Page:Text-book of Electrochemistry.djvu/240

 mercury to the amalgam, the vapour pressure of which must necessarily be lower than that of the pure substance. The electromotive force is evidently proportional to the depression of the vapour pressure, and this again is proportional to

-jjjr, where n is the number of dissolved molecules, and N the

number of solvent (mercury) molecules. By measuring the electromotive force of such an element, the molecular weight of the dissolved metal can be determined (compare Meyer's concentration element, p. 210).

Solution Pressure of Metals. — In concentration ele- ments we have three electromotive forces, which act at the three contact surfaces. For one of these, namely, that between the concentrated and dilute solution, Nemst has deduced (see p. 218) the expression —

n u + V p%

where p\ and p^ denote the osmotic pressures of the two solutions, u and v the migration velocities, and n is the valence of the ions. For the other two electromotive forces we have obtained (see p. 221) —

TT = TTo + TTa = 1-99. 10-* r. log ?\

It would be of interest to ascertain the value of each of these electromotive forces, e.g. between Cu and dilute CUSO4, and between Cu and concentrated CuSO^, and not only, as the above formula gives us, their sum.

In order to obtain some analogy with the other formulae, the form —

TTo + 7r.2 = 1-99. 10-* r. log - - 1-99. lO"* T . log -'-

has been given to the above one.

p The factors containing the expression log — give the

Q

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