66 MANUAL OF BACTERIOLOGY. 



venient and serviceable for most purposes, it is advisable to use 

 meat when the bouillon is to be employed for the development 

 of bacterial toxins. Meat should also be used in the prepara- 

 tion of either bouillon, gelatin or agar-agar when new species 

 of bacteria are being studied for publication. 



In both of these cases the recommendations of the American Public Health 

 Association should be followed.* 



These recommendations have been largely followed in the directions for the 

 preparation of culture-media given below. The student is referred to the re- 

 port itself for further details than those given below. 



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

 per cent, alcohol, normalf (-^-) and twentieth normal (y^-) solutions of sodium 

 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, 



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



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



*Report of Committee of the Am. Pub. Health Assn. on Standard Methods 

 of Water Analysis, Journ. Infect. Dis. Sup. No. i. May, 1905. 



fA 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 hydro- 

 gen 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 nor- 

 mal 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 nor- 

 mal solutions. Its molecular weight, including a molecule of water of crystal- 

 lization, 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 

 solution will exactly neutralize an equal volume of any - alkaline solution. 

 To make -~- 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 -^- solution of oxalic acid; this amount contains 

 the quantity of sodium hydroxide which should be present in i c.c. of a nor- 

 mal 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 hydrox- 

 ide as a standard, an ~- solution of hydrochloric acid may be prepared. 

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



