V] CARBOHYDRATES 63 



Some of the best-known gums are the following : 



Oum Arabic (arabin). This substance is obtained from an Acacia 

 {Acacia Senegal), a native of the Soudan. The gum exudes from the 

 branches. Other species of Acacia yield inferior gums. Gum arabic is 

 a mixture of the calcium, magnesium and potassium salts of arabic acid, 

 a weak acid of which the constitution is unknown, in combination with 

 araban and galactan. 



Gum Tragacanth. This is a product from several Tragacanth shrubs 

 which are species oi Astragalus (Leguminosae), chiefly A, gummifer. It 

 is obtained by wounding the stem and allowing the gum to exude and 

 harden. On hydrolysis it gives a mixture of complex acids and various 

 sugars such as arabinose, galactose and xylose. 



Cherry Gum (cerasin) occurs in the wood of the stems and branches 

 of the Cherry (Prunus Cerasus), the Bird Cherry (P. Padus), the Plum 

 (P. domestica), the Almond (P. Amygdalus) and other trees of the Rosa- 

 ceae. It exudes from fissures of the bark. On hydrolysis it yields almost 

 entirely arabinose. 



Expt. 62. Reactions of Oum Arabic. Put a little gum arabic into an evaporating 

 dish and add a little water. Heat gently and stir. The gum will slowly dissolve, 

 giving a thick sticky solution which does not solidify or gel on cooling. Make the 

 following tests, using a little of the gum solution in a test-tube each time. 



{a) Add a little alcohol. The gum is precipitated. 



(6) Add a little Fehling's solution and boil. No reduction takes place. 



The three following experiments show the presence of pentosan complexes in the 

 gum (see also Expt. 39, p. 45) : 



(c) Add a little phloroglucinol to the guni and then strong hydrochloric acid. No 

 colour is produced. Now heat, and a cherry-red colour appears. 



{d) Heat the gum solution with a little concentrated hydrochloric acid and then 

 add a trace of orcinol. Warm again and then add one or two drops of strong ferric 

 chloride solution. A green coloration will be produced. 



(e) Heat the gum solution strongly with hydrochloric acid, and, after heating for 

 a few minutes, place a piece of filter-paper soaked in a solution of aniline acetate in 

 the mouth of the test-tube. A cherry-red coloration indicative of furfural will be 

 formed. 



Expt. 63. Hydrolysis of Gum Arabic. Weigh out 10 gms. of gum arabic. Put it 

 into a round-bottomed flask and add 100 c.c. of water and 4 c.c. of strong sulphuric 

 acid. Warm gently until the gum goes into solution. Then fit the flask with an air 

 condenser (see p. 46) and heat on a water-bath for about 4 hrs. Cool the solution, 

 and neutralize with barium carbonate. Filter ofi'the barium sulphate and concentrate 

 the solution on a water-bath. Boil a drop or two of the syrup with Fehling's solution 

 and show that reduction takes place. (The original gum either does not reduce 

 Fehling at all, or, if so, only slightly.) Then add a httle nitric acid (sp. gr. 1-15, see 



