TESTING ACIDITY OF MILK 23 



five drops of phenolphthalein.^ Allow the alkali solu- 

 tion to drop slowly into the milk or cream being 

 tested; meanwhile continually stir until a pinkish 

 color is noticeable which does not disappear imme- 

 diaetly by continued stirring. The number of c.c. 

 of alkali solution required to bring about this result 

 will indicate the number of hundredths per cent of 

 acid, since 1 c.c. of alkah will neutralize .01 per 

 cent of acid when 17.6 c.c. of milk or cream is used, 

 as in Farrington's Test. Thus if 50 c.c. neutrahzer 

 be used the milk has an acidity of .5 per cent. 



Marshall's Acid Test 



Carefully mix milk or cream to obtain an accurate 

 sample as in above cases, and fill a 9 c.c. pipette with 

 one-tenth normal solution alkali as prepared at the 

 factory. Add from two to four drops of indicator. 

 Allow alkali solution to drop into the milk, constantly 



' Phenolphthalein is one of the dyes made by a combination 

 of certain coal tar derivatives (phenols) with phthalic acid; in 

 the presence of free alkali a chemical combination is formed 

 having a red color. When a few drops of phenolphthalein 

 solution are added to milk containing free lactic acid no color 

 is appreciable in the milk; as the alkali is added the lactic acid 

 (which has a greater affinity for the alkali than has the phenol- 

 phthalein) is neutralized; and as soon as it is all neutralized the 

 slightest further addition of alkali changes the indicator to a 

 pink color. Shaking, or stirring, during the test is necessary 

 because the effect of the alkali in producing the red or pink 

 color in the indicator may be seen locally, where the alkali falls 

 into the milk, before the acid in the milk is all neutralized. As 

 soon as all the acid in the sample is neutralized the pink color 

 remains. 



