38 PHYSIOLOGICAL CHEMISTRY 



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

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

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

 acid will be indicated by the appearance of a red color. To differentiate between 

 these bodies make a spectroscopic examination and look for the absorption band 

 between D and E given by pentoses and glycuronic acid. Differentiate between 

 the two latter bodies by the melting-points of their osazones. 



Compare the reaction with that obtained with galactose (page 36). 



3. Orcinol Test. Repeat 2, using orcinol instead of phloroglucinol. A suc- 

 cession of colors from red through reddish blue to green is produced. A green pre- 

 cipitate is formed which is soluble in amyl alcohol and has absorption bands be- 

 tween C and D. 



4. Phenylhydrazine Reaction. Make this test on the arabinose solution 

 according to directions given under Glucose, 3 or 4, pages 22 and 23. 



CH 2 OH 

 XYLOSE, (CHOH) 3 



CHO 



Xylose, or wood sugar, is obtained by boiling wood gums with dilute 

 acids as explained under Arabinose, page 37. It is dextro-rotatory, 

 forms an osazone and has reducing power, but does not ferment. 



EXPERIMENTS ON XYLOSE 

 1-4. Same as for arabinose (see above). 



RHAMNOSE, C 6 Hi 2 O 5 



Rhamnose or methyl-pentose is an example of a true carbohydrate 

 which does not have the H and O atoms present in the proportion to 

 form water. Its formula is C 6 H 12 5 . It has been found that rham- 

 nose when ingested by rabbits or hens has a positive influence upon the 

 formation of glycogen in those organisms. 



DISACCHARIDES, C 12 H 22 On 



The disaccharides as a class may be divided into two rather dis- 

 tinct groups. The first group would include those disaccharides which 

 are found in nature as such, e.g., sucrose and lactose, and the second 

 group would include those disaccharides formed in the hydrolysis of 

 more complex carbohydrates, e.g., maltose and iso-maltose. 



The disaccharides have the general formula Ci 2 H 22 On, to which, 

 in the process of hydrolysis, a molecule of water is added causing the 



