Wien effects

The conductivity also increases in solutions of weak electrolytes. This second Wien effect (or field dissociation effect) is a result of the effect of the electric field on the dissociation equilibria in weak electrolytes. For example, from a kinetic point of view, the equilibrium between a weak acid HA, its anion A" and the oxonium ion H30+ has a dynamic character ... 

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... 

Wien effect phys chem An increase in the conductance of an electrolyte at very high... 

There is a modest increase in the electrical conductance with an increase in the electric-field gradient, an effect that operates with both strong and weak electrolytes (the first Wien effect). More important in the present context is the marked increase in electrical conductance of weak electrolytes when a high-intensity electric field is applied (second Wien effect). The high field promotes an increase in the concentration of ion pairs and free ions in the equilibrium... 

At high field strengths a conductance Increase Is observed both In solution of strong and weak electrolytes. The phenomena were discovered by M. Wien (6- ) and are known as the first and the second Wien effect, respectively. The first Wien effect Is completely explained as an Increase In Ionic mobility which Is a consequency of the Inability of the fast moving Ions to build up an Ionic atmosphere (8). This mobility Increase may also be observed In solution of weak electrolytes but since the second Wien effect Is a much more pronounced effect we must Invoke another explanation, l.e. an Increase In free charge-carriers. The second Wien effect Is therefore a shift in Ionic equilibrium towards free ions upon the application of an electric field and is therefore also known as the Field Dissociation Effect (FDE). Only the smallness of the field dissociation effect safeguards the use of conductance techniques for the study of Ionization equilibria. 

The energy dissipation of a system containing free charges subjected to electric fields Is well known but this Indicates a non-equilibrium situation and as a result a thermodyanmlc description of the FDE Is Impossible. Within the framework of interionic attraction theory Onsager was able to derive the effect of an electric field on the Ionic dissociation from the transport properties of the Ions In the combined coulomb and external fields (2). It is not improper to mention here the notorious mathematical difficulty of Onsager s paper on the second Wien effect. 

The role played by the micro Wien effect in dissociating the disodium spinide with increasing concentration beyond the conductance minimum is strongly indicated by the temperature coefficient of the conductance of solutions of sodium and potassium in liquid ammonia. Here we have data for the temperature coefficient of sodium and potassium from relatively dilute solutions up to saturation. 

The fluidity of ammonia increases about 1.5% per degree, and in the dilute range the temperature coefficient of metal solutions is of this order of magnitude but from a concentration of approximately 0.9AT onward the temperature coefficient of sodium and potassium begins to increase, reaching a maximum of about 3.6% for sodium and 4.6% for potassium. The conductance increase owing to temperature increase can only be caused by an increased dissociation of sodium spinide. It follows that the conductance increase with increasing concentration of sodium solutions is to be expected and conforms with the assumptions of a micro Wien effect. 

The last two factors, which cause the molar conductivity to decrease with concentration beyond the c.m.c., normally outweigh the first factor, which has the reverse effect (see Figure 4.13). When conductance measurements are made at very high field strengths the ionic atmospheres cannot re-form quickly enough (Wien effect) and some of the bound counter-ions are set free. It is interesting to note that under these conditions the molar conductivity increases with concentration beyond the c.m.c. 

Pulsating DC at 80 kilocycles was used by Mach and Geffert (Ml), and it shortened the duration of the run by about one-third, perhaps as the result of the disruption of the Debye and Hiickel ion cloud. This so-called Wien effect has not yet been found by other authors (D17). 

Since the two major results we obtained in our experiments are a nonlinear impedance and a low frequency dispersion effect of polyelectrolytes under high fields, it is logical for us to provide the readers with some background review in the field of nonlinear effects and high-field dispersion effects of electrolytic solutions. Most of the studies in these fields were devoted to Wien effects and DFW dispersion theory which are briefly discussed here. A detailed discussion of these two groups of studies with their recent developments is reported in a separate review chapter in Nonlinear Electromagnetics. 

Wien effect — In large -> electric fields (deviations in the) it is possible to observe deviations from the relationship between the applied -> voltage and the flowing electrical - current in a -> conductance measurement following... 

Fig. 36. Wien effect for salts of different valence types...

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