l8 CULTURE MEDIA. 



Having determined the percentage acidity of the 10 c.c. sample 

 tested, we easily calculate the number of c.c. of N/iNaOH solution 

 required to be added to the 1000 c.c. of bouillon to obtain a reaction 

 corresponding to the neutral point of phenolphthalein. It is more exact 

 to take the average of two titrations. 



As 100 c.c. of medium would require 31/2 c.c., 1000 c.c. would 

 require 10 times as much, or 35 c.c. N/i NaOH solution. Having 

 measured out and added 35 c.c. of the N/i NaOH solution to the 

 meat infusion, containing salt and peptone, we have a solution which is 

 exactly neutral to phenolphthalein, or o. It is usually considered that 

 a reaction of about i percent acid is the optimum reaction for bacterial 

 growth. Hence we should now add i % of N/i HC1 solution to the 

 medium. This would be accomplished by adding 10 c.c. of N/i 

 HC1 solution to the 1000 c.c. of neutralized medium, and we would 

 have a medium with a reaction of + 1. If we desired a reaction of one 

 percent alkalinity we would add an additional c.c. of N/i NaOH 

 solution to every 100 c.c. of the medium at o, or 10 c.c. for the 1000 

 c.c. of medium. The reaction would then be i. 



As a matter of convenience, we usually determine the reaction of the 

 medium, which is always more or less acid, and then add enough N/i 

 NaOH to reduce the acidity to the percentage we desire to set the 

 medium, instead of neutralizing all the acidity present and then, in a 

 second operation, restoring the acidity to the point desired. 



Thus finding the acidity of the medium to be 3 1/2% and desiring to 

 give it an acidity of i%, we would add only 21/2 c.c. of N/i NaOH 

 to every 100 c.c. of medium, or 25 c.c. for the 1000 c.c. of medium. 

 The reaction would then be found to be -fi. 



The neutral point of litmus is not a sharp one, but it corresponds 

 rather closely with a reaction of +1.5 to phenolphthalein. The 

 recommendations of the A. P. H. Association call for making the 

 titration with the medium boiling. This is a very difficult titration 

 and students obtain results varying greatly, which is not the case when 

 the titration is conducted at room temperature and a standard color is 

 at hand. If the color of the end reaction at boiling-point be obtained, 

 it will be found that when cool it deepens until it corresponds to the 

 rich violet-pink of the end reaction in the cold or vice versa. 



