CHAPTER V. 



Reactions Methods for Adjustment Titration Hydrogen-ion Concentra- 

 tion Preparation of Media Bouillon, Gelatin, Agar-agar, Potato, 

 Blood Serum, Blood Serum from Small Animals, Milk, Litmus- whey 

 Milk, Durham's Peptone Solution, Lactose Litmus-agar, Loffler's 

 Blood-serum Mixture, the Serum-water Media of Hiss, Guarniari's 

 Gelatin-agar Mixture. 



REACTION: METHODS OF ADJUSTING. 



OF fundamental importance to the successful cultivation 

 and study of bacteria upon artificial media is the reaction 

 of the media used. For most purposes this should be at 

 or about neutral. 



Reaction may be roughly determined by the use of litmus 

 papers: Acids causing the blue paper to turn red and alka- 

 lies turning the red paper blue. 



It may also be determined by titration, the indicator used 

 being a substance that announces by changes in color slight 

 deviations to the acid or alkaline side of neutral. 



It may also be determined by estimating the total acidity 

 as indicated by the hydrogen-ion content resulting from 

 the dissociation of various electrolytes dissolved in the 

 medium. 



For a long time the simple method of adjusting the reac- 

 tion of culture media by the use of litmus papers was thought 

 all that was necessary. Closer study of the matter, however, 

 revealed, among other facts, that litmus is inconstant in its 

 composition and that it also undergoes changes in color 

 resulting from influences other than those of acids and 

 alkalies. In consequence, for more exact work other methods 

 have been developed. 



(ill) 



