Steady equilibrium potential

Anode Polarization-the difference between the potential of an anode passing current and the steady-state or equilibrium potential of the electrode with the same electrode reaction. 

The electrode s open-circuit potential (steady potential) , depends on the relative values of the exchange CD of both reactions and also on the slopes of the polarization curves. When the exchange CD and slopes are similar, the open-circuit potential will have a value, the mixed (or compromise ) potential, which is intermediate between the two equilibrium potentials (Fig. 13.2a). However, when the exchange CD for one of the reactions is much higher than that for the other, the open-circuit potential will practically coincide with the equilibrium potential of this reaction (Fig. 13.2b). 

Underpotential Deposition of Metal Atoms Because of the energy of interaction between a foreign substrate and the adsorbed metal atoms formed by discharge, cathodic discharge of a limited amount of metal ions producing adatoms is possible at potentials more positive than the equilibrium potential of the particular system, and also more positive than the potential of steady metal deposition. 

It is worthwhile mentioning that the interfacial potential created at the liquid-liquid interface is governed by single ionic or redox equilibrium only in the simple cases. The presence of various, often two, interfacial processes is a source of the steady-state potential, named also the mixed or the rest potential. Its value is situated between the two equilibrium potentials, near that one which corresponds to the higher exchange current... 

What is the quantitative relationship between the steady state, convection-with-diffusion current density and the potential difference across the interface How is the steady-state potential difference at a steady current density related to the zero-current, or equilibrium, potential difference These questions are the relevant ones for steady passage of current in convection-aided situations. 

This is the steady-state current which is theoretically predicted if stage 1 is the rate-determining step in the sub-stages sequence represented in Equations 4.8 1.12. An important parameter to compare both in theory and experimentally is the Tafel slope or the transfer coefficient which results from it. Therefore, Equation 4.30 has to be written in a form that contains only one exponential term. Since the considered I-E curve is an oxidation wave, the effect of the reduction (second term in the right-hand part of Equation 4.30) will be negligible with potentials that are situated sufficiently far away from the equilibrium potential, and for the anodic current the following applies ... 

The steady state potential Ess of the oxide electrode at a particular composition MO lies between the formal equilibrium potentials with respect to oxygen, Eq(h), and metal, E n), and its value is determined by the electroneutrality condition at a particular total current density... 

Fig. 2. Schematic Tafel plots for ITR for steady-state anodic oxide deposition between the equilibrium potentials with respect to metal E (n) and with oxygen Eg (n).

Cyclic voltammetry of LaNi03 in the presence of the hexacyanoferrate system and deconvolution of Faradaic and surface processes by means of the RRDE are depicted in Fig. 5. Steady-state results obtained for the hexacyanoferrate redox couple at LaNiO, in alkaline solutions were similar to those reported for La05Sr05CoO3 with very fast kinetics, comparable with the reaction on platinum electrodes, and convective-diffusional limiting currents which obey the Levich equation are observed close to the equilibrium potential (Fig. 5). 

The understanding gained by considering the Evans diagrams allows us to measure the corrosion current in a straightforward manner. First we must realize that the corrosion potential is in fact the open-circuit potential of a system undergoing corrosion. It represents steady state, but not equilibrium. It resembles the reversible potential in that it can be very stable. Following a small perturbation, the system will return to the open-circuit corrosion potential just as it returns to the reversible potential. It differs from the equilibrium potential in that it does not follow the Nemst equation for any redox couple and there is both a net oxidation of one species and a net reduction of another. 

Because the steady-state membrane potential Lies between the equilibrium potentials for Na+ and K", there is a constant movement of out from the cell and Na" into the cell. To ensure that this does not lead to a progressive decline in the concentration gradients across the membrane, all cells have a Na-K pump, which uses the hydrolyses of ATP to simultaneously pump K+ into the cell and push Na" out. The constant fluxes of and Na constitute electrical currents across the cell membrane, and at steady-state, these currents cancel each other out so that the net membrane current is zero. 

Electrochemistry is full of situations like this one, in which charged species (electrons or ions) cross interfacial boundaries. These processes generally create a net transfer of charge that sets up the equilibrium or steady-state potential differences that we observe. Considering them in more detail must, however, await the development of additional concepts (see Section 2.3 and Chapter 3). 

What aspect of Eq. (31.71) shows that the diffusion potential is a steady state—and not an equilibrium-potential ... 

In analogy to the partial equilibrium approach, the assumption of the degradation sequence of chlorinated ethenes to occur at steady-state can be used also to derive equilibrium potentials for their degradation according to... 

Open circuit potential As defined by the McGraw-Hill Dictionary of Scientific and Technical Terms, open circuit potential (OCP) is the steady-state or equilibrium potential of an electrode in absence of external current flow to or from the electrode. OCP measures the corrosion potential of a corroding metal with regard to a reference electrode. For instance, increased susceptibility of stainless steels to pitting and crevice corrosion in seawater has been attributed to increase in OCP, which could partly be due to biofllm formation. Monitoring of OCP spectra can be used to rank the corrosion vulnerability of metals in comparison with each other. 

Another important point, as far as the steady-state current-potential curves are concerned, is the position on the potential axis of the branch plotted, which in the example above is anodic. As previously underlined, no equilibrium potential can be spotted because the open-circuit potential cannot be defined by the Nernst law. However, it is quite useful to have the thermodynamic constant associated with the redox couple in question, which in this case is the standard potentiali °(Cu VCu), because this enables one to position the current-potential curve, i.e., the potential zone where the current undergoes sharp variations (see figures 2.18 above and 2.21 in section 2.B.2.2) Our aim here is to plot and use the shapes of the curves rather than to estimate their precise mathematical expression or any potential values to the mV. 

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