CURD-MAKING 



61 



known strength, using phenolphthalein as an indi- 

 cator. The object of the indicator is to tell the con- 

 dition of the milk, whether i't is acid, alkaline or neutral. 

 The indicator does not change in an acid solution but 

 turns pink when the solution is or becomes alkaline. 

 To make the test, a known quantity of the material to 

 be tested is placed in a 

 white cup, and to this 

 several drops of indicator 

 are added. As an indi- 

 cator, a 1 per cent solu- 

 tion of phenolphthalein 

 in 95 per cent alcohol is 

 commonly used. As an 

 alkali solution, sodium 

 hydroxide (NaOH) is 

 used in the standardized 

 strength usually either 

 tenth (N/10) normal or 

 twentieth (N/20) normal. 

 This solution should be 

 obtained in some one of 

 the standardized forms 

 commercially prepared. 

 The alkali is added, drop 



by drop, from a gradu- FIG. 9. An acid tester. 



ated burette until a faint 



pink color appears. This shows that the acid in the 

 milk has been neutralized by the alkali. The amount 

 of alkali that has been used can be determined from the 

 burette. Knowing the amount of milk and alkali solu- 

 tion used, it is easy to calculate the amount of acid in 

 the substance tested. The results are usually expressed 



