Reference redox systems

The ferrocene/ferrocenium reference redox system at platinum fulfills these requirements fairly well [4-6]. Another system which has been recommended is bis(biphenyl)chromium (0)/bis(biphenyl)chromium (+1) (BCr+ /BCr) [5, 7]. Several other systems have been suggested, and used sporadically, such as cobaltocene/cobaltocenium, tris(2,2 -bipyridine) iron (I)/tris(2,2 -bipyridine) iron (0), Rb+/Rb(Hg), and so on. 

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. 

Tab. 6.4 Potentials of the Ag/Ag+ and Hg/Hg2+ reference electrodes and Fc/Fc+ reference system in various organic solvents (V vs BCr reference redox system in 0.1 M Bu4NCI04 unless otherwise stated in footnote at 25 °C)... 

In reporting the potential data, the reference redox system used should be indicated by such symbols as i/2(bct) and prc). Detailed information should also be given concerning the cell construction, solvent purification, impurities in the solution, etc. 

DMSO, because Fc+ is rapidly reduced to Fc in these solvents. The Fc+/Fc couple is useful as a reference redox system rather than as a reference electrode. 

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 many cases platinum or silver wires serve as quasireference electrodes, however, they have to be calibrated by a reference redox systems. [The term pseudoreference (literally false reference) electrode is also used in the literature, however, the term quasireference electrode is preferred]. 

Several problems are encountered when potentials in different solvents are sought compared to the potential scale in water. A variety of approaches [186-194] have been followed to attack this problem usually the approach has been to introduce some kind of nonthermodynamic assumption, such as the supposition that certain large, monovalent ions (Rb, CS ) [191] or redox systems [186-188] of the charge type n/n + 1 (preferably 0/ + 1 [186,187]) have a nearly equal free energy of solvation in the two solvents so that the free energy of a transfer of the reference ion is small. The redox couples [194] ferro-cenium/ferrocene and bis(biphenyl)chromium(I)/bis(biphenyl)chromium(0) (BCr" "/BCr) have been recommended as reference redox systems for measurements in nonaqueous solvents however, an investigation concluded [195] that the electrochemistry of ferrocene in MeCN at microelectrodes was far from ideal, as some film formation may occur. 

Ferrocenium ion/ferrocene (Fc" /Fc) and bis(biphenyl)chromium(I)/bis(biphenyl)chrom-ium (O) (BCr" /BCr) have been proposed [194] as reference redox systems for nonaqueous electrochemistry. Both materials have been found suitable for practical referencing, for example, in THF [219], covering complementary potential ranges. 

Cyclic voltammetric measurements were carried out with a Pine Instrument Co. Model AFRDE4 potentiostat. All electrochemical measurements were done in benzonitiile at ambient temperature at a glassy carbon working electrode, with a saturated calomel electrode or Ag/Ag (0.01 M) as a reference, and a platinum-wire counterelectrode. The electrolyte was 0.1 M tetra-n-butyl-ammonium hexafluorophosphate, and ferrocene was added as an internal reference redox system. 

If voltammetric and related techniques are used, the ohmic drop should be either compensated (now this is usually done by the software or hardware of electrochemical devices [69]), or reduced by using, for example, a Luggin (Lu in-Gaber) capillary (see in Ref. [12]). Another important technical detail is that the components of reference redox systems (such as fer-rocene/ferrocenium) are frequently added immediately into the working compartment when voltammetry-hke techniques are applied. 

IRRS Internal reference redox systems/intemal reference redox scale... 

Note Other reference redox systems include Rb/Rb and Fefbpylj / Fe(bpy)3 + (where bpy 2,2 -bipyridine). 

Additionally, one thing has to be respected when comparing the electroactivity ranges of electrode-medium systems. It is the following difficult problem to be solved Any comparison assumes the existence of a reference electrode which has the same potential in all media to be considered. This potential, however, is a function of the free energies of solvation of the two species wliich comprise the reference redox system the free energies vary with the medium. 

Reference redox systems for the determination of redox potentials... 

Reference Redox Systems in Nonaqueous Systems and the Relation of Electrode Potentials in Nonaqueous and Mixed Solvents to Standard Potentials in Water... 

Two different approaches are used to establish electrochemical series in nonaqueous and mixed aqueous-nonaqueous systems (1) a reference redox system [5] or (2) a reference electrode in combination with a bridge to suppress the liquid jtmction potential [6]. 

Studies on Gibbs energies of transfer of Rb" [9-11] showed that the interaction of Rb" with different solvents is not negligible. The Rb IRb couple turned out to be too much solvent dependent to be used as a reference redox system to relate electrochemical properties in different solvents. 

Strehlow and coworkers studied various organometallic complexes. They formulated requirements for suitable reference redox systems [12] (1) The ions or molecules forming the reference redox system should preferably be spherical with as... 

Reference Redox Systems in Nonaqueous Systems and the Relation of. 

Other reference redox systems have been proposed and used, such as tris(2,2 -bipyridine)iron(I)/fr (2,2 -bipyridine)iron(0) [21], 4,7-dimethyl-l,10 phenanthroline iron(II) [4], and redox systems based on polynuclear aromatic hydrocarbOTis and the respective radical ions [22-24],... 

Electrode potentials should only be reported in thermodynamic arrangements. The most convenient way in polarography and cyclic voltammetry is the use of a reference redox system in the same electrol3de as the system under study. The Ag lAg electrode seems applicable to many solvents and may be used as reference electrode in potentiometric investigations. 

There are two aspects to reference redox systems. One point is the possibility of compiling electrode potentials in a variety of solvents and solvent mixtures, which are not affected by unknown liquid junction potentials. Unfortunately very frequently aqueous reference electrodes are employed in electrochemical studies in nonaqueous electrolytes. Such data, however, include an unknown, irreproducible phase boundary potential. Electrode potentials of a redox couple measured in the same electrolyte together with the reference redox system constitute reproducible, thermodynamic data. In order to stop the proliferation of—in the view of the respective authors— better and better reference redox systems, the lUPAC recommended that either ferrocenium ion/ferrocene or bw(biphenyl)chromium(l)/te(biphenyl)chromium(0) be used as a reference redox system [5]. 

The second point is the assumption that the potential of a half-cell containing the reference redox system is—within experimental error— independent of the nature of the solvent. This assumption is outside the realm of exact thermodynamics and thus open to discussion. As for any extra-thermodynamic assumption it is impossible to prove its validity. This point should be kept in mind especially when discussing single-ion transfer properties. 

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... 

In this book, there are other chapters related to nonaqueous systems. Chapter 1 by Inzelt is on the electrode potentials and includes a section on the problem to relate the electrode potentials between different media. Chapter 2 by Gritzner is on the reference redox systems in nonaqueous systems and their relation to water. Chapter 3 by Tsirlina is on the liquid junction potential and somewhat deals with the problem between different solvents. Chapter 7 by Bhatt and Snook is on the reference electrodes for room temperature ionic liquids. See these chapters as well. 

The Fc /Fc couple has been recommended by Gritzner and Kuta as a reference redox system for potentials in nonaqueous solutions (see Chap. 2 or [190]). Because the reference electrode of the couple can be used only under limited conditions, the potential of the couple should be measured, in general, as a half-wave potential in cyclic voltammetry. 

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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