Overvoltage, 6.33

Concentration overvoltage (reaction overvoltage and diffusion overvoltage) If a 

Electrode Hydrogen overvoltage m volts Electrode Hydrogen overvoltage in volts 

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. 

As measurement of minimum overvoltage is marked by an inherent error due to indefinable and thus problematic determination of the beginning of a visible evolution of gases, it is more exact to quote the values whioh are valid for a certain current density (see Tab. 13a, b). 

In Fig. 24 there is a graphical representation of the dependence of hydrogen overvoltage — on electrolyzing 2N—HjSO, at 12 °C — upon current densities on several metals. 

The platinized platinum (platinum coated with platinum black) is the only 

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It is easy to notice, that the protection against a short-circuit failure in the X-ray tube circuit implements due to the "soft" outer characteristic of the apparatus main circuit. The overvoltage protection at emergencies in the control system happens due to the redistribution of the magnetie flow, created by power winding I, between the 3,6 control yokes. Therefore the voltage on the X-ray apparatus anode drops approximately two times. 

The first law of electrode kinetics, observed by Tafel in 1905 [197], is that overvoltage i) varies with current density i according to the equation... 

Overvoltage. Overvoltage (ti. ) arises from kinetic limitations or from the inherent rate (be it slow or fast) of the electrode reaction on a given substrate. The magnitude of this value can be generally expressed in the form of the Tafel equation... 

Overvoltages for various types of chlor—alkali cells are given in Table 8. A typical example of the overvoltage effect is in the operation of a mercury cell where Hg is used as the cathode material. The overpotential of the H2 evolution reaction on Hg is high hence it is possible to form sodium amalgam without H2 generation, thereby eliminating the need for a separator in the cell. 

The electrolytic processes for commercial production of hydrogen peroxide are based on (/) the oxidation of sulfuric acid or sulfates to peroxydisulfuric acid [13445-49-3] (peroxydisulfates) with the formation of hydrogen and (2) the double hydrolysis of the peroxydisulfuric acid (peroxydisulfates) to Caro s acid and then hydrogen peroxide. To avoid electrolysis of water, smooth platinum electrodes are used because of the high oxygen overvoltage. The overall reaction is... 

A conductor with a hydrogen overvoltage exceeding that of smooth platinum can be used for the cathode. 

Fig. 5. Energy requirements of the HaH-Hfiroult cell (23—25). E, decomposition of alumina Eg, depolarization by carbon E, anode overvoltage E, counter electromotive force E, bath voltage drop E, bath bubble voltage F/, anode voltage drop Eg, cathode voltage drop E, external voltage drop ...

The low current efficiency of this process results from the evolution of hydrogen at the cathode. This occurs because the hydrogen deposition overvoltage on chromium is significantly more positive than that at which chromous ion deposition would be expected to commence. Hydrogen evolution at the cathode surface also increases the pH of the catholyte beyond 4, which may result in the precipitation of Cr(OH)2 and Cr(OH)2, causing a partial passivation of the cathode and a reduction in current efficiency. The latter is also inherently low, as six electrons are required to reduce hexavalent ions to chromium metal. 

Mercury Cells. The cathode material ia mercury cells, mercury [7439-97-6] Hg, has a high hydrogen overvoltage. Hydrogen evolution is suppressed and sodium ion reduction produces sodium amalgam [11110-32-4J, HgNa. 

Chlorine—hydrogen ha2ards associated with mercury cells result from mercury pump failures heavy-metal impurities, particularly those with very low hydrogen overvoltage, ie. Mo, Cr, W, Ni excessively low pH of feed brine low NaCl concentrations in feed brine and poor decomposer operation, which leads to high sodium amalgam concentrations in the cell. 

Seconday Current Distribution. When activation overvoltage alone is superimposed on the primary current distribution, the effect of secondary current distribution occurs. High overpotentials would be required for the primary current distribution to be achieved at the edge of the electrode. Because the electrode is essentially unipotential, this requires a redistribution of electrolyte potential. This, ia turn, redistributes the current. Therefore, the result of the influence of the activation overvoltage is that the primary current distribution tends to be evened out. The activation overpotential is exponential with current density. Thus the overall cell voltages are not ohmic, especially at low currents. 

Overvoltages and voltage surges caused by disturbances in an LT system 6/130... 

All these drives are based on pulse width modulation (PWM) and hence would produce overvoltages at the inverter output and require overvoltage protection for cable lengths of 100 m (typical) and above, depending upon the steepness of the wave (Section 6.14.1). 

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