Solvents potential range

FIGURE 8.1 Approximate potential ranges in nonaqueous solvents. 

Electrochemical measurements are commonly carried out in a medium that consists of solvent containing a supporting electrolyte. The choice of the solvent is dictated primarily by the solubility of the analyte and its redox activity, and by solvent properties such as the electrical conductivity, electrochemical activity, and chemical reactivity. The solvent should not react with the analyte (or products) and should not undergo electrochemical reactions over a wide potential range. 

The equilibrium (1) at the electrode surface will lie to the right, i.e. the reduction of O will occur if the electrode potential is set at a value more cathodic than E. Conversely, the oxidation of R would require the potential to be more anodic than F/ . Since the potential range in certain solvents can extend from — 3-0 V to + 3-5 V, the driving force for an oxidation or a reduction is of the order of 3 eV or 260 kJ moR and experience shows that this is sufficient for the oxidation and reduction of most organic compounds, including many which are resistant to chemical redox reagents. For example, the electrochemical oxidation of alkanes and alkenes to carbonium ions is possible in several systems... 

The non-steady-state optical analysis introduced by Ding et al. also featured deviations from the Butler-Volmer behavior under identical conditions [43]. In this case, the large potential range accessible with these techniques allows measurements of the rate constant in the vicinity of the potential of zero charge (k j). The potential dependence of the ET rate constant normalized by as obtained from the optical analysis of the TCNQ reduction by ferrocyanide is displayed in Fig. 10(a) [43]. This dependence was analyzed in terms of the preencounter equilibrium model associated with a mixed-solvent layer type of interfacial structure [see Eqs. (14) and (16)]. The experimental results were compared to the theoretical curve obtained from Eq. (14) assuming that the potential drop between the reaction planes (A 0) is zero. The potential drop in the aqueous side was estimated by the Gouy-Chapman model. The theoretical curve underestimates the experimental trend, and the difference can be associated with the third term in Eq. (14). 

Kakiuchi et al. [75] used the capacitance measurements to study the adsorption of dilauroylphosphatidylcholine at the ideally polarized water-nitrobenzene interface, as an alternative approach to the surface tension measurements for the same system [51]. In the potential range, where the aqueous phase had a negative potential with respect to the nitrobenzene phase, the interfacial capacity was found to decrease with the increasing phospholipid concentration in the organic solvent phase (Fig. 11). The saturated mono-layer in the liquid-expanded state was formed at the phospholipid concentration exceeding 20 /amol dm, with an area of 0.73 nm occupied by a single molecule. The adsorption was described by the Frumkin isotherm. 

As would be expected, the larger titration potential ranges offer much more scope for mutually distinguishing between individual acids or bases in amphiprotic solvents, as a consequence of self-dissociation, the potential ranges are rather limited, whereas in the aprotic protophilic solvents and "aprotic inert solvents these ranges are considerably more extensive. 

Fig. 4.1. Potential ranges of solvents, (a) h.n.p.s of acids. I, Acetic acid II, benzoic acid III, formic acid IV, salicylic acid V, sulphuric acid VI, p-toluenesulphonic acid, (b) h.n.p.s of conjugate acids of I, n-butylamine II, piperidine III, ethylenediamine (1) IV, ammonia V, ethylenediamine (2) VI, pyridine.

As most of the solvents in Fig. 4.1 are amphiprotic and it is interesting to know the correct vertical position of their potential ranges compared with that of water, we shall apply eqn. 4.45 to a solvent HS with self-dissociation ... 

In the meantime, it is of great practical importance to establish how the redox potential ranges can vary from one solvent to another without the occurrence of solvent decomposition. Therefore, we shall first consider the redox potential range in water. Here at the surface of the hydrogen platinized Pt electrode (cf., p. 31) the following reversible reduction can take place ... 

Before mentioning some more literature data on non-aqueous voltammetry, we suggest on the basis of our previous discussions that the choice of the experimental conditions used in the techniques must be a compromise between a sufficient solubility of the analyte in the solution, an ample redox potential range of the solvent, a suitable type of indicator electrode and adequate conductance of the solution with supporting electrolyte added. In this connection Fig. 4.20 may be a useful guide. 

Using solvent-containing triruthenium species 1 as a synthetic precursor, a series of pyridyl-substituted triruthenium derivatives [Ru30(0Ac)6(py)2(L)]+ (L = 4,4 -bpy 5, BPE 6, BPA 7) were prepared by Meyer et al. [9]. Electrochemical studies showed that these triruthenium complexes exhibit four to five reversible one-electron redox waves in the potential range of +2.0 to —2.0 V, suggesting that these complexes can... 

The accessible potential ranges for platinum and mercury electrodes, in the commonest organic solvents are reported in Table 1. It is noted that gold exhibits characteristics very similar to platinum. 

A similar conclusion arises from the capacitance data for the mercury electrode at far negative potentials (q 0), where anions are desorbed. In this potential range, the double-layer capacitance in various electrolytes is generally equal to ca. 0.17 F Assuming that the molecular diameter of water is 0.31 nm, the electric permittivity can be calculated as j = Cd/e0 = 5.95. The data on thiourea adsorption on different metals and in different solvents have been used to find the apparent electric permittivity of the inner layer. According to the concept proposed by Parsons, thiourea can be treated as a probe dipole. It has been cdculated for the Hg electrode that at (7 / = O.fij is equal to 11.4, 5.8, 5.1, and 10.6 in water, methanol, ethanol, and acetone, respectively. 

Examination of the behaviour of a dilute solution of the substrate at a small electrode is a preliminary step towards electrochemical transformation of an organic compound. The electrode potential is swept in a linear fashion and the current recorded. This experiment shows the potential range where the substrate is electroactive and information about the mechanism of the electrochemical process can be deduced from the shape of the voltammetric response curve [44]. Substrate concentrations of the order of 10 molar are used with electrodes of area 0.2 cm or less and a supporting electrolyte concentration around 0.1 molar. As the electrode potential is swept through the electroactive region, a current response of the order of microamperes is seen. The response rises and eventually reaches a maximum value. At such low substrate concentration, the rate of the surface electron transfer process eventually becomes limited by the rate of diffusion of substrate towards the electrode. The counter electrode is placed in the same reaction vessel. At these low concentrations, products formed at the counter electrode do not interfere with the working electrode process. The potential of the working electrode is controlled relative to a reference electrode. For most work, even in aprotic solvents, the reference electrode is the aqueous saturated calomel electrode. Quoted reaction potentials then include the liquid junction potential. A reference electrode, which uses the same solvent as the main electrochemical cell, is used when mechanistic conclusions are to be drawn from the experimental results. 

Double-layer properties in aqueous, propylene carbonate and formamide solutions have been studied at room temperature for liquid Ga-Pb alloy (0.06 atom % of Pb) [15], as a model of Pb electrode with renewable surface. The electrode behaves as an ideally polarizable electrode in a wide potential range, and its capacitance is intermediate between that of Ga and Hg electrodes and is independent of the solvent. This electrode is much less lipophilic than Ga. Adsorption of anions on this electrode increases in the sequence -BP4 = S042 < Gl < Br < r. 

---