PROPERTIES 



Riboflavine is sparingly soluble in ethyl alcohol (4-5 mg. per 100 

 ml. at 27-5° C), amyl alcohol, cyclohexanol, phenol or amyl acetate, 

 but insoluble in acetone, ether, benzene or chloroform. 



Riboflavine is amphoteric in nature, with an isoelectric point at 

 pH 6.^2 jhg dissociation constants are : 



Ka = 63 X 10-^2 and Kj, = 0-5 X lO"^. 



On acetylation a tetraacetate, m.p. 242° C, is formed. 



As already stated above (page 136), when an alkaline solution is 

 irradiated, lumiflavine, m.p. 330° C., is produced, and this, being 

 sparingly soluble in water, separates out from the irradiated solution. 



Riboflavine is reduced to a colourless leuco-com.pound on treat- 

 ment with sodiimi dithionite solution, and the colour and fluores- 

 cence are restored on exposure to air. 



Riboflavine has a characteristic absorption spectrum, the peaks of 

 the absorption bands being situated at 221, 266, 359 and 445 m/z. 



Crystalline riboflavine is stable in the dark at ordinary tempera- 

 tures, but slowly decomposes on exposure to light. In solution it is 

 unstable, especially when alkaline solutions are exposed to light. It 

 is moderately stable to heat, and no appreciable destruction occurred, 

 for example, when milk was incubated for twenty- two hours at 31 to 

 37° C.,^^ or during the cooking of foods .^* When, on the other hand, 

 milk in bottles was exposed to sunlight, more than half the riboflavine 

 was destroyed within two hours. ^^« ^® The rate of destruction by 

 light increased as the temperature and pK were increased.^* 



So sensitive is riboflavine to the action of light that riboflavine 

 assays (see page 159) should be carried out in dim light and preferably 

 in a red light ; a 150 -watt lamp screened with a red cellophane filter 

 has been recomnaended.^' The light from the lamp normally employed 

 in a Coleman spectrophotometer, however, does not cause appreciable 

 destruction. 



Riboflavine was said to be rendered more stable to light by the 

 presence of sodium dithionite ^^ or by heating with boric acid.^® 

 Solutions containing boric acid are recommended for injection, being 

 said to be self -sterilising as well as photo-stable. 



Riboflavine was included in the Sixth Addendum (1945) to the 

 British Pharmacopoeia 1932, which laid down tests for identity and 

 purity. The monograph was slightly modified in the British Pharma- 

 copoeia 1948. The prophylactic and therapeutic doses are given as 

 I to 4 mg. and 5 to 10 mg. daily respectively. 



References to Section 6 



I. R. Kuhn and H. Rudy, Ber., 1935, 68, 169; P. Karrer and H. 

 Fritzsche, Helv. Chim. Acta, 1935, 18, 1026. 



155 



