Overvoltage variation with

While the hydrogen overvoltage variations with current density may be described by the Tafel equation in general, the experimental results (see Fig. 14) show the overvoltage (rjh) dependence on... 

FIGURE 4.4.12. Chlorine overvoltage variation with current density in 5M NaCl solutions (pH = 3.5) at 95°C [39]. 

Hydrogen Overvoltage.—In view of the foregoing discussion of the dependence of polarization phenomena on the nature of the slow stage of the electrode process, it is evident that in order to give a satisfactory quantitative interpretation of hydrogen overvoltage it is necessary to know the exact nature of its variation with current density. 

FIGURE4.5.1. Variation of the chlorine overvoltage, tjcij, with current density on various anodes in SMNaCI solutions at 95°C at pH = 3.5 [20]. 

SOME APPLICATIONS OF THE VARIATION OP HYDROGEN OVERVOLTAGE WITH THE PRESSURE... 

Figure 19.17. Overvoltage and distribution of voltage drops in cells (Jtiine, 1985). (a) Overvoltage of hydrogen on some metals, (b) Voltage distribution in two kinds of cells for electrolysis of brine, (c) Variation of voltage distribution with current density in the electrolysis of HC1. (d) Schematic of voltage profile in a bipolar cell with five pairs of electrodes.

Kinetic factors may induce a variation of electrode potential with current the difference between this potential and the thermodynamic equilibrium potential is known as the overvoltage and the electrode is said to be polarized. In a plating bath this change of potential can be attributed to the reduced concentration of depositing ions in the double layer which reduces the rate of transfer to the electrode, but the dissolution rate from the metal increases. Since the balance of these rates determines the electrode potential, a negative shift in the value occurs the concentration polarization Olconc)- Anodic effects are similar but in the opposite direction. 

Fig. 118. Variation of overvoltage with current density (Bowden and Rideal)...

The results recorded below for the variation of the overvoltage with current density at a mercury cathode in dilute sulfuric acid at 25 have been estimated from the data of Bowden and Rideal [ Proc. Roy. Soc. 120A, 59 (1928)]. 

Plot the variation of the overvoltage with log 7, where I is equal to the c.d., and determine the value of the slope 6 estimate the values of a and 7o for the mercury cathode. 

The energy for ignition varies with voltage and overvoltage, but the trend of variation is not the same for all explosives... 

The variation of the overvoltage with the current density has been studied by a number of workers including Tafel,19 Lewis and Jackson,20 Harkins and Adams,31 and by Bowden.22 The main conclusion from these studies is that the hydrogen overvoltage, which may be represented by Ea, varies with the current density i according to the equation... 

ZAP corrections (Tables 5-7). ZAP corrections for certain elements (Si, Ca, Y, U, Pb) are inherently large due to overvoltage requirements and typical monazite compositions resulting in large [A] factors for unknowns. However, variation in compositions resulting from application of different ZAF models is equal to or less than the compositional variation associated with analytical uncertainty. 

Steel. This behaviour is explained by a higher overvoltage for the cathodic reaction of oxygen reduction on stainless steel with respect to carbon steel [14,30], as shown, for instance, by the cathodic polarization cxirves of Figure 15.5. Only minor changes in the cathodic behaviour occur due to variation of the cdiemical... 

We will consider only the influence of activation overpotential or overvoltage on secondary current distribution. It is useful to regard the slope of the polarization curve dE /di (if any effect of concentration overpotential can be ignored) as a polarization resistance R. This represents the slowness of charge transfer across the interface and is based on the electrode kinetics of the reaction. If acts in series with R, the resistance of the electrolyte, we can distinguish between two situations. If R R, then the kinetics of charge transfer and not electrolyte resistance determine the current distribution, i.e., secondary current distribution dominates. Conversely, if R R, primary current distribution dominates. Secondary current distributions tend to smooth out the severe nonlinear variations of current associated with primary distributions and they eliminate infinite currents associated with electrode edges. 

Starting with a general expression for the variation in voltage E ev - nacovation, we only have overvoltages because the terms in ln(l-//// ) are negative. 

Figure 4.2 Normalized reversible cathodic voltammograms simulated at different stability of ML+ complexes (log K values are given at the respective curves). Variations of the overvoltage at i/i = 0.75 with log K (see dotted lines) are given in the inset.

The Tafel equation applying to large overvoltages predicts a straight line for the variation of the logarithm of current density with potential. 

Since absolute Acp values are not accessible, the exchange current density characterizes the rate of an electrode reaction. The current-voltage curves can be related to io and the overvoltage rj = Acp - Acpo, as it was done for redox reactions. The variation of the activation energies with rj is illustrated in Figure 2.29. One obtains for the partial currents... 

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