138 MODERN METHODS OF TESTING MILK 



desired strength by dilution. (4) Alkali tablets and 

 powders containing a definite amount of carbonate 

 can be purchased and dissolved according to directions. 



In preparing alkali solutions, observe the following 

 precautions: (a) Measuring. The measuring-flask or 

 graduated cylinder used in making dilutions must be 

 accurate, (b) Water used for dilution. The water 

 used must be neither acid nor alkaline; water care- 

 fully distilled and kept free from impurities should 

 always be used, if possible; next best is clean rain- 

 water, (c) Use of dry alkali When pure sodium 

 hydroxide (caustic soda) is used, it must be care- 

 fully transferred without loss from the bottle to 

 the vessel in which solution and dilution are to take 

 place; the bottle is rinsed with water several times, 

 the rinsings being poured into the vessel holding 

 the alkali. The dissolving of caustic soda in water 

 produces heat and the solution should be allowed to 

 cool to room temperature before it is finally diluted to 

 the desired mark. Bottles containing alkali in the dry 

 form should come tightly sealed, and be kept so, and 

 should not be opened until the solution is to be made. 

 It is necessary to use the entire contents of the bottle 

 at one time, (d) Use of concentrated alkali solution. 

 When a normal alkali solution is purchased and di- 

 luted, the same precautions must be observed as when 

 the dry form is used. 



(3) Precautions in keeping alkali solution. The al- 

 kali solution should be kept from contact with air in 

 order to prevent weakening by absorption of carbon 

 dioxide and moisture. A weakened alkali solution has 

 the effect of giving higher results of acidity than the 



