Electrode internal polarization

Figure 6. Bipolar precipitates consisting of an inner anion-selective layer and an outer cation-selective layer.19 When the electrode is polarized to the more noble side, protons and chloride ions are kept from permeating through the film, so that anodic dissolution of the substrate metal is blocked. (Reproduced from N. Sato, Corrosion, 45 354, 1989, Fig. 24 with permission of NACE International.)...

The interfacial charge-related capacity, Cum, due to the internal polarization of adsorbed water molecules remains constant in the potential range where no reorganization of adsorbed water molecules occurs. On the other hand, the interfacial capacity related to water dipoles, Cs.di i, on the aqueous solution side depends on the orientation of adsorbed water molecules which changes with the interfacial charge and, hence, with the electrode potential. Further, the dipole capacity. Cm, dtp > on the metal side appears to slightly depend upon the interfacial charge and, hence, the electrode potential. Equation 5-11, then, yields Eqn. 5-12 ... 

To avoid serious errors, the cell current and internal cell resistance must be kept as small as possible, and the reference electrode must be designed to have low internal resistance and a metal-solution interface of sufficient area to minimize internal polarization. Under ordinary polarographic conditions (10-/tA current and 1000-Q internal cell resistance) the error amounts to 10 mV. 

To minimize errors in voltammetric work, the three-electrode configuration (Figure 5.2b) is commonly used. The cell current flows between the working electrode and the counter or auxiliary electrode, while the potential of the working electrode is measured with respect to the reference electrode using a high-impedance measuring device. This avoids internal polarization of the reference electrode and compensates for the major portion of the iR drop in the cell. 

As shown in Figure 33-6, the instrumentation for capillary electrophoresis is simple. A buffer-filled fused-silica capillary, typically 10 to 100 xm in internal diameter and 40 to 100 cm long, extends between two buffer reservoirs that also hold platinum electrodes. Sample introduction is performed at one end and detection at the other. A potential of 5 to 30 kV dc is applied across the two electrodes. The polarity of this high voltage can be as indicated in Figure 33-6 or can be reversed to allow rapid separation of anions. 

The polarization curves in the current density range greater than icm are called external polarization curves, and those in the current density range less than icMT are called internal polarization curves. By sweeping the electrode potential from the corrosion potential to the cathodic or anodic direction, the external polarization curve can be determined. The internal polarization curve cannot be measured directly by the electrochemical technique since it is impossible to pick up the current separately from the anode and cathode, which exist in the elecdode. By analyzing the metallic ions dissolved and the oxidizer reaction, the internal polarization curves can be determined. 

Let the net (overall) current density of the porons electrode be i. Under conditions of uniform work of the full internal surface area, the value of i would be y times larger than the current density t o of a smooth electrode, working at the same valne of polarization and in an electrolyte having the same composition. This case is rare in practice, and it is more common to And that i is smaller than its maximum value i = yi. The ratio between these parameters. 

Unlike the earlier case, here decreases with increasing current not only at large but also at small values of polarization, since depends on polarization. For d >Ldiff (particularly when polarization is significant), the electrode will work under internal diffusion control. 

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