37 



MANNS'S ACIDITY TEST. 



Apparatus required: 

 One 50 cubic centimeter glass burette graduated to 



tenths, with stopcock. 

 One 50 cubic centimeter pipette. 

 One 250 cubic centimeter beaker, or a white teacup. 

 One support for burette. 

 Glass stirring rods. 



One-tenth normal solution of caustic soda, each cubic 

 centimeter of which will neutralize 0.009 gram of 

 lactic acid. 



An alcoholic solution of phenolphthalein made by 

 dissolving 10 grams in 300 cubic centimeters of 90 

 per cent alcohol. 



One who has not had training in chemistry should not 

 attempt to make the tenth-normal solution of caustic soda, 

 as it can be purchased to better advantage from any chem- 

 ical supply house. 



Conducting the test. With the pipette 50 cubic centi- 

 meters of the milk or cream is measured into the beaker or 

 cup and 2 or 3 drops of phenolphthalein solution added. 

 If the cream is thick, it may be slightly warmed. The 

 burette is filled with the tenth-normal caustic-soda solu- 

 tion so that the lowest part of the meniscus is level with 

 the zero point on the graduations. The solution is now 

 run slowly from the burette into the milk or cream, stir- 

 ring with a glass rod at the same time. It will be noticed 

 that the alkali at once produces a pink color where it 

 strikes; this, however, disappears on stirring. As more 

 and more of the alkali is added, it will be noticed that the 

 pink color is slower in disappearing until finally it becomes 

 permanent for a time. Toward the end, the alkali should 

 be added drop by drop and the very first appearance of a 

 permanent faint pink is the signal that the neutral point 

 has been reached. This color, on account of absorption 

 of carbon dioxid from the air, will disappear after standing 

 a short time. The number of cubic centimeters of alkali 

 used can be learned by referring to the burette, remember- 

 ing that the reading is taken from the lowest point of the 

 meniscus. 



