158 PHYSIOLOGY OF THE FUNGI 



rise to the same potential difference have the same pH value. Modern 

 pH meters are calibrated in pH units so that direct readings are obtained. 

 Color or turbidity does not affect potentiometric measurement of pH. 



The glass electrode in conjunction with the calomel half cell is the most 

 commonly used for liquids of biological interest. The glass electrode 

 consists of a bulb blown from a special glass. The bulb is filled with 

 O.IA'^ hydrochloric acid. A potential difference develops between the 

 inside and the outside of the electrode; the magnitude of this potential 

 difference depends upon the hydrogen-ion concentration of the liquid 

 in which the bulb is dipped. Measuring the potential difference which 

 develops between the glass electrode and the calomel half cell is equivalent 

 to determining the pH value of the unknown solution. Sensitive auxiliary 

 electrical equipment is required to measure this potential difference. 

 For a discussion of the glass electrode, see Dole (1941). 



Many suitable pH meters are available. The trend appears to be 

 toward instruments which use alternating current rather than batteries 

 as a source of power. Since the details of operation are somewhat dif- 

 ferent for the various makes, the directions of the manufacturer should 

 be consulted. 



The pH of media should be determined before autoclaving and the 

 reaction adjusted by the addition of acid or alkali if necessary. The pH 

 of a sample of a medium should also be determined after autoclaving and 

 before inoculation. The pH value at this time is known as the initial 

 pH. Alkaline media absorb carbon dioxide from the atmosphere, causing 

 a slow decrease in pH, Pritham and Anderson (1937) reported that the 

 pH of uninoculated alkaline media may decrease as much as two units 

 during the course of an experiment. This factor is of particular impor- 

 tance when upper pH limits are being investigated. For methods of 

 adjusting pH, see Suggested Laboratory Exercises. 



EFFECTS ON FUNGI 



Hydrogen and hydroxyl ions are present in all media and in substrates 

 upon which fungi grow in nature. The pH of the medium exerts a 

 decided effect upon the rate and amount of growth and many other life 

 processes. A medium may have a pH which is favorable for growth and 

 unfavorable for sporulation or other processes. The production of pig- 

 ments, vitamins, and antibiotics may be influenced by the pH of the 

 medium. As a result of metabolic activity a fungus ordinarily changes 

 the pH of the medium upon which it grows. 



pH limits. The upper and lower pH values between which a fungus 

 grows form the pH range of that species. The pH values which inhibit 

 growth vary with the species. Between the limiting pH values there is 

 a pH range which allows optimum growth. An initial pH of 5 to 6 is 



