Page:Text-book of Electrochemistry.djvu/236

 If the work which can be gained from this be measured in gram-calories, the corresponding electromotive force can be calculated from the equation —

23,070 . /J,. TT = 1-99. T. In ^,

1-997. 10-*, »i or TT = log —.

To this we must add the process (2), which is analogous to that already described (p. 218), but which takes place in the opposite direction. In the former case the cation moves from the solution of osmotic pressure, pi (osmotic pressure with reference to the cation), to that of osmotic pressure, p^ ; here the motion takes place in the opposite direction, and gives rise to the electromotive force —

1-997. 10-* V --U, pi

TTl =. 1 . log -.

By adding together tt and tti, we obtain the total electro- motive force P —

„ . 1-997.10-* 2v, pi

n u + V ^ p%

The direction of the current is always from the dilute to the concentrated solution, since it is in this way that the concentration difference can disappear.

Instead of an element of this type with unpolarisaUe electrodes of the first order ^ we may consider a type with un- polarisable electrodes of the second order, say the combination —

Hg I HgCl I 0-01 HCll^Ol HCl | HgCl | Hg.

When a quantity of electricity n. 96,500 coulombs (in this

an unsaturated solution of a salt containing a cation, the same as the metal of the electrode ; by an nnpolarisable electrode of the second order, is meant a metal in a solution of one of its salts, which by being in contact inrith excess of solid salt, is always kept saturated.
 * By nnpolarisable electrodes of the first order, we mean electrodes in

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