146 HYDROGEN ION CONCENTRATION 



The difference between k' and k was interpreted in the sense of the 

 change of activity of the acid-ions because of the presence of the 

 neutral salt. If we should furthermore assume that the cation of 

 the neutral salt can form a true salt with the anion of the weak acid 

 as we have done formerly, and, as we still must assume in certain 

 cases — then we find the following: 



Let us consider only the dissociation residue, p. The first approxi- 

 mation for p was 



P = -^ (1) 



The activity theory demands that, in the presence of neutral salt^ 

 we substitute for the constant k the constant k' reduced to the cor- 

 responding salt concentration, and hence, 



P=^ (2) 



If we now assume that the anion of the buffer acid and the neutral 

 salt cation (of concentration i) form a salt whose dissociation constant 

 is constant kg, then according to (I), page 140, 



p = rr-T-- r (3> 



Equation (3) differs from (2) inasmuch as the value of k' in (2) 



is substituted in (3) by k' (t- +1). Since with a constant salt 



content the value within the parenthesis is a constant, this entire 

 difference can be summarized as k", and equation (3) becomes 



P = -^ (3a> 



In the formal sense, nothing is changed, except that another 

 value of k is used, i.e., the p-curve is only horizontally displaced; 

 this is a formal procedure similar to that when k' was earlier sub- 

 stituted for k. 



