14 



in the sample taken. To make such a determination we require the foK 

 lowing- : 



1st. A standard solution of caustic soda, usually made of the 

 streng-th known as .111 normal. 



2nd. An indicator — some chemical which added to th«:; milk indicates 

 by changfe of color when enoug-h of the alkaline solution has been added 

 to render the milk neutral. Phenolphthalein is the one most commonly 

 used for this purpose. It is made by dissolving 10 grams of phenolph- 

 thalein in 300 c.c. of 80 per cent, alcohol. 



3rd. A burette, graduated to 1-10 of a cubic centimeter, in which to 

 measure the amount of solution used. 



4th. A pipette, to measure the milk or cream. 



5th. A glass or porcelain cup, and a stirring rod. A complete out- 

 fit suitable for use in butter and cheese factories may now be procured 

 from almost any of the dairy supply firms. 



For the information of those who want to make their own alkaline 

 solution or who may wish to check the strength of a solution on hand, 

 the following directions are given : 



Preparation of Solutions. The caustic soda solution may be pre- 

 pared by a druggist or one who has a delicate balance at hand by care- 

 . fully weighing out 4.4 grams of pure sodium hydroxide ^nd dissolving 

 in one litre (1000 c.c.) of water. But impurities in the sodium hydroxide 

 and lack of delicate enough balance make this method unreliable. 



The most accurate way of preparing this solution is by standardiz- 

 ing it against an acid diluted to the same strength as the alkaline solu- 

 tion wanted. As it requires an experienced chemist to prepare this acid 

 of the strength required, it is important that it be got from a reliable 

 source. 



Having on hand then a .111 normal solution of acid, the object is to 

 make a solution of the alkali, one c.c. of which will exactly neutralize 

 one c.c. of the acid. For this purpose, dissolve about 5 grams sodium 

 hydroxide (NaOH) in one litre of water. If the soda contains much car- 

 bonate, it must be removed by adding a little of a solution of barium 

 hydroxide, boiling, and filtering off the precipitated carbonates. The 

 relative strength of the acid and alkali solution is next determined. 

 This is done ,as follows : 



Rinse out a clean burette two or three times with the acid solution, 

 and then fill it with the same. Note the exact point at which the surface 

 of the liquid stands in the burette; measure out 10 c.c. of the alkaline 

 solution, and deliver into a clean beaker, glass or porcelain cup. Dilute 

 with about 50 c.c. of water, add three or four drops of the phenolphtha- 

 lein indicator, and then stirring all the time let the acid from the burette 

 drop slowly into the alkaline solution, until the color first produced by 

 the indicator is just destroyed. This is the neutral point. Now, again 

 note the exact point at which the surface of the liquid stands in the bur- 

 ette. The difference between the two readings is the amount of acid re- 

 quired to neutralize the 10 c.c. of alkali. If care be taken in coming ta 

 the neutral point slowly, it will be seen that one drop finally destroys 



