H YDROGEN-ION CONCENT RA TION 



loo 



If a medium contains several buffer systems, as is frequently the case, 

 each buffer system will play its role over the pH range in which it is active 

 alone. If two buffers with overlapping pH ranges are present, it will 

 require more acid or alkali to effect a change of unit pH than if one buffer 

 were present. The effect of buffer concentration has not been considered 

 in the above discussion. ^Vhile the pH of a buffer depends solely upon 

 the ratio of the concentrations of the weak acid and its salt, it is obvious 

 that the amount of an acid or base required to change the pH value of a 



7.0 



I 



a. 



6.0 



5.0 



^4.0 



I 



1.0 0.8 0.4 



Ml O.IN HCl 



0.4 



2.0 



0.8 1.2 1.6 



Ml. O.IN NAOH 



Fig. 21. Buffer-capacity curve.s of two media. The dotted line was obtained by 

 titrating 20 ml. of glucose-asparagine medium with O.IjV hydrochloric acid and O.liV 

 sodium hydroxide. The pH was determined after each addition of acid or base. The 

 solid line was obtained in the same way on the above medium to which 10 mg. of 

 neutralized glutamic acid had been added, (Courtesy of Robbins and Schmitt, Am. 

 Jour. Botany 32 : 324, 1945.) 



buffer one unit wdll depend upon the concentrations of the buffer acid and 

 salt present. The hufer capacity of a medium is measured by titrating a 

 definite volume of medium (usually 100 ml.) with standard acid and 

 alkali. The pH is measured after each addition of acid or alkali. From 

 the curve drawn from these data the buffer capacity for any range of pH 

 A^alues may be obtained. The curves in Fig. 21 illustrate the buffering 

 capacity of two media (Robbins and Schmitt, 1945). These media 

 differed in the buffers present. The unsymmetrical nature of the curves 

 is due to the presence of overlapping buffers. 



The pH of culture media may be controlled within desirable limits, in 

 some instances, by adding calcium carbonate to the medium. Calcium 

 carbonate is essentially insoluble in neutral and alkaline media but acts 

 as a neutralizing agent for acids. The calcium carbonate is used up as 

 acid is produced by a fungus. The degree of neutralisation achieved 

 depends upon the amount of calcium carbonate added and whether the 



