PROPERTIES OF MATTER 



TABLE 2-2. Relationship between Hydrogen-ion Concentration 



and pH at 25°C 



Hydi^xyl-ion 

 Hydrogen-ion ConcenUalion, Concentralion, 



41 



acid or base. A mixture of sodium acetate and acetic acid serves as an 

 illustration. In solution the following species are present: 



CH3COO- + Na+ 

 CH3COOH + H.O 



When Na+OH- is added, 



CH3COO- + H:30+ 



OH- + H3O+ ±=^ 2H2O 



disturbing the equilibrium dissociation of acetic acid to form more 

 acetate and hydrogen ion in satisfaction of the dissociation constant. 

 In this way hydrogen ions lost by surrender of protons to hydroxy! ions 

 are largely replaced. As a result the pH change is quite slow until 

 most of the acetic acid has been converted to acetate ion. Addition of 

 more hydroxyl ion then changes the pH extensively. This buffer 

 system is effective then in one pH range but not others. The disso- 

 ciation constant of the acid determines the effective range, so other 

 weak acids would be used in other ranges. 



It is readily apparent that acetic acid itself is a good buffer when 

 hydroxyl ion and other bases are being added. However, it is not 

 effective when strongly acidic solutions are added instead, for the 



