The Wien Effects

Fig. 2.8 The Wien effect shown by the percentage increase of equivalent conductivity in dependence on the electric field in Li3Fe(CN)6 solutions in water. Concentrations in mmol dm-3 are indicated at each curve... 

The Wien Effect. If, instead of using potentials of the order of one volt per centimeter in the measurement of electrolytic conductance, voltages of several hundred thousand times this are employed, the conductances of solutions of electrolytes are no longer constant but tend to increase with the potential. Under these conditions Ohm s law is evidently no longer valid. This increase of conductance at high potentials is called the Wien effect. The passage of high potentials... 

Fig. 9. Barretter and Oscillation Circuits for Measuring the Wien Effect.

Fig. 10. The Wien Effect for Lithium Ferricyanide at Three Concentrations.

Fig-. 11. The Wien Effect for Different Valence Type Salts at Concentrations having the Same Low Field Conductance. 

There are two experiments which beautifully illustrate the correctness of the idea of an ionic atmosphere and its manifestation in terms of the relaxation and electrophoresis which occur when the ion moves under the influence of an external field. These are the Debye-Falkenhagen effect and the Wien effect. 

The discussion in this section and in the previous one on the Debye-Falkenhagen effect is qualitative, but it does illustrate the physical significance of the Debye-Falkenhagen and the Wien effects. 

The concept of the ion atmosphere is further substantiated by the Wien effect and the Debye-Falkenhagen effect. In very high fields, > 10 V/m, an increase in conductivity is observed (Wien effect), resulting from the fact that a finite time (the relaxation time) is required for the atmosphere to form about an ion. In very high fields the ion moves so quickly that it effectively loses its atmosphere the atmosphere does not have time to form and so cannot slow the ion. The asymmetry effect disappears and the conductance increases. 

For the measurement of concentration changes and for the recording of orientational changes in solutions of optically anisotropic molecules, optical techniques have proven to be widely applicable. If ionic species are involved, conductivity measurements are suitable to monitor concentration as well as orientation changes in electrically anisotropic molecules. The Wien effects are directly accessible from the conductivity change 5/c/k (0) relative to the k value at E = 0. For a 1 1 ionic equilibrium like that in... 

This is displacement of positive ions with respect to negative ions. To clarify the division between electronic and ionic polarization electronic polarization is the displacement of the electron cloud with respect to the nucleus ionic polarization is the displacement of ions relative to each other. The hydrated sheath around an ion at rest is symmetrical (not really at rest, everything is bumping around at room temperature, we are talking statistically). When current flows, the sheath will lag behind the migrating ion, and the sheath is no longer symmetrical to the ion, cf. the Wien effect (Section 8.4.1). This is local polarization of charges bound to each other. 

For an electrolytic solution this phenomenon Is known as the Wien effect. 

The conductivity of a strong electrolyte is also increased at high field strengths because the ions are moving so fast that the ionic atmospheres are unable to form completely. This is known as the Wien effect it will also operate in solutions of weak electrolytes, but it is considerably smaller than the dissociation field effect and can be eliminated at sufficiently high field strengths. 

The Wien effects are more than just curiosities. They significantly affect the kinetics of electrode processes, particularly in solvents of low... 

Another nontrivial application of the Wien effect is in discriminating between CO2 (aq) and H2CO3 in order to accurately measure the true equilibrium constant for the aqueous ionization ... 

Another mechanism is the phenomenon first called the Wien effect.A drifting ion is surrounded by a solvatation well. Under the motion the structure of this well undergo changes and then reconstruction. The time of this process, called the relaxation time, T, is characteristic for a given liquid. Above the threshold electric field the ionic motion becomes greater in comparison with t, reconstruction does not take place, and the mobility increases greatly. The onset of the electric field strength for this process is -1 MV. ... 

A quantitative theory of the Wien effects has been given by Onsager and Wilson they show that a small electrophoretic effect remains, so that at high voltages the molar conductivity of a dilute solution increases to a value only a little below the normal A°° value. 

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