222 PHYSIOLOGICAL CHEMISTRY 



about 3 c.c. or less. The fluid should be clear at this point, and 

 a fine white precipitate of mucic acid should form. 



II. PENTOSES. Only two pentoses are of physiological in- 

 terest, arabinose and xylose. These pentoses give the reduction 

 tests and form osazones. They may be distinguished from dex- 

 trose and levulose by two color reactions, the orcin and phlo- 

 roglucin tests. 



a. Arabinose. The arabinose may be obtained by hydrolyzing 

 gum arabic by boiling for several hours with 1-2% sulphuric 

 acid. 



i. Orcin Test. Mix equal volumes (about 2 c.c.) of arabinose 

 solution and concentrated hydrochloric acid, add a few grains 

 of orcin and heat on the water bath. In the presence of a pen- 

 tose, galactose, or glucuronic acid a red color develops which 

 gradually passes through reddish blue to green. The color alone 

 is not sufficient proof of the presence of a pentose. To confirm 

 the test it is necessary to extract the liquid with amyl alcohol; 

 if pentoses are present, this extract will contain the colored 

 compound which should show an absorption band between the 

 Fraunhofer lines C and D. 



ii. The pJiloroglucin test, commonly called Tollen's reaction, 

 is similar to the above test except that phloroglucin is used in 

 place of orcin. The absorption band of the amyl alcohol extract 

 is between D and E. 



iii. Final proof of the presence of a particular pentose is ob- 

 tained- by determining the melting point of its osazone. 



II. Disaccharides 



a. Saccharose (Sucrose). Study the properties of saccharose 

 as follows : 



i. Solubility. (See note on solubility determinations under 

 Dextrose) . Test the solubility of cane sugar in water, cold alco- 

 hol, hot alcohol (do not warm over a flame, dip the test tube 

 in hot water) and ether. 



