CARBOHYDRATES 259 



G-alactose. 



This sugar gives the usual reduction tests and forms an osa- 

 zone. It may be distinguished from dextrose and levulose by 

 the mucic acid test. Lactose also gives this test, as galactose 

 makes up one-half of the lactose molecule. To distinguish be- 

 tween lactose and galactose one may use the Barfoed test, which 

 reacts with lactose only after prolonged boiling. 



15. Mucic Acid Test. To 50 c.c. of galactose solution add 10 

 c.c. concentrated nitric acid and evaporate on the water bath to 

 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. Three pentoses are of physiological interest, 

 arabinose, xylose and ribose. 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. 



Arabinose. The arabinose may be obtained by hydrolyzing 

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

 acid. 



16. 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. 



17. The phloroglucin 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. 



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

 tained by determining the melting point of its osazone. 



