m CARBOHYDRATES [ch. 



Then neutralize the liquid with calcium carbonate, filter from calcium sulphate, and 

 concentrate on a water- bath. Some of the solution of arabinose should be tested with 

 all the tests given in Expt. 39. The sugar can be extracted from the s}Tup with 90 ^/q 

 alcohol, but it crystallizes only with difficulty. 



Xylan. This pentosan occurs in lignified cell-walls, and is the chief 

 constituent of " wood gum." It is found in the wood of many trees (not 

 Coniferae), in bran, in wheat and oat straw, in maize cobs, in the shells 

 of coconuts and walnuts, in the testa of the cotton (Gossypium) and in 

 many other tissues : also in some gums. On hydrolysis, xylan yields 

 xylose ; hence wood shavings, bran, straw, etc., will give the pentose reac- 

 tions on hydrolysis. 



Bxpt. 49. Detection of pentose from pentosans in bran, sawdust and straw. Take 

 a small quantity of bran and boil it up several times with 98 % alcohol, filtering oft' 

 the alcohol after each treatment. This should remove any sugars or glucosides 

 present. Allow the alcohol to evaporate off from the bran, and then make the following 

 tests for pentoses (see Expt. 39) : 



(a) Heat, for about one minute, a small quantity of the bran in a test-tube, with 

 sufficient concentrated hydrochloric acid to cover it. Care should be taken not to 

 char the material. Then add as much solid orcinol as will lie on the tip of a penknife. 

 Heat gently again for a few seconds. Then add one or two drops of strong ferric 

 chloride solution ; a green coloration will be produced. Add amyl alcohol and the 

 green colour will pass into the alcohol. 



(6) Heat again another portion of the bran with the same quantity of concentrated 

 hydrochloric acid in a test-tube, but this time heat more strongly. After heating 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 will denote the formation of 

 furfural. 



The above method and tests with bran may be repeated in exactly the same way 

 using sawdust or straw. 



Expt. 50. Preparation of xylan from sawdust. Extract one kilo of sawdust with 

 4 litres of 1-2% ammonia solution for 24 hrs. Then filter off" the ammoniacal solution 

 through muslin and repeat the extraction. The xylan is insoluble in ammoniacal 

 solution, and in this way colouring matters are removed. Finally wash the sawdust 

 well with water and press dry from the liquid. Then add to the sawdust sufficient 

 6% caustic soda solution to make a thick mush (about 1000-1500 c.c.) and allow it 

 to stand for 24 hrs. in a warm place. The alkaline solution is then pressed out 

 through calico and filtered through filter-paper. To the clear filtrate add an equal 

 volume of 96% alcohol which will precipitate the xylan as a sodium compound. 

 Filter off this precipitate, wash with alcohol, and decompose with alcohol to which 

 a little strong hydrochloric acid has been added to remove the sodium. The free 

 xylan is again washed with alcohol, and can be dried by washing with absolute 

 alcohol and ether and finally in a desiccator. It is a dirty-white powder which is 

 almost insoluble in wafer. Make the tests for pentoses (see Expt. 39) on a little of 

 the solid xylan. The reaction will be given in each 



