DISSOCIATION OF STRONG ELECTROLYTES 135 



fore, the concentration of acetate-ions, Cacetate = 0.1 A''. The 

 activity of the acetate ions aacetate = fa X Cacetate, where fa, the 

 activity factor, is < 1. Furthermore, what is measured by means 

 of the concentration chain is not the concentration of H-ions, Ch+, 

 but their activity, aH+, and the relation between the two is 



a.Tj+ = I a X Oh+ 



where again f'a < 1, but is not necessarily = fa. The buffer equation 

 must therefore be restated as 



k [acetic acid] 

 ^^ fa' [Na — acetate] 



If we let f'a = 0.766, then we obtain the desired value an^ = 2,48 X 

 10~^. The concentration chain measurement does not yield anything 

 du-ectly concerning the value Ch+, so long as there is no way of 

 determining the value of f'a. 



3. The third explanation says: As a solvent, the solution rich in 

 salt content does not behave in the same way as pure water. Con- 

 sequently, the solution pressure of the Ho-Pt electrode towards it is 

 not quite equal to that towards water. Hence the second of the 

 above chains does not represent, strictly speaking, a concentration 

 chain, for even two solutions of the same [H+], one in salt-poor 

 water, the other containing much salt, will give rise to an E.M.F. 

 of a few volts. This adventitious E.M.F. must be subtracted from the 

 observed E.M.F. of the second chain before we can utihze it as a 

 true concentration chain for the calculation of pH. The calculation 

 carried out without this correction gives a value of pH which is in a 

 certain respect too low, the calculated [H+] is hence too great by a 

 definite factor. 



Besides this, it is possible that in a solution containing much salt 

 the dissociation constant of acetic acid is greater than in a salt-poor 

 solution. 



These two conditions bring about the result that pH calculated in 

 the usual way has the value of only a calculation figure, which may 

 deviate to the extent of about 0.1 from the original definition of pH. 

 But since the correction which we would have to introduce demands 

 the knowledge of certain data which are not yet completely available, 

 it is for the time being more practicable to define the uncorrected 

 figure of our calculation as the pH value. 



