BACTERIOLOGICAL TECHNIC 77 



with the hydrant are properly made and that the flow is properly regu- 

 lated, in order to guard against any back pressure, which may cause the 

 receiver to fill with hydrant water. This accident is best avoided by 

 interpolating a flask or bottle. Agar may also be clarified by precipita- 

 tion. Pour the hot agar into an ordinary percolator used by pharmacists. 

 The dirt particles and other impurities will gradually settle to the bottom. 

 When cool, take out the solid medium and cut away the lower portion 

 containing the sediment. 



C. Titration of Culture Media. As already stated, most bacteria 

 grow best in neutral or very slightly alkaline (to litmus) media, and since 

 most media are quite decidedly acid in reaction, it is desirable to alkalinize. 

 This is done by means of normal sodium hydroxide solution. In order to 

 understand the method of procedure clearly, it is necessary to make certain 

 explanations. 



A normal (N/i) solution of any substance contains as many grams per 

 liter of the substance as there are units in its molecular weight, if the sub- 

 stance contains one atom of replaceable hydrogen. If it contains two atoms 

 of replaceable hydrogen, the number of grams used equals the molecular 

 weight divided by two, and so on. According to this, a normal solution of 

 sodium hydroxide contains 40 gm. of sodium hydroxide in a liter. Exact 

 normal solutions are, however, not prepared by weight. Crystallized 

 oxalic acid is used as the basis for making normal solutions. This acid 

 has a molecular weight (including a molecule of water of crystallization) 

 of 126, and, since it is dibasic, 63 gm. per liter are taken. Any normal 

 acid solution will exactly neutralize an equal volume of normal alkaline 

 solution. To make a normal sodium hydroxide solution, add about 

 14 gm. of pure caustic soda to one liter of distilled water. Determine the 

 amount of this solution required to just neutralize i cc. of normal oxalic 

 acid solution. This volume contains the quantity of sodium hydroxide 

 which should be present in i cc. of normal solution, and from this we may 

 calculate the volume of distilled water to be added in order that i cc. of 

 sodium hydroxide solution will neutralize i cc. of normal oxalic acid solu- 

 tion. Having a normal solution of sodium hydroxide, it is now possible to 

 prepare a normal solution of hydrochloric acid, etc. A tenth- (N/io), 

 twentieth- (N/2o), fiftieth- (N/5o) normal solution is a normal solution 

 diluted ten, twenty, and fifty times. 



An acid reaction is indicated by +, and an alkaline by . The degree 

 of acidity of any culture medium in preparation may be indicated by the 

 amount of normal sodium hydroxide solution required to render it neutral 

 to phenolphthalein. Neutralization by titration is done as follows: 

 Place 5 cc. of the medium to be neutralized in a dish, add 45 cc. of distilled 

 water, stir, and bring to a boil. Add i cc. of phenolphthalein solution 



