

CARBOHYDRATES. 37 



and permit it to stand for some time before the precipitate will form. 

 It is impossible to differentiate between galactose and lactose by 

 this test, but the reaction serves to differentiate these two sugars 

 from all other reducing sugars. Differentiate lactose from galac- 

 tose by means of Barfoed's test (p. 31). 



3. Phenylhydrazine Reaction. Make the test according to 

 directions given under Dextrose, 3 or 4, pages 24 and 25. 



Pentoses, C 5 H 10 5 . 



In plants and more particularly in certain gums, very complex 

 carbohydrates, called pentosans, occur. These pentosans through 

 hydrolysis by acids may be transformed into pentoses. Pentoses 

 do not ordinarily occur in the animal organism, but have been 

 found in the urine of morphine habitues and others, their occur- 

 rence sometimes being a persistent condition without known cause. 

 They are non- fermentable, have strong reducing power and form 

 osazones with phenylhydrazine. Pentoses are an important constitu- 

 ent of the dietary of herbivorous animals. Glycogen is said to be 

 formed after the ingestion of these sugars containing five oxygen 

 atoms. This, however, has not been conclusively proven. On 

 distillation with strong hydrochloric acid pentoses and pentosans 

 yield furfurol, which can be detected by its characteristic red reac- 

 tion with aniline-acetate paper. 



CH 2 OH 



ARABINOSE, (CHOH) 3 . 



CHO 



Arabinose is one of the most important of the pentoses. The 

 /-arabinose may be obtained from gum arabic, plum or cherry gum 

 by boiling for several hours with 1-2 per cent sulphuric acid. 

 This pentose is dextrorotatory, forms an osazone and has reducing 

 power. The z-arabinose has been isolated from the urine and 

 yields an osazone which melts at 166-168 C. 



EXPERIMENTS ON ARABINOSE. 



i. Tollens' Reaction. To equal volumes of arabinose solu- 

 tion and hydrochloric acid (sp. gr. 1.09) add a little phloroglucin 

 and heat the mixture on a boiling water-bath. Galactose, pentose 

 or glycuronic acid will be indicated by the appearance of a red 



