150 



The two principal methods now employed to determine the reaction 

 of media are the determination of the hydrogen ion concentration by 

 means of the hydrogen electrode, and the total titratable acid present 

 as determined by titration. The hydrogen electrode was applied to bio- 

 chemistry by Sorensen (7). Since 1912 several investigators have used 

 the hydrogen electrode in the study of bacterial activities. Among these 

 are Michaelis and Marcola (8) ; Brunn (9) ; Clark (1) ; Itano (5) ; and 

 Clark and Lubs (10). 



The advantages of the hydrogen electrode in bacteriological work 

 are claimed to be that it gives the hydrogen ion concentration the bac- 

 teria are in contact with and that it can be used advantageously in col- 

 ored solutions. Its disadvantages are that it can not be used in solid 

 media and that for every grouping of chemicals there is a new electro 

 chemical problem. Different investigators working with the hydrogen 

 electrode, from a purely scientific point of view, have not agreed on the 

 contact potential between 0.1 N. HCl.— 0.1 N. KCl. (11). 



This paper is written not to find fault with the hydrogen electrode 

 in its applications to bacteriology but to point out some factors in the 

 making of culture media and in the controlling of its reaction that are 

 as important as the method by which the reaction is determined. It is 

 (so-called) acidity due to the crude methods of making media that is 

 discussed in the following paragraphs. 



Hot Solutions. 



The usual procedure followed in titrating culture media is crude. 

 Titrations are conducted in hot solutions (12). Hydrolysis increases 

 with temperature and titrations of culture media containing meat, 

 peptone, gelatine, agar agar or plant extracts when made at high tem- 

 peratures are much greater than they would be at lower temperatures. 

 The difference between hot and cold titrations is often more than the 

 titration of the media at loom temperature. Clark (1) mentions a 10 

 per cent gelatine, 1 per cent peptone, and 5 per cent meat media titrat- 

 ing plus 1.0 per cent acid when hot and plus 0.5 per cent acid at room 

 temperature. 



Small Aliquots. 



Too small aliquots of media are generally used. Aliquots are pi- 

 petted or poured out from graduated cylinders. These methods of taking 



