Chemically 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. 

All ECi adsorption coupled mechanisms have been verified by experiments with azobenzene/hydrazobenzene redox couple at a hanging mercury drop electrode [86,128,130]. As mentioned in Sect. 2.5.3, azobenzene undergoes a two-electron and two-proton chemically reversible reduction to hydrazobenzene (reaction 2.202). In an acidic medium, hydrazobenzene rearranges to electrochemically inactive benzidine, through a chemically irreversible follow-up chemical reaction (reaction 2.203). The rate of benzidine rearrangement is controlled by the proton concentration in the electrolyte solution. Both azobenzene and hydrazobenzene, and probably benzidine, adsorb strongly on the mercury electrode surface. 

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... 

U(III) species and a second three-electron reduction to give U(0) metal. The first reduction, U(IV)/U(III) couple, is elec-trochemically and chemically irreversible except in hexamethylphosphoramide at 298 K where the authors report full chemical reversibility on the voltammetric timescale. The second reduction process is electrochemically irreversible in all solvents and only in dimethylsulfone at 400 K was an anodic return wave associated with uranium metal stripping noted. Electrodeposition of uranium metal as small dendrites from CS2UCI6 starting material was achieved from molten dimethylsulfone at 400 K with 0.1 M LiCl as supporting electrolyte at a platinum cathode. The deposits of uranium and the absence of U CI3, UCI4, UO2, and UO3 were determined by X-ray diffraction. Faradaic yield was low at 17.8%, but the yield can be increased (55.7%) through use of a mercury pool cathode. 

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)... 

This criterion is good for establishish whether a process is under thermodynamic control. Care should be taken however to understand the term reversibility in this case. The folding of a protein is generally per se a chemically irreversible process, in the sense that the chemical equilibrium is overwhelmingly shifted towards the folded form - there is not a low activation energy barrier between the native folded and the unfolded form and a corresponding chemical equilibrium in the native state between the two forms. Thus, in the case of the thermodynamic hypothesis of... 

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... 

The diffusion length of photogenerated charge carriers is one of the important parameters governing the efficiency of a solar cell. In conventional cells, this is an intrinsic property of the semiconductor and its purity [34]. However, in DSSCs, the diffusion length is a function of the rate of reaction (4) and, thus, varies with different redox couples, surface treatments, and so forth. When the oxidation of R [reaction (2)] is chemically irreversible, the diffusion length of electrons is effectively infinite, whereas with kinetically fast, reversible redox couples (see Section VI), it approaches zero with unpassivated interfaces. 

Although the one-electron reduction of nitrobenzene to its radical anion in dipolar aprotic solvents is a classical example of a chemically reversible redox couple, the reductions of many organic compounds are chemically irreversible. The redox behavior of /7-chlorobenzonitrile is typical of those systems in which the initial electrode product undergoes rapid, irreversible chemical reaction to give another reducible species. 

B.. .. chemically irreversible at this scan rate. In addition, the ring-opening reaction,... 

Electrochemical studies on 134 and [CoPdPt( -dppm)2(CO)3X]+ (X = I, PPh3) in DMSO show electrochemically reversible, but chemically irreversible, two-electron reductions attributed to the cationic species [Co-PdPt( -dppm)2(CO)3(L)] + (L = DMSO and PPh3) and several ill-defined oxidation steps (203). For X = CO an irreversible reduction is observed, indicating a differing electrochemical mechanism (204). 

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