36 PHYSIOLOGICAL CHEMISTRY 



examination to differentiate lactose and galactose from other reducing 

 sugars. 



EXPERIMENTS ON GALACTOSE 



1 . Phloroglucinol-Hydrochloric Acid Reaction (Tollens) . To equal volumes of 

 galactose solution and hydrochloric acid (sp. gr. 1.09) add a little phloroglucinol, 

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

 acid will be indicated by the appearance of a red color. Galactose may be 

 differentiated from the two latter substances in that its solutions exhibit no 

 absorption bands upon spectroscopical examination. 



2. Mucic Acid Test. Treat 100 c.c. of the solution containing galactose with 

 20 c.c. of concentrated nitric acid (sp. gr. 1.4) and evaporate the mixture hi a broad, 

 shallow glass vessel on a boiling water-bath until the volume of the mixture has 

 been reduced to about 20 c.c. At this point the fluid should be clear, and a fine 

 white precipitate of mucic acid should form. 



If the percentage of galactose present is low it may be necessary to 

 cool the solution 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 

 galactose by means of Barfoed's test (page 29). 



3. Phenylhydrazine Reaction. Make the test according to directions given 

 under Glucose, 3, page 22. 



Pentoses, C 5 Hi 5 



In plants, and more particularly in certain gums, very complex car- 

 bohydrates, called pentosans (see page 50), occur. These pentosans 

 through hydrolysis by acids may be transformed into pentoses. Pen- 

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

 found in the urine of morphine habitues and others, their occurrence 

 sometimes being a persistent condition without known cause. They 

 may be obtained from the hydrolysis of nucleoproteins being present 

 in the nucleic acid complex of the molecule. Pentoses are non- 

 fermentable have strong reducing power and form osazones with phenyl- 

 hydrazine. Pentoses are an important constituent 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 reaction with aniline-acetate paper. 



CH 2 OH 



ARABINOSE, (CHOH) 3 



CHO 



