230 BERTHA VAN H. ANTHONY AND C, V. EKROTH 



is marked, that the error is not appreciable. In solutions of organic 

 substances, especially when considerable amounts of feebly dissociated 

 substances such as are contained in peptone or gelatin, are present, 

 this error becomes very appreciable. The discrepancy between the 

 end point for litmus and for phenolphthalein will vary for different 

 lots of media. 



Naturally those media which contain litmus as an indicator 

 to show acid production by the growth of bacteria, must be 

 alkaline to litmus yet not too alkaline or the indicator is ren- 

 dered useless. The testing of such media by the use of litmus 

 paper is an unsatisfactory and crude method useful for only 

 the roughest work. The use of a litmus solution (Merck's 

 purified in 5 per cent aqueous solution) is far more satisfactory. 



"Neutral to litmus" is "so and so" acid to phenolphthalein, 

 the figure given varying with the writer. Muir and Ritchie 

 place it at + 2.5, Stitt at + 1.5 boiling titration and about 

 + 0.7 with the cold titration. Abbott gives + 2.5, Abel + 1.5 

 to 2.5 and Heinemann + 2, all depending on the shade of pink 

 considered by the worker as suitable and the length of time 

 the sample is boiled in the casserole. 



In the modified method used in our laboratory, the figure is 

 as low as + 0.6 or + 0.7 with media prepared with 1 per cent 

 peptone. (This figure rises to + 1 when the boiling titration 

 is used.) A 1 per cent peptone (Witte) solution in water is 

 about + 0.2 to + 0.3 with phenolphthalein at room tempera- 

 ture and about + 0.4 at boiling figure. 



It has been shown by Hildebrand (1913) and others (Clark, 

 1915; Bovie, 1915) that an indicator does not indicate the point 

 of actual neutrality but merely a definite degree of hydrogen 

 ion concentration. Where these two points coincide, i.e., where 

 the hydrogen ion concentration at which the indicator changes 

 color, is within the zone of absolute neutrality for a particular 

 mixture of substances, this change of color in an indicator will, 

 of course, be of significance. It would therefore seem desirable 

 to select a specific indicator for each class of media. This 

 could be accomplished only by determining the effect of added 



