Potential reference, 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. 

Both in acetonitrile and in other non-aqueous solvents, a major problem arises in terms of the manner in which the potential values are reported by various investigators. Koepp, Wendt, and Strehlow [6] noted that hydrogen ion is the poorest reference material on which to base nonaqueous potentials because of the extreme differences in its solvation in various solvents. On the basis of an investigation of the solvent dependence of 18 redox couples, these investigators concluded that ferrocene/ferrocenium ion (i.e. bis(cyclopentadienyl)iron(III/II), abbreviated as Fc+ /Fc°) and/or cobal-tocene/cobalticenium ion represented optimal potential reference materials for nonaqueous studies. On the basis of their minimal charge (+1, 0) and their symmetry (treated as though they were roughly spherical), the potentials of these two redox couples are presumed to be relatively independent of solvent properties. 

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. 

Some redox couples of organometallic complexes are used as potential references. In particular, the ferrocenium ion/ferrocene (Fc+/Fc) and bis(biphenyl)chromium(I)/ (0) (BCr+/BCr) couples have been recommended by IUPAC as the potential reference in each individual solvent (Section 6.1.3) [11]. Furthermore, these couples are often used as solvent-independent potential references for comparing the potentials in different solvents [21]. The oxidized and reduced forms of each couple have similar structures and large sizes. Moreover, the positive charge in the oxidized form is surrounded by bulky ligands. Thus, the potentials of these redox couples are expected to be fairly free of the effects of solvents and reactive impurities. However, these couples do have some problems. One problem is that in aqueous solutions Fc+ in water behaves somewhat differently to in other solvents [29] the solubility of BCr+BPhF is insufficient in aqueous solutions, although it increases somewhat at higher temperatures (>45°C) [22]. The other problem is that the potentials of these couples are influenced to some extent by solvent permittivity this was discussed in 8 of Chapter 2. The influence of solvent permittivity can be removed by... 

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. 

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. 

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]. 

A bis(pentamethylcyclopentadienyl)iron reference electrode was described its potential was pH independent in aqueous buffer solutions. Unlike any other ferrocene electrodes previously reported, these electrodes were stable in aqueous solvents in air as well. 

To compare redox potentials of aqueous and nonaqueous systems, a variety of internal references were investigated. In 1984, Gritzner and Kuta recommended two systems for nonaqueous electrolytes that are also accepted by lUPAC [244]. The solvent-independent organometallic redox couples are fer-rocene/ferrocenium (Fc/Fc+) (E = 0.158 V vs saturated calomel electrode (SCE)) and bis(biphenyl)chromium(0)/(l) (BCr/BCr+) (E = —0.82 V vs SGE) [245]. Very stable electrode redox potentials Ei/i vs Ag/Ag+-cryptand electrode of 0.478 V for Fc/Fc+ and —0.616 V for BCr/BCr+ in EMIm tetrafluoroborate were measured [246]. 

The establishment of a solvent-independent reference electrode for the comparison of redox potentials in nonaqueous systems has a long story. It began with the concept of RblRb+ or Rb(Hg)IRb, followed by the proposal of using organometallic redox couples [2], In order to limit the number of redox systems used and then make the comparison easier, lUPAC recommended that the systems ferrocenelferrocenium, Fc° , and bis(biphenyl)chromium(0)lbis(biphenyl)chromium(l), BCi , need to be used as internal reference redox systems in nonaqueous media [2], These two complexes were selected arbitrarily from several published redox systems [2],... 

Online CD detectors are now commercially available for use with HPLC that are inherently more sensitive than corresponding OR detectors and not affected by solvent changes to the same extent and are thus more gradient compatible [121]. Provided Ae and the concentration of an analyte are known with good precision/accuracy, the measurement of CD will allow the determination of enantiomeric purity. In addition, with CD-based detection systems, both chiroptical and ordinary absorbance can be determined simultaneously allowing the measurement of the g-factor (or dissymmetry factor), which is defined as the ratio of the CD to the absorbance (AA/A) [122]. The g-factor is concentration independent and its measurement allows a more reliable determination of enantiomeric purity (without using a CSP) with reference to standards of known enantiomeric composition irrespective of their concentration [123]. A small number of recent literature examples have suggested the potential use of achiral HPLC with online CD detection for the determination of extreme enantiomeric ratios [121, 124-126] however, chiral separation techniques currently provide a more reliable measurement of enantiomeric purity. 

Throughout this discussion we have considered cells in which the electrolytic solution is an aqueous solution. The same methods can be used to define standard half-cell potentials in any solvent system. However, it is important to remember that when the reference state is defined as the infinitely dilute solution of a solute in a particular solvent, the standard state depends upon that solvent. The values so obtained are not interchangeable between the different solvent systems. Only if the standard states could all be defined independently of the solvent would the values be applicable to all solvent systems. 

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