PLANT CONSTITUENTS — CARBOHYDRATES 273 



on hydrolysis with boiling acid the pentose sugar is obtained 

 and the pentose sugar on boiling with acid (hydrochloric) is 

 converted into furfural. The furfural forms an insoluble com- 

 pound with phloroglucinol, which is weighed and the pentose 

 and pentosan calculated from this. 



EXPERIMENT STUDY XXXV 



Determination of Pentosans^ 



Place 5.0 g. of finely ground wheat or oat straw in a flask, together 

 with 100 c.c. of 12 per cent hydrochloric acid (specific gravity, 1.06), 

 and several pieces of recently heated pumice stone. Place the flask 

 on a wire gauze, connect with a condenser, and heat, rather gentl}^ at 

 first, and so regulate as to distill over 30 c.c. in about ten minutes, the 

 distillate passing through a small filter paper. Replace the 30 c.c. 

 driven over by a like quantity of the dilute acid added by means of a 

 separatory funnel in such a manner as to wash down the particles 

 adhering to the sides of the flask, and continue the process until the 

 distillate amounts to 360 c.c. To the completed distillate gradually 

 add a quantity of phloroglucinol (purified if necessary) dissolved in 

 12 per cent hydrochloric acid and thoroughly stir the resulting mix- 

 ture. The amount of phloroglucinol used should be about double 

 that of the furfural expected. The solution first turns yellow, then 

 green, and very soon an amorphous greenish precipitate appears, 

 which grows rapidly darker, till it finaUy becomes almost black. 

 Make the solution up to 400 c.c. with 12 per cent hydrochloric acid, 

 and allow to stand overnight. 



Filter the amorphous black precipitate through an asbestos felt 

 in a tared Gooch crucible, wash carefully with 150 c.c. of water in 

 such a way that the water is not entirely removed from the crucible 

 untU the very last, then dry for four hours at the temperature of 

 boiling water, cool and weigh, in a weighing bottle, the increase in 

 weight being reckoned as phloroglucid. To calculate the furfural, 

 pentose, or pentosan from the phloroglucid, use the following formulas 

 given by Krober : 



1 Oflficial Methods of Analysis, A. O. A. C, U. S. Dept. Agr. Bur. Chem. Bui. 107, 

 p. 54 (1912). 

 T 



