Page:Text-book of Electrochemistry.djvu/239

 ��ELECTROMOTIVE FORCE,

��CHAP.

��Electrolyte.

��Cj.

��c obs.

��CtlCi

��e calc.2.

��(HI78<> 0-094O

��In the calculation of c calc.i, Nernst took account of the incomplete dissociation, and set the ratio of the osmotic pressures of the two solutions equal to the ratio of their conductivities, and not to that of their concentrations. The values calculated in this way agree very well with those observed directly.

The activity of a concentration element can be easily shown ; this is best done by the experiment made by Bucholz

in 1804. A glass cylinder is half filled with

a strong solution of stannous chloride (layer

b in Fig. 43), and this is covered by a layer

of pure water (a), A tin rod is immersed in

the liquid. A current is produced which

passes from the lower end of the rod upwards ;

this causes solution of the tin at the upper

end, and dendritic crystals of tin are formed

. at h.

Another kind of concentration cell has been suggested by

von Ttirin (17), This has the combination —

���SrUZg

��Fig. 43.

��Mercury | Mercuric salt in solution | Amalgam.

In order that the mercury may not expel the dissolved metal in the amalgam, it is necessary that this be " nobler " than mercury: for example, gold. In this arrangement mercury will be transported through the solution from left to right. This corresponds exactly with a distillation of

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