314 PHYSICAL CHEMISTRY 



general from two to three per cent higher at all concentrations than 

 the conductivity-ratio. The fact that these differences do not, as a 

 rule, increase with increasing concentration indicates that they may 

 be due to some constant experimental error, or to an error in the 

 extrapolated conductivity value. 



Accurate measurements have been made by Jahn of the electro- 

 motive force of concentration-cells consisting of two silver or mercury 

 electrodes covered with silver chloride or mercurous chloride, one of 

 which is immersed in a weak solution and the other in a strong 

 solution of sodium or potassium chloride. These measured values 

 were compared by him with those calculated from the thermodynamic 

 relation between electromotive force and the concentrations and 

 degrees of ionization of the salt in the cell. Unfortunately, however, 

 the thermodynamic relation employed involved the assumption that 

 the ionization varies with the concentration in accordance with the 

 Mass- Action Law an assumption which is known not to be true of 

 the ionization values derived from conductivity. The assumption is, 

 therefore, an irrational one one by which the question at issue is 

 prejudged. What should be done in calculating the electromotive 

 force so as to determine whether the conductivity-ratio gives ioniza- 

 tion values consistent with the measured electromotive forces is evi- 

 dently to assume that the ionization changes with the concentration 

 in the way that the conductivity indicates that it does. Arrhenius 

 recognized this error and partially corrected for it by a method of 

 approximation. I have repeated the calculations by an exact thermo- 

 dynamic formula based on an empirical law expressing the change of 

 the conductivity-ratio with the concentration, to which I will refer 

 later. The results are summed up in the statement that, when the con- 

 ductivity-ratio is assumed to represent the degree of ionization of the salt, 

 the calculated values of the electromotive force of concentration-cells exceed 

 the measured ones by only about one per cent in the case of potassium 

 and sodium chloride between the concentrations of -g-^ and --$ normal. 

 The measured electromotive force corresponds to an ionization value 

 at the latter concentration about one per cent less than the conduc- 

 tivity-ratio. 



The thermodynamic relation involving heat of solution has been 

 accurately tested with only one salt' potassium perchlorate; but 

 since it is a different salt from those used in the other experiments, 

 and since its concentration was fairly high J normal the result 

 is of interest. It was found that the measured heat of solution was less 

 by only 1.1 per cent than that calculated under the assumption that the 

 conductivity-ratio is equal to the degree of ionization. The measured 

 heat of solution corresponds to an ionization value 2 per cent lower 

 than the conductivity-ratio. 



With respect to these small deviations of the results obtained by 



