TESTING MII^K AND CREAM FOR ACID 597 



more time-consuming work of weighing. While the size of the 

 sample may vary, factory experience has shown that 18 grams 

 makes a very practical sample. It represents enough material to 

 insure reasonable accuracy and it obviates complex calculations to 

 determine the per cent acid. In the case of milk the 18 grams are 

 measured with the standard 17.6 c.c. Babcock test pipette. In the 

 case of cream an 18 c.c. or possibly a 9 c.c., pipette is used. The 

 cream pipette must be rinsed with water and the rinsings added to 

 the sample. 



Example : 7.4 c.c. of tenth normal alkali solution are required 

 to neutralize 18 c.c. of cream to a faint pink color. What is the 

 per cent acid? 



7.4 



Or, for more rapid calculation, simply divide the c.c. alkaline 

 solution required, by 20. 



7-4 ^ 



= .37 per cent acid. 



If instead of an 18 c.c. pipette, a 9 c.c. pipette is used, then only 

 one-half as much alkaline solution, or 3.7 c.c., is required and the 

 per cent acid is calculated as follows: 



37 * - 009 X 100 = .37% acid. 



Or the c.c. alkaline solution required is simply divided by 10. 

 -j-=z .37 per cent acid. 



Phenolphthalein indicator is prepared by dissolving 1 gram dry. 

 phenolphthalein in 100 c.c. of a mixture of one-half alcohol and one- 

 half water. 



Standard Acid Tests and their Equipment for Factory 

 Use. The following are practical and satisfactory acid tests, the 

 equipment and chemicals for which can be readily secured from 

 glassware manufacturers and from dairy and creamery supply 

 houses : 



Mann's Acid Test. Apparatus needed: 50 c.c. graduated 

 burette with burette stand for measuring the alkaline solution ; a 50 

 c.c. pipette for measuring the cream, a white cup and stirring rod or 

 teaspoon. 



N 

 Reagents needed: j^rNaOH (a one-tenth normal solution of 



