CULTURE-MEDIA. 6 1 



These also advise that media be neutralized by tUralion. 



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



I N N 



per cent, alcohol, normal * I ) and twentieth normal ( ) solutions of sodium 



I 20 



hydroxide and of hydrochloric acid. 



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



water; boil three minutes; add i c.c. of phenolphthalein solution, and proceed 



with the titration while still hot. As the reaction will usually be found acid, 



N . 



add from a burette sodium hydroxide solution, stirring constantly, until 



20 



a decided pink color develops in the entire solution. The color reaction indi- 

 cates the more or less arbitrarily adopted neutral point. Repeat this procedure 

 with three different portions of bouillon, and determine the average amount 



JV 

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



20 



N 

 of sodium hydroxide needed to neutralize the whole quantity of bouillon. 



This should be added. The bouillon should then be boiled for ten minutes, 



and again titrated. It will usually be found acid. The deficiency should 



2V 

 be corrected by adding the necessary amount of sodium hydroxide. It 



should be boiled again, and again titrated, and any deficiency made good. 



* A normal solution of any substance contains, in 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 

 crystaUization, is 126. As it is a dibasic acid (having two atoms of replaceable 



hydrogen), half of this weight, or 63 grams, per liter, is taken. Any acid 



N 

 solution vnll exactly neutralize an equal volume of any alkaline solution. 



To make sodium hydroxide solution, add about 41 grams of pure caustic 



1 

 soda to a liter of distilled water. Find the amount of this solution needed 



iV 

 to exactly neutralize i c.c. of 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 solution of sodium 



I I 



N 

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



Twentieth normal solutions have one-twentieth the strength of normal solutions. 



