Page:Text-book of Electrochemistry.djvu/238

 ��1-99. T. 10-* 2w, 2h

-- . - - — .log -.

n u + V ^ p^

��Pi = TT + TTl = - ^ -' • -, -. log

��The negative sign indicates that the electromotive force strives to drive the electricity in the opposite direction to that assumed in the development of the formula, i.e. the electricity goes through the element from the concentrated to the dilute solution.

Experimental Confirmation of the Theory.— In Ids investigations Helmholtz only considered concentration elements with unpolarisable electrodes of the first order. His results were experimentally confirmed by Moser (lo) and Miesler. Similar experiments were later carried out by Nemst (16), Nernst found for the element —

Ag I 0-1 AgNOa 1 0-01 AgNOg | Ag

the electromotive force 0055 volt at 18^, whilst the theory leads to —

oo'O + 61'8 ®

= 1-99 X 291 X 1058 X KM = 00613 volt,

when T = 291, u = 550, and v = 618 (see p. 142).

The agreement is satisfactory. Now, since the dissocia- tion is in reality not complete, as is assumed, we may introduce a correction for this disturbing factor, and then obtain the value 00574 volt.

Nemst has measured a large number of electromotive forces with elements which are reversible with respect to the anion, that is to say, with unpolarisable electrodes of the second order. His results are given in the following table, where ci and c^ denote the nonnalities of the concentrated and dilute solutions, and c obs. and c calcs the electromotive forces, the latter calculated by means of Nemst's formula : —

�� �