Potential, chemical irreversible

A baseline potential pulse followed each current pulse in order to strip extracted ions from the membrane phase and, therefore, regenerated the membrane, making it ready for the next measurement pulse. This made sure that the potentials are sampled at discrete times within a pulse that correspond to a 6m that is reproducible from pulse to pulse. This made it possible to yield a reproducible sensor on the basis of a chemically irreversible reaction. It was shown that the duration of the stripping period has to be at least ten times longer than the current pulse [53], Moreover the value of the baseline (stripping) potential must be equal to the equilibrium open-circuit potential of the membrane electrode, as demonstrated in [52], This open-circuit potential can be measured prior to the experiment with respect to the reference electrode. 

In this solvent, using CV and Osteryoung square-wave voltammetry (OSWV) under high vacuum conditions at room temperature, Cgo displays a one-electron, chemically reversible oxidation wave at +1.26 V vs. Fc/Fc+. TBAPFe was used as the supporting electrolyte. Under the same conditions, the first one-electron oxidation of C70 occurs at +1.20 V, 60 mV more negative (easier to oxidize) than that of Cgo- Both oxidations are electrochemically quasireversible with A pp = 80 mV. In addition, a second oxidation wave is observed for C70 close to the limit of the solvent potential window at+1.75 V. However, this wave appears to be chemically irreversible (see Fig. 3) [36]. 

Two chemically irreversible, multielectron oxidation steps are observed in PhCN at (peak potential) +1.13 and +1.35 V vs. Fc/Fc+ for C60H2. Irreversibility persists even at scan rates up to 8 V s . The first oxidation is consistent with the loss of two protons and two electrons from C60H2 to form Cfio- The second oxidation has been attributed to the formation of Cgo" , since the peak potential for that step is in good agreement with other reported... 

The electrochemical behavior of Np ions in basic aqueous solutions has been studied by several different groups. In a recent study, cyclic voltammetry experiments were performed in alkali ([OH ] = 0.9 — 6.5 M) and mixed hydroxo-carbonate solutions to determine the redox potentials of Np(V, VI, VII) complexes [97]. As shown in Fig. 2, in 3.1 M LiOH at a Pt electrode Np(VI) displays electrode processes associated with the Np(VI)/Np(V) and Np(VII)/Np(VI) couples, in addition to a single cathodic peak corresponding to the reduction of Np(V) to Np(IV). This latter process at Ep —400 mV (versus Hg/HgO/1 M NaOH) is chemically irreversible in this medium. Analysis of the voltammetric data revealed an electrochemically reversibleNp(VI)/Np(V)... 

The cyclic voltammograms show three reduction waves in the potential window between 0 and —1.3 V versus SCE (Fig. 6.20). The first process is fully reversible, while the second one is chemically irreversible since the dianion is subject to a bond breaking of the cyclopropane ring, known as the retro-Bingel reaction.75 The third... 

Catalytic reactions in electrochemistry — When the product of an electrochemical reduction reaction is regenerated by a chemical reoxidation, or when the product of an electrochemical oxidation is regenerated by a re-reduction, the regeneration reaction is called a catalytic reaction. For thermodynamic reasons the chemical oxidant (or the reductant) has to be electro-chemically irreversible in the potential range where the catalyst is electroactive. The reduction of Ti(IV) in the presence of hydroxylamine is an example for an oxidative regeneration [i, ii] ... 

A mixed electrochemical reaction with a mixed potential difference, inevitably of reduced output. Detailed consideration of the breakdown of the oxygen reduction and hydrogen oxidation paths would be involved in a calculation, including any chemically irreversible cul-de-sacs, such as platinum particles insulated from the electrode, which catalyse irreversible combustion rather than reversible electrochemical oxidation. 

The thermochemical cycle in Scheme 4 can be used to estimate the effect of one-electron oxidation on metal-hydride acidities. The method is analogous to one that has been extensively used to investigate organic cation radicals [10c]. Eq. 29 shows that measurements of °ox(MH) and °ox(M ) provide relative p a data for metal hydrides and their cation radicals. Absolute values for p a(MH +) are obtained if the acidities of the neutral hydrides are known. The oxidation potentials of 18-electron hydrides can be readily obtained by cyclic voltammetry. In our experience, the waves that are obtained are frequently chemically irreversible, even at rather high scan rates. Consequently, the oxidation peak potentials will be kinetically shifted and represent minimum values for the true °ox(MH) data, the estimates for p a(MH +) represent maximum values, and calculated Ap a are minimum values. 

The respective redox potentials of the first and second reduction step in THF [18, 21] are -2.68 and -3.31 V. The corresponding values in DMF [101] are -2.57 and -3.13 V. [All redox potentials in this chapter are reported relative to the ferro-cenelferricinium couple.) Both reduced forms of bpy are intrinsically stable in strictly aprotic and anaerobic media. For bpy and some other polypyridines, the radical anions and/or dianions have been characterized by UV-Vis absorption, resonance Raman, and/or EPR spectroscopy [18, 21, 70, 71, 102, 103]. Bipyridine derivatives with strongly electron-accepting groups -C(0)OEt or -Ph in 4,4 positions have a third reduction step at very negative potentials [18, 21]. Free phen [104] and tpy [101] are reduced by one electron at potentials similar to that of the first bpy reduction. The second reduction steps of phen and tpy are chemically irreversible. 

The complexes can be both oxidized and reduced reduction potentials for many of the complexes are shown in Table 6. Cyclic voltammograms of Re(a-diimine)(CO)3X show that in most cases the first oxidation is chemically irreversible at scan rates of 0.1-0.2 V s however, at much faster sweep rates (>100 V s ) a reversible wave is observed at 1.32 V (vs. SCE) in MeCN for Re(bpy)(CO)3Cl [60]. The first oxidation is metal based and is followed by the rapid loss of carbon monoxide due to the weakening of the Re 7r-backbonding... 

All of the quarternary arsonium salts are reduced in a chemically irreversible two-electron process at mercury electrodes in aqueous solution and thereby resemble the analogous phosphonium salts " . In contrast, the stibonium salts, with the exception of the tetramethylstibonium ion, are reduced in two distinct one-electron steps at mercury electrodes . The arsonium salts are in aqueous solution reduced at potentials which are shifted approximately 300 mV in the positive direction compared with those of the corresponding phosphonium salts , whereas the first electron transfer to the stibonium salts takes place at a further 0.5-1.0 V in the positive direction cf Table 1. 

TABLE 7. Half-wave potentials, ( 1/2) for the chemically irreversible reduction of RjM in aprotic solvents"... 

The mechanism proposed above is in disagreement with a later, more detailed study of the related 1,2,5-triphenylarsole (4) in MeCN . Cyclic voltammetry of 4 shows two reduction peaks (cf Table 8), of which the first is a chemically reversible one-electron reduction on the time scale of the voltammetric experiment (v = 0.05-0.5 V s ), whereas the second is chemically irreversible . Changing the As substituent from Ph to Me shifts the reduction potentials in the negative direction (cf Table 8) and diminishes the chemical stability of the anion radical (the reduction is not completely reversible at v = 1V s ) . 

Exhaustive electrolysis of 4 in MeCN at the potential of the first reduction requires 2 F mol", and not 1F mol as suggested from the CV experiments the final product is l-//-2,5-diphenylarsole (5) . In contrast, chemical reduction by Li or K in DME gives a stable solution of the anion radical (4 ) as confirmed by ESR spectroscopy . The difference between the electrochemical and the chemical reduction has been explained by the difference in the media due to small amounts of residual water in MeCN during the electrochemical reduction, the disproportionation equilibrium (equation 62), which is strongly displaced to the left, is pulled to the right by protonation of the dianionic product (equation 63) in accord with the observation of a chemically irreversible second electron transfer in CV . 

With only few exceptions, all of the electrochemical oxidation studies have been carried out in MeCN containing varying amounts of water. The compounds of the structure R R R M, R, R, R = aryl, alkyl, M = As, Sb (Bi), arc in most cases (for exceptions see below) oxidized in a chemically irreversible process in MeCN. Half-wave potentials for the chemically irreversible oxidations of R R R M are given in Table 14. 

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