152 



changes to very close to the hydrogen ion concentration at which phe- 

 nolphthalein changes from colorless to pink. 



The last report (13) of the committee on standard methods for bac- 

 teriological analysis of milk makes no recommendation as to the adjust- 

 ing of the reaction of the media. This is taken as an indication of a 

 growing realization by this committee that proper selection of materials 

 in making media gives a media near to neutral in reaction. Other evi- 

 dence that most bacteria will thrive when media are somewhere near 

 neutral is brought out in the fact that most ensymes function when 

 kept close to neutral. 



Carbon Dioxide. 



Usually some carbon dioxide is present in the alkali used, and many 

 bacteriologists consider freshly distilled water carbon dioxide free. Cai*- 

 bon dioxide has affected the accuracy of some titrations, for we have 

 reference to where investigators advise against titrating the media to 

 a low per cent of aciditiy for fear of volatilizing ammonia from the 

 ammonium salt used in making the media, (14). Ammonia is not easily 

 volatilized from acid solutions but is slowly evolved by alkaline solu- 

 tions even at low temperatures (40'C.), therefore these investigators are 

 making their media nearer neutral than they think. Slightly alkaline 

 media saturated with carbon dioxide is acid to phenolphthalein. 



Apparatus supply houses are advertising water stills which, ac- 

 cording to the advertisements, give pure distilled water. Quoting from 

 the advertisement of one of the leading firms, we have "water of the 

 highest purity — free from ammonia and all gaseous and organic im- 

 purities." These stills, as shown by the titrations given in the following 

 table do not give carbon dioxide free water where the water used in 

 them is hard. Freshly distilled water made from the same local hard 

 water supply with different stills gave the following titrations with N/10 

 carbonate free alkali and phenolphthalein.* 



* The water from which distilled water is prepared in many localities is as hard 

 or harder than that in this locality. 



