38 MILK ANALYSIS 



Boric acid and borax. — These may be detected by the follow- 

 ing test : A few drops of the sample are mixed with a drop of 

 hydrochloric acid and a drop of strong alcoholic solution of 

 turmeric, evaporated to dryness at a gentle heat, and a drop 

 of ammonium hydroxid added to the residue when cold. A 

 dull green stain shows that boric acid is present. 



Formaldehyde. — Hehner found that when milk containing this 

 substance is mixed with sulfuric acid containing a trace of ferric 

 salt, a blue color appears. Richmond & Boseley showed that 

 the delicacy of the test is much increased by diluting the milk 

 with an equal bulk of water and adding sulfuric acid of 90 to 

 94 per cent., so that it forms a layer underneath the milk. 

 Under these conditions, milk, in the absence of formaldehyde, 

 gives a slight greenish tinge at the junction of the two liquids, 

 while a violet ring is formed when formaldehyde is present even 

 in so small a quantity as i part in 200,000 of milk. The color 

 is permanent for two or three days. In the absence of formalde- 

 hyde, a brownish color is developed after some hours, not at 

 the junction of the two liquids, but lower down in the acid. 



One of the most delicate and positive tests for formaldehyde 

 is as follows : To a few c.c. of the suspected liquid, a pinch of 

 phenylhydrazin hydrochlorid is added, the liquid shaken and a 

 drop of a dilute solution of sodium nitroprussid added and then 

 a few drops of sodium hydroxid. Milk containing formalde- 

 hyde gives a grayish-green. If the test is applied to the pure 

 solution obtained by distilling the sample, as noted below, a 

 characteristic deep blue is produced. This is a delicate and 

 useful test and should always be applied. It avoids the fallacy 

 noted below in the testing of ice-cream and desserts. 



Another test is the addition of a small amount of a solution of 

 I per cent, of phloroglucol and about 25 per cent, of sodium 

 hydroxid in water. This produces a rose-red. The test is best 

 applied by running the test solution by means of a pipet under 

 the suspected liquid. 



