduction was observed in acetonitrile and in A,A-dimethylformamide (equation 34)103. 

It is clear that the decrease of the rate of the electron transfer operated by the temperature makes the oxidation of ferrocene become quasi-reversible for both the electrode materials. Moreover, it is noted that for both types of electrode the faradaic current increases with temperature. For both the electrodes the oxidation process is governed by diffusion, since in both cases the plot of log(/p) vs. 1/T is linear. Furthermore, one should note in particular that, contrary to the naive expectation, for the superconducting electrode one does not observe any abrupt change in the response upon crossing the barrier from superconductor (that should exchange pairs of electrons) to simple conductor (that should exchange single electrons). 

Usually, however, electron transfers at the electrode are denoted by E , while chemical steps not involving the electrode are denoted by C . The ET may further be characterized as Er , Eqr , or Ej in the reversible, quasi-reversible, or irreversible case. It is usually not indicated how transport occurs. If the C-step is a dimerization, the symbol D is common, while an ET between two species in a (homogeneous) solution is denoted SET (for solution electron transfer) [18] or DISP (see, e.g. [19]). 

The effects of mercury film electrode morphology in the anodic stripping SWV of electrochemically reversible and quasi-reversible processes were investigated experimentally [47-51], Mercury electroplated onto solid electrodes can take the form of either a uniform thin film or an assembly of microdroplets, which depends on the substrate [51 ]. At low sqtrare-wave frequencies the relationship between the net peak crrrrent and the frequency can be described by the theory developed for the thin-film electrode because the diffusion layers at the snrface of microdroplets are overlapped and the mass transfer can be approximated by the planar diffusion model [47,48],... 

---