LABORATORY WORK IN DAIRY BACTERIOLOGY 267 



they are ready for use. All glassware subsequently used should 

 be treated in the same way. 



*No. 2. Normal NaOH Solution. To make this with absolute 

 accuracy special chemical methods are necessary. A solution 

 which is nearly accurate, sufficiently so for all purposes of this 

 work, can be made by dissolving 40 grams of pure NaOH (in 

 sticks) in one liter of water. This should be kept in a tightly 

 closed bottle. It does not maintain its strength long and should 

 frequently be made fresh. To make i/io normal NaOH, one 

 part of normal is mixed with nine parts of distilled water. 



*No. 3. Normal HC1. This is more difficult to make, and it 

 will usually be best to buy it, already made, of dealers in chemical 

 supplies. It can be made as follows: Add 50 c.c. of chemically 

 pure HC1 to 450 c.c. of distilled water. If it is of the right 

 strength, one c.c. of it will exactly neutralize one c.c. of normal 

 NaOH. Place one c.c. in an evaporating dish, dilute with 10 

 times its bulk in water, add a few drops of phenolthalein, and 

 then exactly one c.c. of normal NaOH. If the mixture turns 

 red, it means that there is not enough acid in the solution, and 

 a little more should be added. If no red appears, add a drop 

 or two more of i/io normal NaOH, and see if the red color 

 appears. If it does, the HC1 solution may be regarded as 

 normal. If, however, it requires several drops to produce the 

 red color, the HC1 solution is too strong and must be weakened 

 by adding water. By adding a little acid or water as may be 

 indicated, and testing repeatedly, the HC1 solution may be 

 brought to a strength where it will exactly neutralize the same 

 amount of NaOH. When this point is reached, it is to be 

 labeled Normal HC1. 

 *No. 4. Preparation of Bouillon. Measure out the following: 



Water I liter 



Liebigs extract of beef ...... 3 grams 



Common salt 5 grams 



Peptone . . 10 grams 



