A Brief Rerun through the Conduction Sections

We learned early on that equivalent conductivity and specific conductivity differed in that the former was not directly proportional to concentration but only secondarily so. However, it turned out that this secondary dependence was considerable and arose because the mobility of the ion itself decreased with an increase in concentration. Thus, as ions get near enough to feel each other through the interaction of their electric fields, they slow down. 

At a molecular level, this slowdown is described in terms of two effects. One of these effects is called electrophoretic and the other, relaxational. The electrophoretie effect is easy to understand because it is a kind of electrical friction as one ion passes the other within electrical hailing distance, both ions slow down in recognition of the electrical existence of the other. 

The relaxation effect is a bit more difficult to explain. It has to do with the fact that when an ion moves in a given direction, inertia causes the ionic cloud around it to become egg shaped, and this dissymmetric ionic cloud has more counter charge toward its rear than toward its front. The dissymmetry of charges acts to counteract the effect of the directional electric field applied through the solution, and so this also slows the ion down. Both these effects combine to explain why mobility falls with increasing concentration, for the two effects increase in strength with the square root of the concentration. 

having broached the subject of the relaxation of the ion s atmosphere—its taking up a dissymmetric shape when the ion moves—we went on to tackle the subject of relaxation quite generally. For example, if an electric field is suddenly applied to a solution, it would orient the solvent dipoles therein. A new equilibrium would then be set up. The relaxation time is a measure of the time it takes to set up this new equilibrium. At first it seems peculiar that one should call it a measure of and not the time itself. However, the situation is similar to that of radioactive decay because in changing from state 1 to state 2, the concentration of aradioactive nucleus decreases exponentially with time, taking an infinite time to disappear completely. Since this is not a practical measure, we agree to use another measure of the rate of decay—the time to decline by 63%. 

These ideas about relaxation times are applied to several phenomena, including the changes in asymmetry of the ionic atmosphere and the unusual behavior of the dielectric constant of water, which has three values according to the frequency with which it is measured. They are 78, 5, and 2. 

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