150. Estimation of Pentose Sugars and Pentosans as Furfurol.—The production of furfurol by distilling carbohydrates with an acid has already been mentioned. Tollens and his associates have shown that with pentose sugars, and carbohydrate bodies yielding them, the production of furfurol is quantitive.
The production and estimation of furfurol have been systematically studied by Krug, to whose paper the reader is referred for the complete literature of the subject.[123] The essential principles of the operation are based on the conversion of the pentoses into furfurol by distilling with a strong acid, and the subsequent precipitation and estimation of the furfurol formed in the first part of the reaction.
The best method of conducting the distillation is as follows:
Five grams of the pentose substance are placed in a flask of about a quarter liter capacity, with 100 cubic centimeters of hydrochloric acid of 1.06 specific gravity. The arrangement of the apparatus is shown in [Fig. 46]. The flame of the lamp is so regulated as to secure about two cubic centimeters of distillate per minute.
Figure 46. Distilling Apparatus for Pentoses.
The distillate is received in a graduated cylinder and as soon as thirty cubic centimeters are collected, an equal quantity of hydrochloric acid, of the strength noted, is added to the distilling flask, allowing it to flow in slowly so as not to stop the ebullition. The process is continued until a drop of the distillate gives no sensible reaction for furfurol when tested with anilin acetate. The test is applied as follows: Place a drop of the distillate on a piece of filter paper moistened with anilin acetate. The presence of furfurol will be disclosed by the production of a brilliant red color. Usually about three hours are consumed in the distillation, during which time a little less than 400 cubic centimeters of distillate is obtained. The distillate is neutralized with solid sodium carbonate and, in order to have always the same quantity of common salt present, 10.2 grams of sodium chlorid are added for each fifty cubic centimeters of water necessary to make the total volume to half a liter.[124]
The reactions with pentosans probably consist in first splitting up of the molecule into a pentose and the subsequent conversion of the latter into furfurol according to the following equations:
(C₅H₈O₄)ₙ + (H₂O)ₙ = (C₅H₁₀O₅)ₙ
Pentosan. Water.Pentose.
and