DISSOCIATION OF STRONG ELECTROLYTES 133 



measurements and commonly accept as the desired [H+]- or pH value. 

 An error was then made by using an incorrect formula for the calcula- 

 tion of the H-ion concentration, and we designate this erroneously 

 calculated concentration as the H-ion activity. 



And yet this calculated value appears to be of significance in the 

 physical sense. In many cases in which H-ions exercise any effect 

 whatever the effect is proportional not to the true H-ion concentra- 

 tion but apparently rather to their activity. Stated in other words: 

 two solutions having an equal potential towards an Ho-electrode 

 frequently exhibit an equal effectiveness of their respective H-ions. 

 The limits within which this provisional statement is valid must be 

 established by future investigation, now that the underlying prin- 

 ciples are to a certain extent clarified. In any event we have derived 

 our justification to retain for the present, as a measure of the actual 

 acidity, the values for [H+] and Hkewise pH calculated from the 

 E.M.F. of the concentration chain in place of the true hydrion con- 

 centration. 



AN EXAMPLE FOR THE DIFFERENT METHODS OF REPRESENTATION OF 

 THE EFFECT OF STRONG ELECTROLYTES 



It may be useful to clarify further the differences between the 

 various explanations by means of an illustrative example. Let us 

 take the concentration chain. ^^ 



H, 



0.001 N HCl 



0.001 iV acetic acid 

 + 0.001 AT Na - acetate 



H2 (1) 



In this case all the solutions are of such great dilution that in accord 

 with any theory whatever the HCl and Na-acetate may be assumed 

 to be completely dissociated, and any possible effect of the ions upon 

 the properties of the solvent may be neglected. We can, therefore, 

 say that in the 0.001 A^ HCl, hi = 0.001 N, and for the calculation 

 of hi in^the acetate mixture we may apply Nernst's formula (at 19°) 



hi 



E = 0.579 log — 



n2 



" The two solutions in the actual arrangement are not in direct contact, 

 but are connected by means of a saturated KCI solution, in order to avoid the 

 diffusion potential which will otherwise arise. 



