

3i)b Dr. E. A. Lehfeldt on Electromotive 



for indefinitely great dilution, be that given by Nernst's 

 equation, 



„ 3BT, C, OOAOfl 3RT, , 



E= -^~ log, jj- = 2*3026 x^— ■(+r-fs); 



i. #., graphically a straight line with a slope — =87 millivolts 



for unit difference in the common logarithm. Since, how- 

 ever, the dissociation of the salt is sensibly incomplete even 

 for very dilute solutions, the straight part of the curve does 

 not come within the range of experiment ; the electromotive 

 differences are those calculated from equation (2) above, and 

 given in table. These are: from j~ normal (^ = 7) to 

 y-— normal (yfr=6) 84 millivolts, from milli- to centi-normal 

 (^=6 to yjr = 5) 80 millivolts, from centi- to deci-normal 

 (\fr = 5 to i/^ = 4) 72 millivolts, the curve consequently being- 

 convex upwards. The experimental results agree satisfactorily 

 with the calculated over the range centi- to deci-normal, 

 showing — as might be expected — that the deviation from the 

 value given by the logarithmic formula is accounted for by 

 the incomplete dissociation of the salt, and does not imply 

 any departure from Boyle's law as applied to the solution. 



Goodwin's observations made over the range from i^ = 6 

 to ^ = 4*3, which, to save space, are not shown on the 

 diagram, follow satisfactorily on my own, extending the 

 curve into regions of greater dilution. Goodwin compared 

 his observations with the results of the formula (due to 

 Nernst) 



^ 11T. 7 A 



E=-^log^-r 2 , 

 re °7 i l 1 



where 7 is the degree of dissociation. This formula is 

 contrary to that arrived at thermodynamically above, and I 

 think is wrong. Unfortunately there is no means of de- 

 ciding between them, since for great dilution they give 

 values not far apart, and the experimental numbers lie 

 between, while for strong solutions both formulae are vitiated 

 by the inapplicability of Boyle's law. It may be noted, 

 however, that strong solutions of Z11SO4 and ZnCI 2 show a 

 maximum of conductivity (and so presumably of ionic con- 

 centration), and thus Nernst's formula would make the 

 concentrated solution anodic to the dilute, which is not the 

 case. Beyond this point, however, the curve, which on 

 account of incomplete dissociation should continue to bend 

 downwards, is found instead to pass through a point of 



