Electrochemical potential transients

The chemiosmotic theory postulates that protons moving back into the matrix via an ATP-synthase drive the formation of ATP. Evidence for this is that an electrochemical potential gradient for protons can support the formation of ATP in the absence of electron-transfer reactions. A transient pH gradient that pulls protons into the matrix can be set up by first incubating mitochondria at pH 9, so that the inside becomes alkaline, and then quickly lowering the pH of the suspension medium to 7 (fig. 14.21). 

Secondly, selectivity is not always achievable. For example, permselectivity of ion-exchanging polymer films fails at high electrolyte concentration. We have shown that even if permselectivity is not thermodynamically found, measurements on appropriate time scales in transient experiments can lead to kinetic permselectivity. To rationalise this behaviour we recall that the thermodynamic restraint, electrochemical potential, can be split into two components the electrical and chemical terms. These conditions may be satisfied on different time scales. Dependent on the relative transfer rates of ions and net neutral species, transient responses may be under electroneutrality or activity control. 

The cardiac action potential can be divided into five phases, numbered 0-4. These phases result from the subsequent or parallel activity of ion channels (Figure 3.2) and/or transporters. Initially, the cell is polarized to near the electrochemical potential for potassium ions because of high K+ conductance at rest. A rapid depolarization is initiated by the activation of the fast inward Na+ current (phase 0). This depolarization is followed by a brief partial repolarization mainly resulting from the activation of the transient outward current (/t0) and the inactivation... 

A quantitative interpretation of transient or periodic photocurrents in nanoporous networks requires a physical and mathematical description of the generation and collection of charge carriers. The exact treatments of the problem that have appeared in the literature are based on the assumption of either diffusion or migration as the predominant transport mechanism [78, 90]. A more general treatment that accounts for both diffusion and migration in response to a photoinduced gradient of the electrochemical potential is not yet available. Recently an attempt has been made to treat the problem within the framework of statistical mechanics [187]. 

Most commonly situations of this kind are attacked by variants of the method of finite differences. A numerical model of the electrochemical system is set up within a computer, and the model is allowed to evolve by a set of algebraic laws derived from the differential equations. In effect, one carries out a simulation of the experiment, and one can extract from it numeric representations of current functions, concentration profiles, potential transients, and so on. 

The complexity of the reaction rate transients, which consist of one fast and one slow stage, is in agreement with the cyclic voltammetric evidence abont the existence of differently accessible regions for surface charging. The first rapid step (a) is believed to be dne to accumulation of promoting species over the gas-exposed catalyst surface by the mechanism of backspillover, while the second step (b) is due to current-assisted chemical surface modification. Since no correlation between potential transients and reaction rate transients was manifested, a dynamic approach is justified and the applied current —rather than the catalyst overpotential— may be an appropriate parameter to describe the transient behavior of ethylene combustion rate at electrochemically promoted Ir02AfSZ film catalysts. For the interpretation of the fast transient steps (a) and (c), a dynamic model of electrochemical promotion has been developed, as presented in detail in Section 11.3. 

Iodide was expected to be a better hole scavenger since its formal oxidation potential is of the order of 0.4 V vs SCE however the use of aqueous iodide solutions led to instability of the photoanodic response of the sensitized anodes due to adsorption of triiodide onto the Ti02 surface which, according to evidence gained by both electrochemical and transient spectroscopy techniques, acted as a recombination center, causing a tenfold photocurrent drop from an initial 5 mA/cm to about 0.5 mA/cm. ... 

The living nature of the dermis also creates an electrochemical potential called a skin battery. This battery is associated with the function of the sweat glands. It is very pressure-sensitive. Small movements or forces on the electrodes cause transient millivolt-order changes in the skin-battery magnitude. These changes appear as motion artifacts and wandering baselines on biopotential traces. 

One basic oscillatory system uses the kinetic behavior of protein channels for sodium and potassium ion in a nerve membrane. Before elecflical excitation, the sodium channels are closed to sodium ion flow and the electrochemical gradient of 110 mV possible with the sodium ion gradient does not develop. The more permeable potassium channels tap the potassium ion gradient to produce an internal electrical potential of —60 mV relative to the external solution as ground. On excitation, the transient channels open and then close. As the sodium channels open, the sodium electrochemical potential of 110 mV appears to produce a peak internal potential of -60-1-110 = + 40 mV, The one-shot oscillation is completed as the sodium channel closes, the sodium-ion induced potential is lost, and the internal potential returns to the potassium channel dominated potential of -60 mV. The entire oscillation is controlled only by the sodium and potassium channels and the sodium and potassium ion concentration gradients across the membrane. 

At 1 = 1 the faradaic current is equal to the applied current If = I. The derivative of the potential with respect to time, at current switch-off, therefore permits to evaluate the double layer capacity. Figure 5.14 shows an idealized potential transient. Unfortunately, in practice, other capacitances present in the electrochemical system often attenuate the sharpness of the potential jump and thus complicate the distinction between ohmic and capacitive contributions. 

The choice of electrochemical techniques that can be implemented in a tribocorrosion test and the development of relevant models for the interpretation of the tribocorrosion mechanism are determined by the mechanical contact conditions being continuous or reciprocating. Electrochemical measurements can be performed with both types of tribometers. However, to be implemented under conditions that allow the interpretation of results, some methods require stationary electrochemical conditions, at least prior to starting up the measurements. In the case of continuous sliding, a quasi-stationaiy electrochemical surface state can often be reached, and all the electrochemical techniques available for corrosion studies (polarization curves, impedance spectroscopy, electrochemical noise,...), can be used. On the contrary, when reciprocating contact conditions prevail, the interpretations of experimental results are more complex due to the non-stationary electrochemical conditions. Measuring techniques suitable for the recording of current or potential transients will be used preferentially (Mischler et al., 1997 Rosset, 1999). 

When an electrochemical potential step is applied to the electrode coated with the conjugated polymer film in the neutral state, which is in contact with a polar solvent, the film may act in a way closely similar to that of the insulating barrier film shown in Fig. 20.39. The observed current flow, characterizing the rate of film oxidation, shows typically a nucleation-type behavior. The current transients of Fig. 20.44a are the result of stepping the electrode potential from a value of = -0.8 V vs. SCE, at which the polymer (polypyrrole) is in the neutral, hydrophobic state, to = -t-0.4 V vs. SCE, where the polymer is strongly oxidized and contains a high concentration of ions. 

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