160 



I, II and III, has proven satisfactory for the titration of media : Twenty- 

 five gram aliquots of the hot media are weighed out into 350 cc. erlen- 

 meyer flasks (Jena, pryex or non-sol), which have just been rinsed with 

 carbon dioxide free water. Approximately 250 cc. of hot, carbon dioxide 

 free distilled water is added to each flask and the flasks are shaken 

 until after the mixture of water and media appear homogeneous. They 

 are then loosely stoppered and set to one side until they attain room 

 temperature. Titrations are then made with N/10 carbonate free alkali 

 and two drops of a V2 per cent solution of phenolphthalein. The end 

 point is reached on the appearance of the faintest, yet permanent pink 

 color. The fainter the color one is able to tiii'ate to, the more accurate 

 the titration. 



Summary. 



(A) Ideal media for routine bacteriological work, if rightly pre- 

 pared from selected agar agar from stabilized peptone, from stabilized 

 meat extracts and from chemicals which hydrolize but little, does not 

 need to be adjusted in reaction imless the chemicals inter-react (which 

 should lead to a choice of other chemicals) . 



(B) It is fairly well established that most bacteria will thrive in 

 a neutral medium. The standard methods (12) have allowed media to 

 be adjusted to nearer neutral than the figures would indicate. 



(1) Titrations have been carried out in hot solutions where hy- 

 drolysis is great and media corrected to certain standards by these ti- 

 trations is always nearer neutral when at blood heat or a lower tem- 

 perature. 



(2) Many have used alkali and water containing carbon dioxide 

 and the errors resulting have caused media to be adjusted to lower 

 acidity than desired. 



(C) Hydrolyzable chemicals have been used and their use has made 

 results uncertain. 



(D) Meat infusions, peptones, and other extracts have been found 

 to vary greatly in reaction. Those extracts and peptones giving best 

 results happen to be those that are most stabilized. 



(E) Some organizims tolerate more acidity than others (3) and 

 the hydrogen ion concentration must ])e determined if classifications are 

 to be made on the basis of tolerance to H and OH ion concentrations. 



