FORMALDEHYDE 81 



tendency is to prohibit the entire use of particular preservatives 

 and to restrict them generally. 



The preservatives in most general use are boric acid, borax, 

 or mixtures of these two, and formaldehyde. For milk the last- 

 mentioned is the favourite owing to its potency and general con- 

 venience. The presence of boric acid or borax is allowed in 

 cream in England when declared on the label attached to the 

 container and in quantities not exceeding 0.25 per cent when 

 calculated as boric acid. Harden has shown that the addition 

 of an alkali (7 grms. of Na20 per 100 grms. of boric acid) in- 

 creases the efficiency of boric acid as a preservative, and it is 

 now customary to employ such a mixture for the preservation of 

 cream. Such mixtures also contain cane sugar or traces of 

 saccharin, the object of which is to mask incipient sourness. 



Formaldehyde. Formaldehyde may be detected by any 

 of the following tests, but on account of its reUabihty and del- 

 icacy, the author recommends the Shrewsbury and Knapp 

 process. 



Hehner Method. About 10 c.cms. of sample are placed 

 in a test tube and concentrated commercial sulphuric poiu-ed 

 carefully down the side so as to form a layer beneath the nulk. 

 In the presence of formaldehyde, a violet ring is formed at the 

 junction of the two hquids. Richmond and Boseley modified 

 the test by adding an equal volmne of water to the milk and 

 using acid of 90 to 94 per cent strength. One part in 200,000 

 produces a violet colouration which is permanent for several 

 days. In the absence of formaldehyde, a greenish ring is pro- 

 duced and a brick-red colouration in the acid layer. 



Leonard^ points out that the presence of a mild oxidising 

 agent is essential for the success of this test and that such an 

 agent, preferably a trace of ferric chloride, must be added if 

 pure acid is used. Droop Richmond ^ points out that the test 

 is dependent upon the reaction of formaldehyde with the 

 tryptophane of the caseinogen and that other aldehydes, e.g., 

 vanillin, give similar reactions. 



Hydrochloric Acid Test. 10 c.cms. of commercial hydro- 



