TESTING ACIDITY OF MILK AND MILK PRODUCTS 93 



DESCRIPTION OF MANN'S ACID TEST 



Apparatus and materials. — The apparatus and ma- 

 terials used in this test are : ( i ) A 50 cc. glass burette, 

 with stopcock, (2) a 50 cc. pipette for measuring sam- 

 ples, (3) a glass beaker and stirring-rod, (4) 

 an alcoholic solution of phenolphthalein, 

 and (5) a standardized alkali solution la- 

 beled "neutralizer," each cubic centimeter 

 of which contains enough sodium hydrox- 

 ide (.004 gram) to neutralize .009 gram 

 of lactic acid. The outfit is shown in Fig. 



37- 



Operating the test. — (i) Measuring 

 sample. — Into a clean beaker or white por- 

 celain cup, one measures 

 with the pipette exactly 50 

 cc. of the material (milk, 

 cream, etc.) to be tested. 

 The pipette is then rinsed 

 by filling with water, pre- 

 ferably distilled, and this 

 rinsing water is added to the sample. 



(2) Adding alkali. — The indicator (5 or 10 drops) 

 is then added to the sample. The burette is then filled 

 to the zero mark with the neutralizer, and some of it 

 is run into the sample in the beaker or cup. A pink 

 color appears, but disappears on stirring. The addi- 

 tion of alkali in portions is continued with care, the 

 sami)le being stirred after each addition. It will be 

 noticed sooner or later, according to the amount of 

 acid present, that, after each addition of alkali, the 



FIG. 37 — MANN S ACID TEST 



