METHODS OF TESTING THE ACIDITY OF MILK 133 



longer observed any sour taste of acid or soapy feel- 

 ing or odor of alkali, because the acid and alkali have 

 neutralized each other and have combined to form 

 simply common salt, the presence of which is noticed 

 by its taste. The solution is said to be neither alkaline 

 nor acid, but neutral. 



USE OF INDICATORS IN TESTING ACIDITY 



In working with acids and alkalis, it is necessary 

 to have some simple means of knowing when a solu- 

 tion is acid, alkaline or neutral (neither acid nor alka- 

 line). This can be found by using some substance, 

 called an indicator, which is so acted on by alkalis 

 and acids as to undergo changes of color, being 

 changed one color by alkalis and a different color 

 by acids. One substance which finds wide use as an 

 indicator is a chemical compound called phenolphthal- 

 ein, a solution of which is turned pink by alkalis and 

 colorless by acids. For use in testing acidity, one dis- 

 solves 10 grams of the dry powder in 300 cc. of 90 

 per cent, alcohol, or 5 grams in 100 cc. of 50 per cent, 

 alcohol, adding one or more drops of dilute alkali until 

 the solution is slightly pinkish in color. It is necessary 

 to use only 5 or 10 drops of this solution as indicator. 



GENERAL APPLICATION OF PRINCIPLES OF 

 NEUTRALIZATION 



What use can be made of the foregoing facts in 

 finding the per cent, of acid in a solution? For sim- 

 plicity, we will use the following illustrative experi- 

 ment : In a glass or teacup we put 100 cc. of a solu- 

 tion containing .25 per cent, of lactic acid and add 5 

 or 10 drops of indicator solution. Into this mixture 



