108 HYDROGEN ION CONCENTRATION 



From equation II we see that the maximum obtains in mixtures 

 consisting of equivalent amounts of the acid and of the salt; but in 

 such a mixture [H+] = k, the dissociation constant of the acid. The 

 strength of the acid therefore influences only the pH range in which 

 the mixture of the acid and its salt acts as a good buffer. 



It should now be clear as to just what constitutes a poorly or a well 

 buffered solution. The magnitude of the above defined buffer value 

 is the sole measure for it. Thus distilled water and NaCl solutions 

 are poorly buffered; river and sea waters are still relatively poorly 

 buffered; much better in this respect is blood, and still better are the 

 special buffer solutions prepared in the laboratory. 



The first coherent and exhaustive exposition of these relations was 

 given by Koppel and Spiro (1. c.) Their derivation is terminologi- 

 cally slightly different from the one given here, insofar as we have 

 attempted to adhere closely to the derivations contained in the earlier 

 chapters. Koppel and Spiro take also for their point of departure the 

 conception that given two solutions of the same pH, of which one is 

 only a strong acid and the other a buffer mixture, the first will be 

 more susceptible to the addition of base, as far as change in pH is 

 concerned, than the second. They designated the action of the 

 buffer as a "moderating" one, and the buffers themselves they named 

 "moderators." 



By the "yielding power" of a solution we have hitherto understood 

 its behavior toward the addition of small amounts of either strong 

 acid or base. But practically even of greater interest is the yielding 

 power of a buffer solution to the addition of a small amount of the 

 buffering acid itself. The commonest physiologically encountered 

 buffer is the system CO2 + NaHCOs. There is a constant formation 

 in the metabolic processes of CO2, and the obvious question arises — 

 what is the special buffer effect of such a carbonate buffer towards the 

 addition of free CO2? In a mixture of a free weak acid in concentra- 

 tion A and of its alkali salt in concentration S we have the relation 



therefore, 



pH = — log k — log A + log S 



