TITRATION OF MEDIA 1 9 



Having determined the percentage acidity 'of the 10 c.c. sample tested, we easily 

 calculate the number of c.c. of N/i NaOH solution required to be added to the 1000 

 c.c. of bouillon to obtain a reaction corresponding to the neutral point of phenol- 

 phthalein. It is more exact to take the average of two titrations. 



As 100 c.c. of medium would require 3 1/2 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% 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 +i. If we 

 desired a reaction of i% 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 2 1/2 c.c. of N/i NaOH to every 100 c.c. of 

 medium, or 25 c.c. for the zoooc.c. of medium. The reaction would then be found 

 to be +i. 



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. 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. 



To summarize: 



Take Peptone, 10 grams 



Sodium chloride, 5 grams 



50% meat infusion, 1000 c.c. 



Dissolve the peptone and sodium chloride in the meat infusion and 

 add enough N/i NaOH to make the reaction +i. 



Put the solution in the inner compartment of a rice cooker and bring 

 to the boiling-point and maintain this temperature for twenty minutes. 

 The calcium chloride or sodium chloride in the outer compartment of 

 the rice cooker enables us to secure a boiling temperature for the con- 

 tents of the inner compartment. Do not stir the bouillon that is being 

 heated, as the pultaceous membranous mass of coagulated albumin 

 makes nitration easy. Filter. The filter-paper in the funnel should be 



