CULTURE-MEDIA. 73 



In both of these cases the recommendations of the American Public 

 Health Association should be followed. 1 



These also advise that media be neutralized by titration. 



The following solutions are required : $ per cent, phenolphthalein in 50 

 per cent alcohol, normal 2 (') and twentieth normal (|^) solutions 

 of sodium hydroxide and of hydrochloric acid. 



To 5 c.c. of bouillon in a porcelain evaporating dish add 45 c.c. of dis- 

 tilled water ; boil three minutes ; add I c.c. of phenolphthalein solution, 

 and proceed with the titration wrwle still hot. As the reaction will 

 usually be found acid, add from a burette /^ sodium hydroxide solu- 

 tion, stirring constantly, until a decided pink color develops in the 

 entire solution. The color reaction indicates the more or less arbi- 

 trarily adopted neutral point. Repeat this procedure with three differ- 

 ent portions of bouillon, and determine the average amount of ^ 

 sodium hydroxide required. It is now possible to calculate the amount 



1 See the Report of the Committee of the American Public Health 

 Association entitled Procedures Recommended for the Study of Bac- 

 teria. 1898. Rumford Press, Concord, N. H. 



2 A normal solution of any substance contains, fn a liter, as many 

 grams of the substance as there are units in its molecular weight, in 

 case it contains a single atom of replaceable hydrogen. If it has two 

 atoms of replaceable hydrogen the number of grams used equals the 

 molecular weight divided by two ; and so on. Thus the molecular 

 weight of sodium hydroxide is 40, and its normal solution contains 40 

 grams of sodium hydroxide in a liter. It is not expedient to prepare 

 normal solutions of sodium hydroxide by weight. For convenience, 

 crystallized oxalic acid is used as a starting point in making normal 

 solutions. Its molecular weight, including a molecule of water of 

 crystallization, is 123. As it is a dibasic acid (having two atoms of 

 replaceable hydrogen), half of this weight, or 62.5 grams, per liter, is 

 taken. Any y" acid solution will exactly neutralize an equal volume of 

 any ~ alkaline solution. To make j" sodium hydroxide solution, add 

 about 41 grams of pure caustic soda to a liter of distilled water. Find 

 the amount of this solution needed to exactly neutralize I c.c. of y 

 solution of oxalic acid; this amount contains the quantity of sodium 

 hydroxide which should be present in I c.c. of a normal solution. It 

 is now possible to calculate the amount of distilled water to be added 

 in order that I c.c. of the sodium hydroxide solution may neutralize 

 i c.c. of the solution of oxalic acid. With an f solution of sodium 

 hydroxide as a standard, an ^ solution of hydrochloric acid may be 

 prepared. Twentieth normal solutions have one-twentieth the strength 

 of normal solutions. 



