128 Milk and Its Products 
Place in the agate cooking-pan and weigh. Dissolve 
carefully at a temperature not over 150° F. (60° C.) 
It has been found that organisms develop best at 
a reaction slightly but distinctly acid. That is, the 
media should be + 1 per cent to phenol phthalein. In 
order to standardize the media to this degree of 
acidity, take 5 ¢.-c. of bouillon, and place in a por- 
celain evaporating dish er white cup with about 45 c.c. 
of warm water. Add a few drops of phenol phthalein 
(8 per cent dry in 50 per cent alcohol). This is a 
solution which is colorless when acid, but tinted pink 
when alkaline. Stir and add to the solution in the 
cup enough of the twentieth normal sodium hydroxid 
solution from a burette to give it a clear, bright pink 
color. This is the amount needed to neutralize 5 c. c¢. 
of the bouillon. In order to bring the entire amount 
to +1 per cent, subtract 1 from the amount of twen- 
tieth normal sodium hydroxide used; multiply this by 
the number of cubic centimeters of bouillon, and 
divide by 100. This result represents the amount of 
normal sodium hydroxide to be added. After adding 
the normal hydroxide, test the reaction again. It 
should take 1 c. c. of twentieth normal sodium hydrox- 
ide to bring a pink color with 5 ce. ec. of bouillon. 
This process is called titration. 
After obtaining the desired acidity, boil briskly for 
twenty minutes, restore weight, with distilled water, 
cool and filter through filter paper into sterile flasks. 
The bouillon is now ready to be made into either gela- 
tine or agar, or it can be sterilized and set aside 
until needed. (See directions for sterilizing media.) 
