CARBONIC ACID 145 



ionized hj'drogen is always almost exactly 

 proportional to the ratio of free acid to salt, 

 and is equal, in very close approximation, 

 to the product of this ratio by the ionization 

 constant of the acid. That is to say, repre- 

 senting free acid by HA and salt by BA, 



+ 



whence, if h = (H) 



HA , ' 

 BA 



From this relationship, therefore, follows the 

 conclusion, fully established by experiment, 

 that whenever in such a solution the excess 

 of acid, HA, is chemically equivalent to the 

 quantity of salt, BA, the hydrogen ion concen- 

 tration is almost exactly equal to the ioniza- 

 tion constant of the acid. But the ionization 

 constant of carbonic acid (first hydrogen atom) 

 is 0.0Q00003. Hence in a solution containing 

 exactly equivalent quantities of free carbonic 

 acid and, for example, sodium bicarbonate, 

 the hydrogen ion concentration must be 

 0.0000003 N. Further, since 



HA = (H) 

 BA k 



