Overvoltage, definition

These values are not accurate and cannot be reproduced in a reliable manner. It is due to the fact that apart from the easily definable factors (such as current density, material of electrodes, temperature, pressure etc.) the overvoltage also depends to a great degree on factors that cannot be controlled so easily i. e. on the state of the electrode surface, way of the mechanical treatment of the same, its past (i. e. whether it had been polarized beforehand or not) even traces of impurities prove important. The difficulties connected with the definition of surface properties of electrodes are aggravated by the variability of the overvoltage in the very course of electrolysis. 

The method which seems theoretically most nearly sound is that of Bowden and Ridcal.4 These authors measured the quantity of electricity which had to be passed in order to build up a definite overvoltage on a metal surface. There is reason to believe that this quantity, i.e. the number of ions per square centimetre for each millivolt of overpotential is independent or nearly so of the nature of the metal therefore the true area of the nietal surface is proportional to the quantity of electricity needed to build up a given overpotential. It was first assumed that the real area of a mercury surface was equal to the apparent area with this assumption the real area of various smooth or polished metals appeared... 

Since the point of bubble evolution represents a more or less indefinite rate of discharge of hydrogen and hydroxyl ions, recent work on overvoltage has been devoted almost exclusively to measurements made at definite c.d. s it is then possible to obtain a more precise comparison of the potentials, in excess of the reversible value, which must be applied to different electrodes in order to obtain the same rate of ionic discharge in each case. The details of the methods of measurement and a discussion of the results will be given after the general problem of the mechanism of electrode processes has been considered. 

For the measurement of overvoltage at definite c.d. s, the method adopted is, in principle, the same as that described on page 436, employing a cathode of known area. The solution should be completely free from dissolved oxygen or other reducible material for this reason it... 

Undervoltage and overvoltage relays are often used on the generators to protect against prolonged overload, seen as undervoltage and excessive stress on insulation, seen as overvoltage. These relays are usually chosen with adjustable definite time delays. 

From the definition of overvoltage presented above, E = + rj. For anodic... 

The sum of the surface and concentration overpotentials is called the total overpotential or overvoltage r) of the electrode which can be measured by standard reference electrodes, as already shown in Figure 21.1. All electrode potentials are related to the standard hydrogen electrode (SHE), the potential of which is set by definition at zero at all temperatures. The saturated calomel electrode (SCE), Hg/Hg2Cl2/Cr, is the most widely used reference electrode for potential measurement. Other electrode systems used are Hg/Hg2S04/S04, Hg/HgO/OH , and Ag/AgCI/Cr. 

The second holds that metals passive by Definition 1 are covered by a chemisorbed film—for example, of oxygen. Such a layer displaces the normally adsorbed H2O molecules and decreases the anodic dissolution rate involving hydration of metal ions. Expressed another way, adsorbed oxygen decreases the exchange current density (increases anodic overvoltage) corresponding to the overall reaction M -1- ze. Even less than a monolayer on the surface is... 

The current density at the pore wall, j, depends of the local overvoltage, t], according to some Butler-Volmer kinetics, which are not given here explicitly. The first boundary condition [Eq. (28.72)] is equivalent to the definition of (< —

electrode pore, so it defines the electric potential there. The second boundary condition [Eq. (28.73)] demands that no charge flux exits the electrolyte at the top of the pore. This differential equation can be solved in combination with a reaction rate expression, for example, Butier-Volmer kinetics. 

Definition Transient and oscillatory overvoltage Momentary undervoltage or overvoltage Power outage... 

There are other reasons for grounding not implicit in the IEEE definition. Overvoltage control has long been a benefit of proper power system grounding, and is described in IEEE Standard 142, also known as the Green Book [3]. With the increasing use of electronic computer systems, noise control has become associated with the subject of grounding, and is described in IEEE Standard 1100, the Emerald Book [4]. 

Ibl carries the argument on equalizing action of overvoltage a step further. As we have seen, current distribution is more uniform when ratio RJRg is greater. Now R is the resistance of the solution per m of cross-sectional area, and is given by the specific resistance times the distance between the two electrodes (d). Expressed in terms of the specific conductivity K, Rg = d/K. Remembering the definition of R , we obtain ... 

Two features of this definition are worth noting. One is that EPH is defined as the heat of a reversible reaction, which essentially eliminates the various uncertainties arising from the irreversible factors such as overvoltage. Joule heat, thermal conductivity, concentration gradient and forced transfer of various particles like ions and electrons in electrical field, and makes the physical quantity more definite and comparable. This indicates that EPH is a characteristic measure of a cell reaction, because the term 8 (AG)/8T) p is an amoimt independent on reaction process, and only related to changes in the function of state. That is to say, EPH is determined only by the initial and the final states of the substances taking part in the reaction that occurs on the electrode-electrolyte interfaces, although other heats due to irreversible factors are accompanied. EPH is, unlike the heat of dissipation (Joule heat and the heats due to irreversibility of electrode processes and transfer processes), one of the fundamental characteristics of the electrode process. 

Definitions. Define briefly (a) difference of potential, (b) electromotive force, (c) salt bridge, (d) anode, (e) positive electrode, (f) reference half-cell, (g) standard electrode potential, (h) decomposition potential, (i) overvoltage, (j) sacrificial anode. 

By definition, the polarization = E - E indicates the deviation of the electrode potential from the corrosion potential. The polarization is related to the overvoltage of the partial electrode reactions by (57) and (58). 

It is assumed that the contribution of potential differences due to liquid junctions and the contribution of the I drop are properly eliminated in the experimental determination of the relative electrode potential L/ref Frequently the word relative is omitted. The designation relative electric tension has been suggested [1] for L/ref- The polarization of an electrode is equal to the value of with current flow less the value of L/ref the absence of a current. When, at a given current density, a metallic electrode is the site of a definite and unique reaction, its overvoltage rj (overtension [2]) at a given instant is ... 

These definitions are particularly helpful in the case of complex processes. The parameter io then refers to the rate determining step. At large overvoltages the current-voltage relationship described by Eq. (7.59) departs from linearity and takes on what is known as Tafel behaviour ... 

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