122 MILK AND DAIRY PRODUCTS PART 



reagent, which is a solution of fuchsin from which the colour 

 has been discharged by sulphur dioxide. 1 



One c.c. of dilute sulphuric-acid (1 : 3) is added to 100 c.c. of 

 milk, which is then distilled from a 500 c.c. distillation flask, a 

 ring burner being used, until 20 c.c. of distillate has been 

 collected. A part of the distillate (10 c.c.) is then treated with 

 the above-mentioned SchifFs reagent. If, after 5 or 6 minutes, 

 an intense carmine colour appears, which passes into violet-blue 

 on shaking with 2 c.c. of hydrochloric acid, there was formalde- 

 hyde present in the milk. 



Simpler, and yet quite delicate, is the method recommended 

 by Utz, in which Winckel's reagent vanillin and hydrochloric 

 acid is employed : 10 c.c. of hydrochloric acid (sp. gr. T19) and 

 a few grains of vanillin are warmed in a test-tube with 10 c.c. of 

 milk. With ordinary milk a beautiful deep violet coloration 

 is given, but if only traces of formaldehyde are present in the 

 milk the colour is yellow instead of violet. 



6. Hydrogen Peroxide. 



Where so little hydrogen peroxide is added to the milk that 

 it is split up entirely into water and free oxygen, no test will 

 detect the preservative, for it is no longer there. If, on the 

 other hand, a small excess of hydrogen peroxide is present, it 

 can easily be determined both qualitatively and quantitatively. 



(a) Qualitative Estimation. 



The method generally recommended in text-books for the 

 detection of hydrogen peroxide by means of potassium 

 bichromate and ether (blue coloration of the ether due to 

 perchromic acid) is not sufficiently sensitive, and is easily 

 surpassed by Arnold and Mentzel's test with vanadic acid. 

 This acid is used in the form of a solution made by dissolving 

 1 g. of the acid in 100 g. dilute sulphuric acid. Ten drops 01 

 this yellow solution are added to 10 c.c. of milk, which turns red 

 if hydrogen peroxide is present. As little as O'Ol g. hydrogen 

 peroxide in 100 c.c. of milk will be shown by this test. 



The most sensitive reaction, however, is that of Wilkinson and 



1 For the preparation of Schiff's reagent, see p. 232. 



