128 HYDROGEN ION CONCENTRATION 



or in the urine : 



Primary phosphates 1 



Secondary phosphates I about 0.01 molar 



NaCl 0.1-0.2 molar 



In the first portion of this book it has been stated that NaCl has 

 no influence on the hydrion concentration of a solution and that 

 equation (1) above remains unaffected. But at this juncture we 

 must correct this statement. For we can no longer let the denomina- 

 tor in (1) stand for the concentration of the salt of the weak acid, or, 

 what amounts to the same thing, for the concentration of the anions 

 of this acid. What we now want is the activity of these anions. 

 Equation (1) must now be restated as 



[free acid] 



a,rT+, = k (2) 



'^ ' Activity of the acid anions 



a[H+] is the activity of theH-ions. The true concentration of the 

 acid anions is known from the composition of the buffer and from the 

 chemical analysis; it is S' to retain our formerly used symbol. Then, 

 activity of the acid anions = fa X S'. 



The true concentration of the H-ions is, on the other hand, entirely 

 unknown. We have no direct method of measuring it, and probably 

 it is of no great interest to us, for the effectiveness of the H-ions 

 appears to depend only upon the arn+j, "the H-ion activity." At 

 this point therefore we shall agree to accept a change of significance 

 of our sjanbols. From this point on the symbol [H+] will represent not 

 the concentration but the activity of H-ions, and pH its negative log- 

 arithm. Then equation (1) becomes: 



[H+] = k X [free acid] ^^^ 



fa X [salt of this acid] 



where the brackets still represent the concentration as usual. 



What determines then the value of fa? Since the Na-salt of the 

 weak acid is present in but a small amount in comparison with the 

 neutral salt (NaCl), it is evident that this excess of the NaCl exercises 

 almost the sole effect upon the activity of the buffer acid ions. The 

 electrostatic forces to which each ion is subjected determine the 

 activity coefficient; and since the ions of the NaCl are present in 

 excess, they alone must determine the activity factor of the buffer 

 acid ions. Therefore, when in a series of such solutions the amount 

 of NaCl is kept constant, but always in excess, while that of the 



