144 THE FITNESS OF THE ENVIRONMENT 



Whenever a weak acid is present in aqueous 



solution in company with such bases sa 



sodium, potassium, calcium, magnesium, etc., 



which are invariable constituents of the ocean, 



blood, protoplasm, etc., provided the acid be 



in excess, it is a simple matter to determine 



the reaction, which can best be measured by 



+ 

 the values of (H) and (OH), following the 



considerations above. 



Now there is a certain characteristic prop- 

 erty of an acid, its ionization constant, k, 

 which measures its tendency to dissociate 

 in aqueous solution, thereby to produce hydro- 

 gen ions, and hence to increase the intensity 

 of acidity. Strong acids have ionization con- 

 stants which are of the order of magnitude of 

 1.0, weak acids of the order of magnitude of 

 0.0001, the weakest acids, 0.00000001, or less. 



Table of Ionization Constants 



HC1, HN0 3 , etc 1. 



H 3 P0 4 0.011 



H 3 As0 4 0-005 



HNO2 0.0005 



H 2 C0 3 0.0000003 



NaH 2 P0 4 0.0000002 



H 2 S 0.000000091 



H3BO3 0.0000000007 



Na 2 HP0 4 0.00000000000036 



It has been discovered that in the general 

 case above discussed the concentration of 



