Page:Text-book of Electrochemistry.djvu/249

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��POTENTIAL DIFFERENCE.

��CHAP.

���IV

���FiQ. 47.

��metal (see below), i.e. it possesses a low solution pressure.

Now, if a liquid, W(Fig. 47), in which there hangs a mercury- drop, A, from a capillary electrometer, K, contains mercury ions, even in minute quantity, then, provided that its

atmos., this pressure is higher than the

solution pressure of the mercury. A

sufficient number of mercury ions pass

into solution to establish tMs condition

by oxidation and solution of the small

amount of the mercury which has fallen

from the dropping electrode and rests

at the bottom of the liquid TV. This

+ + determines that mercurous ions, Hg2, must separate from the

liquid W and deposit on the falling drop, which thereby

becomes positively charged; this positive electricity is

carried to the layer of mercury at the bottom, and the solution

becomes negatively charged. By this process the liquid near

the place where the drops are formed loses mercurous ions,

and consequently mercury salt, whilst that at the bottom of

the vessel becomes more concentrated, as Palmaer (7) has

directly proved.

This process should theoretically continue until the con- centration of the mercury ions in the liquid at A has decreased to a certain value corresponding with the solution pressure of the mercury ; this condition can be nearly attained by allowing the drops to form quickly, but it cannot be perfectly reached on account of diffusion of mercurous ions from the lower to the upper part of the vessel. Suppose this condition has been established, then evidently no more ions would pass from the solution to the drop, no double layer would be produced, and the mercury would have the same potential as the liquid.

An analogous process takes place when any other metal of low solution pressure — copper, silver, etc. — is brought into

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