Shifting potential

However, for a constant potential shift e the relative probability... 

Other Coordination Complexes. Because carbonate and bicarbonate are commonly found under environmental conditions in water, and because carbonate complexes Pu readily in most oxidation states, Pu carbonato complexes have been studied extensively. The reduction potentials vs the standard hydrogen electrode of Pu(VI)/(V) shifts from 0.916 to 0.33 V and the Pu(IV)/(III) potential shifts from 1.48 to -0.50 V in 1 Tf carbonate. These shifts indicate strong carbonate complexation. Electrochemistry, reaction kinetics, and spectroscopy of plutonium carbonates in solution have been reviewed (113). The solubiUty of Pu(IV) in aqueous carbonate solutions has been measured, and the stabiUty constants of hydroxycarbonato complexes have been calculated (Fig. 6b) (90). 

One example of a sequence determinant of redox potentials that has been identified in this manner is an Ala-to-Val mutation at residue 44, which causes a 50 mV decrease in redox potential (and vice versa) in the rubredoxins [68]. The mutation was identified because the sum of the backbone contributions to ( ) of residues 43 and 44 change by 40 mV due to an —0.5 A backbone shift away from the redox site. This example points out the importance of examining the backbone contributions. The corresponding site-specific mutants have confirmed both the redox potential shift [75] and the structural shift [75]. 

A second example is that of an Ala-to-Cys mutation, which causes the fonnation of a rare SH S hydrogen bond between the cysteine and a redox site sulfur and a 50 mV decrease in redox potential (and vice versa) in the bacterial ferredoxins [73]. Here, the side chain contribution of the cysteine is significant however, a backbone shift can also contribute depending on whether the nearby residues allow it to happen. Site-specific mutants have confirmed the redox potential shift [76,77] and the side chain conformation of cysteine but not the backbone shift in the case with crystal structures of both the native and mutant species [78] the latter can be attributed to the specific sequence of the ferre-doxin studied [73]. 

The relates to the faet that this is not the true standard deviation, but an estimate to measure the potential shift in the distribution. 

Therefore, Cp as predieted by ean be used to measure the potential shift in eomponent toleranee distributions due to its formulation. 

Fig.l. Results for the system Zn/Cu. Calculated charge transfer from (shown as positive) or towards (shown as negative) the impurity site obtained according to eqn.(2) of text (dashed line) as a function of the potential shift applied on the impurity potential. The variation given by eqn.l is indicated by the solid line while the dotted line indicates the solution which includes corrections due to the redistribution of the impurity charge. 

Langenegger, E. E. and Callaghan, B. G., Use of an Empirical Potential Shift Technique for Predicting Dezincihcation Rates of a/3-brasses in Chloride Media , Corrosion, 28, 245 (1972)... 

To prompt remedial action when stainless-steel agitators in a phosphoric-acid-plant reactor show a potential shift towards a value associated with active corrosion due to an increase in corrosive impurities in the phosphate rock. 

For normal field work the potentiometric voltmeter is the more popular instrument, being usually of lighter construction and not requiring calibration against a standard cell. Where extremely small potentials (usually potential shifts) of the order of 1 mV are to be measured, however, the potentiometer is more suitable and accurate. 

Explain clearly why only a fraction of the energy shift (associated with a potential shift) is used for increasing the activation energy barrier. 

Data are also available with a-acetylenic aliphatic sulphones, which involve only two steps i.e., saturation of the triple bond without subsequent cleavage of the Caliphalic—S bond, since it is not reactive. However, the introduction of an aromatic ring to the S02 group does not lead, contrary to what is observed with enones, to a potential shift toward less reducing potential values. Thus, the aromatic moiety introduced apparently does not bring any additional conjugation effect but even seems to decrease the activation of the unsaturated bond, as shown by data in Tables 6 and 7 where most of the potentials refer to the same saturated calomel electrode under similar experimental conditions. 

While Eq. (36) is valid for 9 = 1, a qualitatively similar equation is obtained at any value of 9. Since the condition 9 - 1 is difficult to reach experimentally, the value of AEaB0 (adsorption potential shift) is often estimated by means of extrapolation to 9 = 1. This procedure is very delicate and the result is often misleading. The variation of EOm0 with 9 may be linear or nonlinear, depending on lateral interactions between... 

A comparison of the adsorption of a given molecule at the air/solution and at the metal/solution interface is a convenient way of obtaining some information on the role of the metal surface.93,94 At the air/solution interface the potential shift is simply... 

Equation (38) still includes the electronic term. On the other hand, SB(dip) ir may differ from gB(dip) at the metal surface as a consequence of different interactions with the environment. Therefore the interpretation of adsorption potential shifts is always subject to a number of assumptions that cannot be easily checked. 

Figure 8. Typical adsorption potential shifts as a function of adsorbate surface concentration. (1) At the free surface of a solution (real behavior), (2) ideal behavior, and (3) at a metal (Hg)/solution interface. Experimental points for adsorption of 1,4-butanediol from Ref. 328.

More recently, the curvature at air/solution interfaces has been accounted for by Nikitas and Pappa-Louisi98 in terms of a specific molecular model that predicts a linear dependence of (lM/ ) on (1/0). The same model also reproduces the behavior at metal/solution interfaces, specifically Hg electrodes, for which most of the experimental data exist. Nikitas treatment provides a method for an unambiguous extrapolation of the adsorption potential shift to 0= 1. However, the interpretation of the results is subject to the difficulties outlined above. Nikitas approach does provide... 

A constant current flowing across the electrolyte solution/eleetrode interface causes a potential shift because of the changing concentrations of educts and produets, which arc consumed and generated respee-tively. The change of the electrode potential as a funetion of time is recorded in a ehronopotentiometrie experiment. Depending on the rate of the electrode reaetion various mathematical treatments are possible providing access to rate constants for details see e.g. [OlBar]. (Data obtained with this method are labelled CH.)... 

The most striking electrochemical feature of Rieske proteins is the pH dependence of the redox potential. This pH dependence has first been demonstrated for the Rieske protein from Chlorobium, where the redox potential shifted from +165 mV at pH 6.8 to +60 mV at... 

C60 has been used to produce solvent-cast and LB films with interesting photoelec-trochemical behavior. A study of solvent-cast films of C60 on Pt rotating disc electrodes (RDEs) under various illumination conditions was reported [284]. Iodide was used as the solution-phase rednctant. The open-circuit potential shifted by 74 mV per decade of illumination intensity from a continuous wave (cw) argon-ion laser. The photocurrent versus power was measured at -0.26 V under chopped illumination (14-Hz frequency, vs. SCE) up to 30 mW cm and was close to linear. The photoexcitation spectrum (photocurrent versus wavelength) was measured at 0.02 V (vs. SCE) from 400 to 800 mn and found to be... 

An electrical potential shifts the energy levels in a metal. The arrows show the direction of electron flow. 

According to the macroscopic model, the adsorption potential shift is due to the removal of some solvent molecules, s, from the surface region and accommodating there the oriented molecules of adsorbate, B."" Using the assumptions listed in Ref 114, the dependence for A% is of the form... 

Adsorption potential shifts are higher at the air/solution than at the Hg/solution interface. This aspect has been discussed in terms of nonlocal electronic effects in the metal surface and different molecular orientation atthetwo interfacee. " "... 

For many electrodes it is fonnd that one H+ or OH ion is involved in the reaction per electron hence, the electrode potential becomes 0.059 V more negative when the pH is raised by 1 unit this is the same potential shift as found for the hydrogen electrode. For such electrodes a special scale of electrode potentials is occasionally employed These potentials, designated as E refer to the potential of a reversible hydrogen electrode (RHE) in the same solution (i.e., at the given pH). For the electrodes of the type considered, potentials in this scale are independent of solution pH. 

In the case of anodic currents, the potential shifts in a positive direction, and AE has positive values in the case of cathodic currents, AE has negative values. In expressions such as high polarization and the polarization increases, the absolute value of polarization denoted simply as AE is implied in the case of cathodes. 

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