132 HYDROGEN ION CONCENTRATION 



or 



E = 0.05S log -^ 



E and h2 being known, hi is easily obtained. 



Criticism is invited when the question is raised as to whether it 

 is justifiable to assume always that the two ho values are identical. 

 If one of these solutions were aqueous and the other in anilin, it 

 would never occur to anyone to regard the ho values as identical. 

 But also in the case when both solutions are aqueous but the one is 

 relatively poor and the other relatively rich in salt content, it is not 

 correct to assume the same ho value for both. In such a case, as of a 

 gas chain consisting of a 0.001 N HCl on the one side and 0.001 A^ 

 HCl + 1 A^ KCl on the other, we find, after allowing for the diffusion- 

 potential, an E.M.F. of a few (2.5) millivolts. Previously, when it 

 had been assumed that the degree of dissociation of the HCl in the 

 two solutions was unequal, this E.M.F. was explained by the differ- 

 ence in the [H+] concentrations of the two solutions. But, as we 

 learned above, this assumption rests on an erroneous interpretation, 

 and we are no longer justified in accepting such an explanation. If 

 we should actually suppose, as in the case above, that hi = ho exactly, 

 then the E.M.F. of such a chain can be explained only by assuming 

 that the two ho values are unequal. If we designate this value as ho 

 in the electrolyte-poor solution and h'o in the electrolyte-rich solu- 

 tion, then the E.M.F. of this chain should rather be stated as 



/ hi h" \ 



E = TTi - 7r2 = 0.05S ( log }^ - log ^1 j 



but since hi = ho, 



ho' 



E = 0.05S • log — 



ho 



Hence, we can express this condition by saying that: The electro- 

 lytic solution pressure of the Ho in an aqueous solution of low electro- 

 lyte content is somewhat different from that in an aqueous solution 

 rich in electrolytes. And yet, when we apply, as we always do, the 

 earlier incorrect formula for the calculation of the ho value from the 

 E.M.F. of concentration chains, we obtain a purely calculated figure 

 which cannot represent the actual concentration of the H-ions. But 

 it is this calculated value which we designate as the result of our 



