HYDROGEN-ION CONCENTRATION 153 



sodium hydroxide is added to an acetate-acetic acid buffer, the sodium 

 hydroxide will react with the acetic acid. 



(9) CH3— COOH + NaOH -* CHsCOONa + H2O 



This reaction will convert sodium hydroxide, which is a strong base, 

 into a salt of a weak acid and water. A slight increase in pH will occur, 

 owing to the hydrolysis of the sodium acetate formed. After most of the 

 acetic acid has reacted with sodium hydroxide, the addition of more of 

 the base will result in a rapid increase in the pH value of the solution. 



In culturing fungi, it is important to choose buffers which retain the 

 pH of the medium in the desired range. The effective yH range of buffers 

 prepared from weak acids and their salts is related to the degree of ion- 

 ization of the acids. The more highly an acid ionizes, the lower will be 

 the pH range of a buffer prepared from it and one of its salts. The degree 

 of ionization of weak acids is designated by a term called the ionization 

 constant {Kg). Mixtures of weak bases and their salts are also buffers. 

 A few ionization constants of w^eak acids are acetic, 1.8 X 10~^; carbonic 

 (first hydrogen), 3.5 X 10"^; phosphoric (first hydrogen), 1.1 X 10"^; 

 phosphoric (second hydrogen), 7.5 X 10^^. Extensive data of this kind 

 may be found in various handbooks of chemistry. 



The ionization constants of weak acids may be used to calculate the 

 effective pH range of buffers prepared from these compounds and their 

 salts by means of the following relation: 



[salt] 



(10) pH = p2v„ + log 



[acid] 



The symbol p/C is equivalent to log {l/Ka). When the mole concen- 

 trations of the weak acid and its salt are equal, Eq. (10) becomes: 



(11) pH = log -^ = p/Va 



The p/va value of a weak acid is thus the pH of a buffer which contains 

 equivalent quantities of a weak acid and one of its soluble salts. The 

 pH of an acetate buffer containing equivalent amounts of acetic acid and 

 an acetate may be calculated using Eq. (11). 



(12) pH = log ^ g ^ ^Q_, = log 1 + log 105 - log 1.8 = 4.74 



In an analogous manner it can be shown that the pH of a buffer com- 

 posed of equivalent amounts of a weak base and one of its salts is related 

 to the ionization constant of the base {Kh) by the following equation: 



(13) pH = 14 - log ^ = pA^t, 



For a derivation of the formulas relating pH and ionization constants 



