METHODS OF TESTING THE ACIDITY OF MILK 137 



If, therefore, an i8-gram (17.6 cc.) sample is used for 

 testing, i cc. of tenth-normal alkali solution corre- 

 sponds to 0.05 per cent, (one-twentieth of one per 

 cent.) of lactic acid in the solution tested; o.i cc. of 

 alkali solution corresponds to 0.005 (one two-hun- 

 dredth of one per cent.) of acid. When a Q-gram 

 sample is used, I cc. of tenth-normal alkali solution 

 corresponds to o.i per cent, (one-tenth of one per 

 cent.) of lactic acid in the solution tested; and O.I per 

 cent, of alkali corresponds to o.oi per cent, (one one- 

 hundreth of one per cent.). 



In a one-fiftieth normal solution, I cc. of alkali cor- 

 responds to 0.0018 gram of lactic acid. Therefore, 

 in case of an i8-gram (17.6 cc.) test sample, I cc. of 

 such alkali solution corresponds to o.oi per cent, (one 

 one-hundredth of one per cent.). 



(2) Methods of preparation. The standard alkali 

 solution is obtained in one of several ways, (i) It 

 can be purchased ready-made in any strength desired 

 from dairy-supply or chemical-supply houses. It costs 

 most in this way. (2) The chemically pure dry sodium 

 hydroxide (caustic soda) can be purchased from re- 

 liable chemical-supply houses in exact amounts and dis- 

 solved in water. This should be the cheapest way of 

 obtaining the alkali solution. For example, 4 grams 

 of the pure compound dissolved in 1000 cc. (i liter) 

 of water makes a tenth-normal solution ; 40 grams dis- 

 solved in 1000 cc. of water makes a normal solution ; 

 and 100 cc. of this normal solution, diluted to 1000 cc. 

 makes a tenth-normal solution or, diluted to 5000 cc. 

 makes a fiftieth-normal solution. (3) The normal 

 alkali solution can be purchased and made to any 



