156 



PHYSIOLOGY OF THE FUNGI 



cultures are agitated. See Foster (1949) for a discussion of the use of 

 calcium carbonate in industrial microbiological processes. 



For fungi which have an extremely narrow pH range, the special cul- 

 ture flask devised by Cantino (1949) for culturing Blastodadia pring- 

 sheimii may be used (Fig. 22). A base (or acid) is placed in the side arm 



and an internal indicator of the desired pH 

 range is added to the medium. A little of the 

 base is tipped into the culture flask as desired. 

 Flasks with two side arms may be used so that 

 either acid or base maybe added to the culture 

 medium. 



Fig. 22. Flask designed for 

 the study of glucose dis- 

 similation by Blastodadia. 



A, the side arm containing 

 NaOH for neutralization; 



B, sintered-glass aerator; C, 

 inlet for aeration with differ- 

 ent gas mixtures; D, the 

 outlet for removal of media. 

 (Courtesy of Cantino, Am. 

 Jour. Botany 36 : 100, 1949.) 



METHODS OF DETERMINING pH VALUES 



Only two general methods of measuring pH 

 values will be discussed. The colorimetric 

 method is simple, inexpensive, and sufficiently 

 accurate for most purposes, but it cannot be 

 used with highly colored or turbid media. 

 The potentiometric method using the glass 

 electrode is more accurate and is often the 

 preferred method. 



Colorimetric methods. The use of indi- 

 cators w^hich change color in response to 

 varying concentrations of hydrogen ion is 

 the basis of this method. Indicators may 

 be considered as weak acids or bases, and as 

 such they act as buffers, but the amounts used are so small they do not 

 affect the accuracy of a determination. For methods of measuring the 

 pH of unbuffered solutions see Snell and Snell (1948). The property of 

 these indicator buffers which distinguishes them from other buffers is 

 that the colors of the salts and free acids or bases (nondissociated) are 

 different. Within the usable pH range, the color of the indicator is a 

 function of the hydrogen-ion concentration of the medium. For exam- 

 ple, bromocresol purple (p/va, 6.3) is yellow in solutions having pH values 

 of 5.2 or less and purple at pH 6.8 or more. Within the pH range 5.2 

 to 6.8 the color changes from yellow to purple. To determine the pH 

 value of an unknown solution within this range, the indicator is added to 

 equal amounts of standard buffers and the unknown solution, and from 

 the color of the standard buffers of known pH, the pH value of the 

 unknown may be estimated to within 0.1 pH unit. By a suitable choice 

 of indicators the pH range of interest may be covered. A few indicators 

 with their pH ranges are listed in Table 32. 



Two methods of color comparison are in general use. The first involves 



