Solvent-independent reference electrode

In the second approach to the electrochemical determination of the ion solvation energy, a cell without a liquid junction is used. Such a cell consists of an electrode formed by the ions to be studied, and as a reference a solvent-independent reference electrode (SIRE) ... 

Equation (4.5) is also valid in this case. Reactions of this type are realized in polarography at a dropping mercury electrode, and the standard potentials can be obtained from the polarographic half-wave potentials ( 1/2)- Polarographic studies of metal ion solvation are dealt with in Section 8.2.1. Here, only the results obtained by Gritzner [3] are outlined. He was interested in the role of the HSAB concept in metal ion solvation (Section 2.2.2) and measured, in 22 different solvents, half-wave potentials for the reductions of alkali and alkaline earth metal ions, Tl+, Cu+, Ag+, Zn2+, Cd2, Cu2+ and Pb2+. He used the half-wave potential of the BCr+/BCr couple as a solvent-independent potential reference. As typical examples of the hard and soft acids, he chose K+ and Ag+, respectively, and plotted the half-wave potentials of metal ions against the half-wave potentials of K+ or against the potentials of the 0.01 M Ag+/Ag electrode. The results were as follows ... 

The Fc+/Fc or BCr+/BCr couple is selected as reference redox system and the electrode potentials in any solvent and at any temperature are reported as values referred to the (apparent) standard potential of the system. Which of the two couples to select depends on the potential of the system under study the potential of the reference redox system should not overlap that of the system under study. Because the difference between the standard potentials of the two couples is almost solvent-independent (1.132 0.012 V, see Table 6.4), the potential referred to one system can be converted to the potential referred to the other. 

A method in which the potential of an appropriate reference electrode (or reference redox system) is assumed to be solvent-independent. 

In fact, it is not easy to get a reference electrode whose potential is solvent-independent. Therefore, we use a reference redox system (Fc+/Fc or BCr+/BCr) instead. We measure the potentials of the M+/M electrode in S and R against a conventional reference electrode (e.g. Ag+/Ag). At the same time, we measure the half-wave potentials of the reference redox system in S and R using the same reference electrode. Then, the potentials of the M+/M electrode in S and R can be converted to the values against the reference redox system. In this case, the reliability of the results depends on the reliability of the assumption that the potential of the reference redox system is solvent-independent. 

In order to use Eq. (6) in electrochemical studies of ion solvation, the problems related to the liquid junction potential have been presented in Sec. 2.2.2. Equation (11) may also be used in such studies, but the measured potentials should be expressed versus the same solvent-independent reference electrode. Such electrodes, which give a basis for the formation of a uniform scale of electrode potentials in different solvents, are available. A scale of this kind is also needed for a correlation of equilibrium potentials (E° 1/2) of electrode systems in various solvents. 

Krishtalik et al. [64] also arrived at a conclusion that cobaltocene and ferrocene electrodes are equivalent as the reference electrodes. The difference of potentials between these two electrodes is practically independent of the nature of the solvent and is given in Table 2. 

We only list some other redox systems which have been proposed for the construction of solvent-independent reference electrodes. In the collection of such systems, one finds tris(2,2-bipyridine)iron(I)/(0) [69], tris(phenantroline)iron(III)/(II) [72], redox systems composed of polynuclear aromatic hydrocarbons [66, 71], 13 /I2 [72], and cryptate complexes [73, 74]. The use of these systems in practice is quite limited. 

There is also, as expected, a correlation between the formal potentials of various systems with DN or the Kamlet-Taft, Lewis basicity parameter [84] shown in other papers (see for instance [85]). Also, in this case, with increasing donor properties of the solvents, the formal potential is moving to more negative values expressed versus a solvent-independent reference electrode, the Foe /Foe system. 

In conclusion, there is no fully satisfactory system for the construction of a unified potential scale which could be used for mixed solvents of different compositions. In fact, the scales used are the same as those applied for pure solvents, but in the case of mixed solvents the extrathermodynamic assumption may be even less strictly obeyed, especially if there are even small preferential interactions of the reference electrode components with one of the solvents of the mixture. In our work we used the Foe /Foe system as a solvent-independent reference electrode. The consequent use of one reference electrode in a series of experiments with mixed solvents of different composition should diminish the error. 

Now we shall draw the reader s attention to an interesting fact. As shown in Sect-tion 2.3, the difference between the delocalized and solvated electron levels for hexamethylphosphotriamide is by almost the same value, i.e. about 0.4 eV higher than for water or liquid ammonia, i.e. for the solvents having a branched structure of H bonds. It follows that the introduction of a hydrocarbon residue into the solvent s molecule forces out only the delocalized electrons from the polar medium the solvated electron energy level in all the enumerated solvents has almost the same value. (An independent confirmation to this is the closeness of equilibrium potentials of the electron in water, hexamethylphosphotriamide, and liquid ammonia — see Section 5 — vs. the reference electrode whose potential is independent of the solvent.)... 

This paper reviews efforts to establish single ion activities for aqueous electrolytes. Nevertheless, a closely related problem, that of the energies of transfer of single ionic species from one solvent to another, has received much attention. Among the chief approaches on which these efforts are based are the following choice of a reference electrode the potential of which may be independent of the solvent, such as Rb /Rb or the ferrocinium/ferrocene couple assumption of the equality of the transfer energies of certain large ions such as tetraphenylarsonium and tetraphenylborate and efforts to nullify the liquid-junction potential between ionic solutions in different solvents. 

In practice the main requirement of a reference electrode is that it has a stable potential and that it is not substantially polarised during the experiment. Hence it is common to use the highly convenient aqueous calomel electrode in many experiments in all solvents. Even so, a very wide range of reference electrodes have been used in non-aqueous solvents. Where there is any doubt about the potential of the reference electrode, it is recommended to check the potential of a standard couple, e.g. ferrocene/ferrocinium ion, by cyclic voltammetry. This is also the easiest way to compare potential scales in different solvents it is assumed that the potential of this couple, where both halves of the couple are poorly solvated, is independent of solvent [2j. 

The conversion to the aqueous standard hydrogen electrode as reference half-cell requires an extra-thermodynamic assumption, either the assumption of a solvent independent reference redox system or other assumptions employed in calculating single-ion transfer properties. Details about the procedure and data for univalent cationimetal systems were published [13]. The redox couple ferrocenium ion/ferrocene as reference electrode system is not very suited for such a conversion as the ferrocenium cation undergoes interactions with water and thus impairs the extra-thermodynamic assumption for aqueous solutions. This becomes apparent when... 

Efforts have been made to prepare oxygen-stable ferrocene-type reference electrodes the most promising is a decamethylferrocene (DMFc) reference electrode. It had a structure PtlO.004 M DMFc -1- 0.004 M DMFcPFg + 0.1 M BU4NBF4 (AN) and could be used in the presence of oxygen [34]. It was prepared for AN solutions, but similar reference electrodes should be possible for a variety of solvents. Moreover, the DMFc /DMFc couple can be used as a potential reference redox system and its potential is considered to be more solvent independent than that of the Fc /Fc couple [194]. However, the reduction of oxygen by DMFc may occur in acidic DCE [35]. 

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