Standard Electrode Potentials for Aqueous Solutions

For more comprehensive information, see A. J. Bard, R. Parsons and J. Jordan, Standard Potentials in Aqueous Solution, Dekker, New York, 1985, from which the following E° values are taken. 

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Ibble 1.1 Standard Electrode Potentials for Aqueous Solutions (25°C)... 

Before we discuss standard electrode potential, we will talk about electromotive force (emf). The electromotive force of a cell is the potential difference between the two electrodes. This can be measured using a voltmeter. The maximum voltage of a cell can be calculated using experimentally determined values called standard electrode potentials. By convention, the standard electrode potentials are usually represented in terms of reduction half-reactions for 1 molar solute concentration. The standard electrode potential values are set under ideal and standard-state conditions (latm pressure and 25°C temperature). From the MCAT point of view, you can assume that the conditions are standard, unless stated otherwise. Table 12-1 shows a list of standard electrode potentials (in aqueous solution) at 25°C. 

STANDARD ELECTRODE POTENTIALS FOR REDUCING IONS IN AQUEOUS SOLUTION All values taken from Ref. 19 unless otherwise indicated. 

Chlorine reactions may be classified broadly under two types (i) oxidation-reduction and (ii) substitution reactions. The standard electrode potential for Cr — V2CI2 + e in aqueous solution is -1.36 V. Some examples of both types are highlighted briefly below ... 

The standard electrode potentials for all the rare earths have similar values and are comparable with the redox potentials of alkaline earth metals [144], Thus the lanthanides are strong reducing agents, and form trivalent ions easily. Both europium and samarium can exist in both trivalent and divalent states and the divalent states are not stable in aqueous solutions. Cerium can exist in both tetravalent and trivalent states in solution but Ce(III) is the most stable. 

Consider the following table of standard electrode potentials for a series of hypothetical reactions in aqueous solution ... 

Table 7.2 Standard electrode potentials for the single- and multi-electron reduction of carbon dioxide (in an aqueous solution at pH 7) [126]...

A cognate issue is the establishment of a universal standard electrode potential scale in nonaqueous and mixed solvents, based on the aqueous standard hydrogen electrode (SHE), for which (H, aq/Hj) = 0 at all temperatures. The standard potentials E° are obtained on extrapolation of the EMF of a suitable cell to zero of the concentration of the electroactive electrolyte, the one that responds to the electrodes irrespective of the eventual presence of a constant inert background electrolyte, in the solutions of the two half-cells. A proper procedure for such an extrapolation that assures accuracy has been described by Mussini et al. [15]. The standard electrode potentials for a cation/metal pair M /M in a solvent S vs. the SHE is related to its standard potential in water and the standard molar Gibbs energy of transfer of the cation from water to the solvent (or solvent mixture) ... 

It is of great theoretical interest to relate a Galvani potential difference to the absolute potential of a clearly defined species, e.g., the potential of a free electron in vacuum. This defines an absolute potential scale (see Chap. 15), for which the precise relation to the conventional standard hydrogen electrode scale for aqueous solutions is still debated. Hence, a consistent system based on the standard hydrogen electrode (SHE) definition still serves the fundamental needs of measuring and calculating redox equilibria, and coupled chemical equihbria in aqueous systems. The relation of the electrode potentials in nmiaqueous systems, be they Uquid or solid, to the (aqueous) SHE, is of fundamental importance in chemistiy because nonaqueous systems play an important role in modem technologies and research, and in many cases aqueous and nonaqueous systems are even directly coupled, as, e.g., in ion partition systems. 

When the activity of the ion M"+ is equal to unity (approximately true for a 1M solution), the electrode potential E is equal to the standard potential Ee. Some important standard electrode potentials referred to the standard hydrogen electrode at 25 °C (in aqueous solution) are collected in Table 2.5.5... 

The electrical double layer at pc-Zn/fyO interfaces has been studied in many works,154 190 613-629 but the situation is somewhat ambiguous and complex. The polycrystalline Zn electrode was found to be ideally polarizable for sufficiently wide negative polarizations.622"627 With pc-Zn/H20, the value of Eg was found at -1.15 V (SCE)615 628 (Table 14). The values of nun are in reasonable agreement with the data of Caswell et al.623,624 Practically the same value of Eff was obtained by the scrape method in NaC104 + HjO solution (pH = 7.0).190 Later it was shown154,259,625,628 that the determination of Eo=0 by direct observation of Emin on C,E curves in dilute surface-inactive electrolyte solutions is not possible in the case of Zn because Zn belongs to the group of metals for which E -o is close to the reversible standard potential in aqueous solution. 

While the laws governing electrode potentials in non-aqueous media are basically the same as for potentials in aqueous solutions, the standardization in this case is not so simple. Two approaches can be adopted either a suitable standard electrode can be selected for each medium (e.g. the hydrogen electrode for the protic medium, the bis-diphenyl chromium(II)/ bis-diphenyl chromium(I) redox electrode for a wide range of organic... 

Thus, these relationships can be used to define a pH scale for non-aqueous protic media, consistent with the pH scale for aqueous solutions. For standard hydrogen pressure, the potential of the hydrogen electrode depends on the pH(s) according to the relationship... 

However, silicon material in an aqueous solution is not a system in equilibrium. It is considered as a mixed system containing two redox couples with standard electrode potentials Ei and Ei separated by a wide interval. Then Eq. (13) must be modified to account for both components ... 

Data on the standard potentials for inorganic redox systems in aqueous solutions have been compiled by IUPAC [1], The standard potentials for some M"+/M and Mn+/M(Hg) couples are shown in Table 4.1 [2]. For alkali metals, the standard potentials of M+/M(Hg) are about IV more positive than those of M+/M. This is because alkali metals have strong affinities to mercury and are stable in the amalgams. It is impossible to measure the potentials of alkali metal electrodes directly in aqueous solutions, because alkali metals react with water. In order to determine the potential of an alkali metal electrode in an aqueous solution, we measure the potential of the corresponding amalgam electrode in an aqueous solution and then the difference between the potentials of alkali metal and alkali metal amalgam electrodes using an appropriate non-aqueous solution [2].2 ... 

The primary reference electrode for aqueous solutions is the standard hydrogen electrode (SHE), expressed by H+(a=l) H2(p=105 Pa) Pt (see 11 in Section 4.1). Its potential is defined as zero at all temperatures. In practical measurements, however, other reference electrodes that are easier to handle are used [24]. Examples of such reference electrodes are shown in Table 5.4, with their potentials against the SHE. All of them are electrodes of the second kind. The saturated calomel electrode (SCE) used to be widely used, but today the saturated silver-silver chloride electrode is the most popular. 

The characteristics of redox reactions in non-aqueous solutions were discussed in Chapter 4. Potentiometry is a powerful tool for studying redox reactions, although polarography and voltammetry are more popular. The indicator electrode is a platinum wire or other inert electrode. We can accurately determine the standard potential of a redox couple by measuring the electrode potential in the solution containing both the reduced and the oxidized forms of known concentrations. Poten-tiometric redox titrations are also useful to elucidate redox reaction mechanisms and to obtain standard redox potentials. In some solvents, the measurable potential range is much wider than in aqueous solutions and various redox reactions that are impossible in aqueous solutions are possible. 

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