12 LABORATORY GUIDE IN BACTERIOLOGY 



6. Adjust the reaction. 



Bacteria, especially pathogenic bacteria, grow prefer- 

 ably in a medium which is neutral or slightly acid to 

 phenolphthalein (alkaline to litmus). Extract of beef 

 always contains a certain amount of acid, which has to 

 be neutralized. Two methods may be employed to 

 accomplish this purpose. 



First method. Gradually add a 4 per cent, solution 

 of sodium hydrate (NaOH) until a sample on the end 

 of the glass rod turns phenolphthalein paper a pale rose- 

 pink color. If accidentally too much NaOH is added, 

 it may be readjusted by means of a 5 per cent, solution 

 of hydrochloric acid (HC1). This method is sufficiently 

 accurate for ordinary purposes. The medium is then 

 strongly alkaline to litmus. 



Second method. A more precise method is as follows: 

 By means of a pipette measure 5 c.c. of the liquid into 

 a porcelain evaporating-dish ; add 45 c.c. of distilled 

 water and i c.c. of a solution of i part phenolphthalein 

 in 200 c.c. alcohol (50 per cent.), then heat to the boiling- 

 point and slowly add from a graduated burette enough 

 -$ normal NaOH to neutralize. (The liquid must 

 have a decided, stable, pale-pink color which does not 

 vanish when heated.) Read the amount used from the 

 burette, and calculate the amount of normal NaOH needed 

 for 1000 c.c. Then add 0.5 per cent, normal HC1. 



Example. By reading the burette we find that it 

 takes 1.2 c.c. -$ normal NaOH to neutralize 5 c.c. of 

 the medium, diluted with 45 c.c. of distilled water. To 

 neutralize i liter, it will take 200 times this amount, 

 which amounts to 240 c.c. of the -^ normal NaOH. 

 Divide 240 by 20, and the result (12 c.c.) is the amount 



