Redox temperature dependence

FIGURE 6.24 Redox behavior of the methano-dimer of a-tocopherol (bis(5-tocopheryl) methane, 28) temperature dependence of the oxidation with bromine. 

Two final concerns must be addressed surface oxidation state and temperature dependence. Whenever one deposits a redox-active species on a metal surface, the oxidation state of the adsorbate (and therefore the OMTS bands) may change. One example is the adsorption of a biaxially substituted dicyano cobalt phthalocy-anine salt, MCoPc(CN)2 (where M = K or Cs), on gold to form the reduced species CoPc [111]. A second example is provided by the adsorption of TCNE on gold, silver, and copper. In that order, the charge state of TCNE on the surface ranges from 0 to 3, and the OMTS reflects these changes. 

In these experiments, AE was obtained from temperature dependent redox potential measurements for the isolated, monomeric couples based on a thermochemical cycle. 

The temperature dependence of ET rates between cytochrome-c and the reaction center in Chromatium (Figure 1), fitted to eqs 3-5 (8), demonstrated that, unlike many redox reactions in... 

We may illustrate this approach to the determination of the nuclear factor by the elegant studies performed by Gray and co-workers, who have determined the thermodynamic properties and the rate temperature dependence for the electron transfer between Ru(NH3) covalently bound to the histidine residues of some proteins, and the redox eenter of these proteins [110, 111, 112, 113]. The experimental results obtained for cytochrome c [110] and azurin [111, 112] are very similar. Using the thermodynamic data and the value or the upper limit of Ea reported in these studies, we deduce from Eq. (23) ... 

A variety of physical methods has been used to ascertain whether or not surface ruthenation alters the structure of a protein. UV-vis, CD, EPR, and resonance Raman spectroscopies have demonstrated that myoglobin [14, 18], cytochrome c [5, 16, 19, 21], and azurin [13] are not perturbed structurally by the attachment of a ruthenium complex to a surface histidine. The reduction potential of the metal redox center of a protein and its temperature dependence are indicators of protein structure as well. Cyclic voltammetry [5, 13], differential pulse polarography [14,21], and spectroelectrochemistry [12,14,22] are commonly used for the determination of the ruthenium and protein redox center potentials in modified proteins. 

Ehrlich et al. (submitted) measured Cu isotopic fractionation between aqueous Cu(II) and covellite between 2 and 40°C (Fig. 10). The temperature-dependent isotope fraction is fairly large 3%o) and hints at a redox control of Cu isotopic variability in abiotic systems. Marechal and Sheppard (2002) conducted experiments at 30 and 50°C between malachite and a chloride solution for Cu isotope fractionation and between smithsonite and a nitrate solution for Zn. They found that, in this temperature range, Cu in malachite is 0.2 to 0.4%o lighter than in the chloride solution. Replacing the chloride by nitrate ion reduces fractionation which indicates that the coordination of the Cu ion dictates isotopic fractionation. In contrast, Zn isotope fractionation between smithsonite and fluid is extremely small (<0.1%o). 

It should be noted that in leaching the rate of reaction is of great importance, since the temperature used is relatively low and many factors combine to keep the rate slow. For practical reasons, leaching studies are performed on a homogenized sample, testing the temperature dependency of the rate of reaction (concentration and time) in various leaching mixtures. The most important variables in aqueous systems are pH and redox... 

The main environmental factors that control transformation processes are temperature and redox status. In the subsurface, water temperature may range from 0°C to about 50°C, as a function of climatic conditions and water depth. Generally speaking, contaminant transformations increase with increases in temperature. Wolfe et al. (1990) examined temperature dependence for pesticide transformation in water, for reactions with activation energy as low as lOkcal/mol, in a temperature range of 0 to 50°C. The results corresponded to a 12-fold difference in the half-life. For reactions with an activation energy of 30kcal/mol, a similar temperature increase corresponded to a 2,500-fold difference in the half-life. The Arrhenius equation can be used to describe the temperature effect on the rate of contaminant transformation, k ... 

Table 3 provides entropies for those species that are needed to determine the temperature dependence of standard potentials for alkali metal redox couples. AU of these entropies were obtained from values published by NIST [11]. The resulting temperature dependences agree well with values tabulated by Bratsch [17]. 

In Table 1 are summarized representative examples of self-exchange rate constant data for a variety of different types of redox couples based on metal complexes, organometallic compounds, organics and clusters. Where available the results of temperature dependence studies are also cited. For convenience, data obtained from temperature dependence studies are presented as enthalpies and entropies of activation as calculated from the reaction rate theory expression in equation (14). 

This is a striking result since it suggests that simple absorption band measurements can be used to calculate Ea for thermal electron transfer. Note that for unsymmetrical cases where AE 0, both Eop and AE must be known AE can sometimes be estimated from temperature dependent redox potential measurements. 

The symbols appearing in this equation are defined in Chapter 3. At 25°C, Equation 17.20 simplifies to Equation 3.18. (However, in molten salt experiments it is extremely rare for this equation to be employed at such a low temperature ) The reversibility of an experimental system is often judged by comparing the experimental values of Ep - Ep/21, Ep/2 - E and AEp to the theoretical values calculated from these equations in the case of a reversible redox system. Therefore, it is important to point out that these parameters are temperature dependent and that they increase with increasing temperature. The complete theoretical expressions for these parameters are given in Equations 17.21 to 17.23, respectively [67]. 

A further example of the use of this technique to introduce a ferrocene redox centre to a platinum surface is given in equation (32). A comparative survey was made of the rates of heterogeneous charge transfer between the platinum electrode and ferrocene both in solution and immobilized on the surface. Both processes show an Arrhenius temperature dependence but AGact(soIii) / A( ACT(surface bound). Absolute rate theory was unsatisfactory for the surface reaction and the need to involve electron tunnelling and a specific model for the conformation of the surface was indicated.66... 

Figure 5.16 Enantioselectivity of the redox reaction of ADH from T. ethanolicus with different alcohols (Pham, 1990). Temperature dependence of free energy of activation differences for 2-butanol and 2-pentanol open squares 2-butanol open circles 2-pentanol filled square reduction of 2-butanone filled circle reduction of 2-pentanone.

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