134 HYDROGEN ION CONCENTRATION 



E being found to be at 19°C = 0.0997 volts, then by calculation 

 pH = 4.722 and [H+] = 1.90 X 10"^ 



Now, the dissociation constant of acetic acid, as determined by 

 the conductivity method, is 1.86 X 10~^ From the buffer equation 

 we should expect to find in the acetate mixture 



[Acetic acid] 

 [Na — acetate] 



which is in very good agreement with the above found value of 1.90 

 X 10~^. As long as the ionic content of our solutions does not 

 exceed 0.001 N, the theory, built upon the basis of the simple buffer 

 equation, will yield a fairly complete picture of the relation of things. 

 Now let us take the concentration chain^^ 



H, 



0.001 N HCl 



0.1 N acetic acid 

 + 0.1 iV Na — acetate 



Ho 



at 19°C, it is found that E = 0.0930 volts. By applying again 

 Nernst's formula we obtain for the acetate mixture pH = 4.606 and 

 [H+] = 2.48 X 10"^. How are we to explain then that in one equi- 

 molar mixture of acetic acid and Na-acetate we found [H+] = 1.90 

 X 10~^andinthe other [H+] = 2.48 X 10~^, according as tO whether 

 the concentration was 0.001 A^ or 0.1 Nf 



1. The first older theory says: in 0.1 N solution Na-acetate is 



[acetic acid] 



not totally dissociated, therefore, in this fH+] is not = k^ , 



[IN a-acetate J 



k [acetic acid] ^ , ^ , , ^ ,. . 



but = — TTz ——.'Where y stands for the degree of dissocia- 



7 [Na-acetate] 



tion of Na-acetate in 0.1 N solution. If we assume that the 



value of k (1.90 X 10~^) found in the first concentration chain to be 



1.90 

 correct, then y = r-— = 0.766. But as we have now come to 



2.48 



believe, in reality y = 1, or very nearly so. Therefore, this inter- 

 pretation is incorrect. 



2. The activity theory says: Because of the Na-acetate content 

 of the solution, the activity of the ions becomes different from their 

 true concentration. The Na-acetate is completely dissociated, there- 



^^ With same arrangement of chain as indicated in preceding footnote. 



