[Vol. 9 



3-60 ANNALS OF THE MISSOURI BOTANICAL GARDEN 



amount of these substances so that it becomes impossible to in- 

 dicate the optimum degree of acidity which would be the same 

 under all experimental conditions. On the other hand, obtaining 

 a measure of the acidity by means of the hydrogen-ion concen- 

 tration, it is possible to show that this is constant and is as 

 definite as the nature and quantity of the enzyme. Thus, while 

 the total acidity necessary to give optimum enzyme action is 

 very different for 3 enzymatic solutions, the optimum concentra- 

 tion of hydrogen ions, on the other hand, is the same for all 3. 



Using a similar illustration for bacteriological media, it may be 

 said that while the optimum acidity for the growth of a particular 

 organism may be -f-15 on a medium containing Witte's peptone, 

 it will be different on a medium containing some other peptone. 

 When an organism is found to respond best to a reaction of Ph 7, 

 it will always respond similarly at that reaction no matter what 

 the peptone may be, other things remaining the same. Occa- 

 sionally it becomes important to know the buffer value of the 

 media and in such cases a determination of the total acidity by 

 ordinary titration is utilized in conjunction with a determination 

 of the hydrogen-ion concentration. Illuminating articles on the 

 buffer values of different peptones have been written by Bronfen- 

 brenner, DeBord, and Orr (II, '21) and by Brown (II, '21). 



Titration 

 (Fuller's scale) 



Ph 



+ 7.5 7.5 



+ 8.0 7.4 



+10.0 7.5 



+ 11.5 7.1 



+ 12.5 _— 7.4 



+ 14.0 6.8 



+ 14.5 6.8 



+ 15.0 6.9 



Not only are different results to be expected because of the 

 difference in principles in titrating for total acidity as compared 

 to a determination of hydrogen ions, but total acidity measure- 

 ments, such as the procedure involved in calculating Fuller's 

 scale, are rather inaccurate for other reasons. As Clark (II, '15) 

 has stated, the use of phenolphthalein as indicator, titrating when 

 the medium is hot, and the difference in end point used by differ- 

 ent investigators due to lack of proper color standards, all tend 

 to make the ordinary titration method variable and inexact. 



