166 



CULTURE MEDIA 



This shows that addition of 0.5 cc portions of N/20 NaOH 

 caused changes in reaction of from as httle as 0.15 pH to 

 as much as 2.20 pH and that changes in yR are 7iot at all 

 proportional to alkali added. 



For this reason other methods of titrating to show the 

 relative H ion concentration have been introduced and 

 are now very generally used in the United States. These 

 methods are two in number, the electrolytic method and the 

 use of indicators other than phenolphthalein, which show 

 color changes within narrow and definite ranges of H-ion 

 concentration, of 2^H. 



Fig 



-Set up of potentiometer outfit for hydrogen-ion determination. 

 Lettering same as Fig. 110, 



The electrolytic method requires a rather delicate set of 

 apparatus — the potentiometer (Figs. 109 and 110) — which is 

 scarcely adapted for students in introductory courses or for 

 class use and will not be discussed here. Every bacteriological 

 laboratory should be equipped with such an outfit, to be 

 used as a standard of reference and for comparison with 

 other methods in accurate work. 



