152 PHYSIOLOGY OF THE FUNGI 



fractions. If a solution contains 4.23 X 10"" mole per liter of hydrogen 

 ion, this concentration of hydrogen ions may be expressed in terms of pH 

 by use of Eq. (5). 



(7) pH = log ^^3 ^ ^Q_, = log 1 + log 10« - log 4.23 = 5.37 



By reversing the above calculations, the hydrogen-ion concentration may 

 be calculated if the pH value is known. 



The pH scale has three features which may be perplexing: (1) alkaline 

 solutions are designated on the same scale as acidic solutions; (2) increas- 

 ing acidity is expressed by smaller pH values; and (3) the logarithmic 

 nature of the scale. A difference of one pH unit indicates a tenfold 

 difference in hydrogen-ion concentration, while a difference of 0.3 pH unit 

 means a twofold difference. A solution having a pH value of 4 contains 

 10,000 times as many hydrogen ions per unit volume as a solution having 

 a pH of 8. A solution having a pH value of 6.2 has twice the hydrogen- 

 ion concentration of a solution having a pH value of 6.5. The student 

 should remember that pH is the name of a logarithmic scale used to 

 measure hydrogen-ion concentration and not an entity in itself. 



BUFFERS AND BUFFER CAPACITY 



A medium having pH values between 5 and 6 at the time of inoculation 

 is suitable for most fungi, but more acidic or more alkaline media are used 

 at times. It is often important that the pH of the culture medium does 

 not change too greatly as the result of metabolic activities of the organism. 

 The ease with which the pH of a medium is modified depends upon the 

 composition of the medium. 



Substances which tend to maintain the pH of a solution relatively 

 constant when an acid or base is added, or when the solution is diluted, 

 are called buffers. In general, the kinds of compounds which act as 

 buffers are mixtures of weak acids or bases and their salts, acid salts of 

 polybasic acids, basic salts of polyacidic bases, and amphoteric compounds 

 such as amino acids and peptides. As an example of a buffer we may 

 consider a solution which contains equivalent amounts of acetic acid and 

 sodium acetate. If a strong acid such as hydrochloric is added to this 

 buffer solution, a reaction will occur between the sodium acetate and the 

 hydrochloric acid. 



(8) CH3— COONa + HCl -^ CH3— COOH + NaCl 



The net effect of adding hydrochloric acid is the formation of an equiv- 

 alent amount of acetic acid. Inasmuch as acetic acid is only slightly 

 ionized, no great change in hydrogen-ion concentration will take place 

 until most of the sodium acetate has been converted into acetic acid. If 



