44 CARBOHYDRATES AND THEIR [CH. 



Expt. 38. Tests for arabinose. For reactions a-e use a 1 % solution of arabinose : 

 for reaction f a 0*2 / solution. 



If pure arabinose is not available, a solution for tests a, b and c can be prepared 

 from gum arabic. Boil 5 gms. of the gum in 100 c.c. of water with 10 c.c. of con- 

 centrated hydrochloric acid for 5 minutes and then neutralize to litmus with alkali. 

 Such a solution is only suitable for the specific tests for arabinose, since it also 

 contains galactose (see p. 61). For tests a, b and c small pieces of solid gum arabic 

 may even be used. 



(a) Heat a few c.c. of the sugar solution in a test-tube with about half its volume 

 of concentrated hydrochloric acid. In the mouth of the test-tube place a piece of 

 filter-paper soaked with aniline acetate (made by mixing equal quantities of aniline, 

 water and glacial acetic acid). A pink colour will be produced in the paper. This is 

 due to the fact that furfural is formed by the action of the acid on the pentose, and 

 the furfural then gives a red colour with aniline acetate solution : 



;OH Hj 



I I CH = CH 



CH CH-iOHi \ 



| .OH ; 3H 2 ^>0 



CH-C< CH = C 



i I N c=o \ r 



JOH "Hi | . = 



H H 



Arabinose Furfural 



This reaction, however, is also given by the hexoses but to a much less 

 extent. 



(b} Warm a few c.c. of the sugar solution with an equal volume of concentrated 

 hydrochloric acid in a test-tube, and add a small quantity of phloroglucin. A bright 

 red coloration is produced. 



(c) To a few c.c. of the sugar solution in a test-tube add an equal quantity of 

 concentrated hydrochloric acid, and then a little solid orcinol. Divide the solution 

 into two equal portions. Heat one portion. The solution will become red changing 

 to violet and finally blue, blue-green or green. To the other portion, after heating for 

 a time, add a few drops of 10% ferric chloride solution. A deep green colour is at 

 once produced. In both cases, on the addition of a little arnyl alcohol, the green 

 colour will be extracted by the alcohol. 



(d) a-Naphthol reaction. Add to a little of the sugar solution a few drops of a 

 1 % solution of a-naphthol in alcohol. Mix the two solutions and then run in about 

 5 c.c. of concentrated sulphuric acid down the side of the test-tube. A violet colora- 

 tion is produced at the junction of the two liquids. The coloration is due to a 

 condensation product of a-naphthol with furfural, the latter being formed by the 

 action of the apid on the carbohydrate. This reaction is likewi.se given by laevulose 

 and cane-sugar (since it yields laevulose, see p. 52), and less strongly by glucose and 

 maltose ; also by some proteins which contain a carbohydrate group. 



(e} Boil a little of the arabinose solution with a few drops of Fehling's solution. 

 Reduction will take place. 



( /*) Make the osazone of arabinose following the instructions given for glucosazone 

 (see p. 49). 



