PREPARATION OF CULTURE MEDIA 31 



N 

 2.4 c.c. of NaOH (twenty times as strong) ; in other words the 



medium is 2.4 per cent acid to phenolphthalein (+2.4). Assum- 

 ing the required reaction is +1 per cent we must add 2.4 c.c. 



N 

 minus 1 c.c. or 1.4 c.c. of NaOH to every 100 c.c. of medium, or 



14 c.c. to a liter. The plus sign is used to indicate an acid and 

 the minus sign an alkaline reaction to phenolphthalein. 



Should, on the other hand, the mixture show a pink color when 



N 

 the indicator is added, the medium is alkaline and HC1 is used 



\J 



instead of the sodium hydroxide solution as above, until only a 

 very faint color persists. If, for example, 0.5 c.c. of the acid has 

 been used then the medium is 0.5 per cent alkaline ( 0.5%) and 



N 

 it will require 0.5 c.c. of HC1 for every 100 c.c., or 5 c.c. for 



every liter, to bring it to the neutral point. If the required reac- 

 tion is +1 or 1 per cent acid, then 5 c.c. plus 10 c.c., or 15 c.c., of 



N 



HC1 must be added to each liter of medium. 



Clearing Media. This is accomplished as follows : One or 

 two eggs for each liter of medium are lightly beaten in a 

 pan and mixed with a little water. The medium is cooled to 

 below 60 C. and the eggs are thoroughly stirred into it. The 

 mixture is then heated in the Arnold Sterilizer or autoclave 

 and as the egg albumin coagulates, it enmeshes and carries 

 down with it as a sediment all the fine particles, thus leaving 

 the medium clear. 



Filtering Media. Fluid media may be filtered through paper ; 

 for media which will solidify on cooling absorbent cotton is better. 

 In the latter case a spiral of copper wire is placed in the bottom 

 of the funnel and two moderately thick squares of absorbent 

 cotton are so arranged over it that the fibers of one are at right 

 angles to those of the other. Paper or cotton should first be mois- 

 tened with water so that any fat in the media will not pass through. 

 The first filtrate may not be clear, in which case it should be passed 



