Part 29

dx ----- = -Cʹdθ. x

Integrating and calling x the quantity of matter changed to the stable form at the moment θ, corresponding to a rotation α₀, we have

1 x₀ Cʹ = ------- log. nap. ----, and taking into consideration θ - θ₀ x

the equation given above, and substituting common for superior logarithms we get

1 α₀ - αₙ C = --------- log. ---------. θ - θ₀ α - αₙ

Experience has shown that such a constant C really exists, and its value can be easily calculated from the data of Parcus and Tollens. The mean value of C from these data is 0.0301 for arabinose; 0.0201 for xylose; 0.0393 for rhamnose; 0.0202 for fucose; 0.00927 for galactose; 0.00405 for lactose; 0.00524 for maltose, and for dextrose, 0.00348 at 11° to 13° and 0.00398 from 13° to 15°. The constant C as is well known, increases as the temperature is raised.

The constant C, at a given temperature, measures the progress of the phenomenon of the change from the unstable to the stable state. It will be noticed that among the sugars possessing multirotation properties the pentoses possess a much higher speed of transformation than the others.

=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. 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.

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.

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

(C₅H₁₀O₅)ₙ = (C₅H₄O₂)ₙ + (3H₂O)ₙ. Pentose. Furfurol. Water.

=151. Determination of Furfurol.=—The quantity of furfurol obtained by the process mentioned above may be determined in several ways.

As Furfuramid.—When ammonia is added to a saturated solution of furfurol, furfuramid, (C₅H₄O)₃N₂, is formed. In order to secure the precipitate it is necessary that the furfurol be highly concentrated and this can only be accomplished by a tedious fractional distillation. This method, therefore, has little practical value.

As Furfurolhydrazone.—Furfurol is precipitated almost quantitively, even from dilute solutions, by phenylhydrazin. The reaction is represented by the equation:

C₆H₈N₂ + C₅H₄O₂ = C₁₁H₁₀N₂O + H₂O. Phenylhydrazin. Furfurol. Furfurolhydrazone. Water.

=152. Volumetric Methods.=—Tollens and Günther have proposed a volumetric method which is carried out as follows: The distillation is accomplished in the manner described. The distillate is placed in a large beaker, neutralized with sodium carbonate and acidified with a few drops of acetic. Phenylhydrazin solution of known strength is run in until a drop of the liquid, after thorough mixing, shows no reaction for furfurol with anilin acetate. The reagent is prepared by dissolving five grams of pure phenylhydrazin and three of glacial acetic acid in distilled water, and diluting to 100 cubic centimeters. The solution is set by dissolving from two-tenths to three-tenths gram of pure furfurol in half a liter of water and titrating with the phenylhydrazin as indicated above. The quantity of the pentose used has a great influence on the result.

With nearly a gram of arabinose about fifty per cent of furfurol were obtained while when nearly five grams were used only about forty-six per cent of furfurol were found. With xylose a similar variation was found, the percentage of furfurol, decreasing as the quantity of pentose increased. The method, therefore, gives only approximately accurate results.

=153. Method of Stone.=—Another volumetric method proposed by Stone is based on the detection of an excess of phenylhydrazin by its reducing action on the fehling reagent. A standard solution of phenylhydrazin is prepared by dissolving one gram of the hydrochlorate and three grams of sodium acetate in water and completing the volume of the liquor to 100 cubic centimeters. This solution contains 1.494 milligrams of phenylhydrazin in each cubic centimeter, theoretically equivalent to 1.328 milligrams of furfurol. The reagent is set by titrating against a known weight of furfurol. Pure furfurol may be prepared by treating the crude article with sulfuric acid and potassium dichromate, and subjecting the product to fractional distillation. The distillate is treated with ammonia and the furfuramid formed is purified by recrystallizing from alcohol and drying over sulfuric acid. One gram of this furfuramid is dissolved in dilute acetic acid and the volume completed to one liter with water. The phenylhydrazin solution being unstable, is to be prepared at the time of use.

The titration is conducted as follows: Twenty-five cubic centimeters of the distillate obtained from a pentose body, by the method described above, are diluted with an equal volume of water, a certain quantity of the phenylhydrazin solution added to the mixture from a burette and the whole heated quickly to boiling. The flask is rapidly cooled and a portion of its contents poured on a filter. The filtrate should have a pale yellow color and be perfectly clear. If it become turbid on standing, it should be refiltered. Two cubic centimeters of the clear filtrate are boiled with double the quantity of the fehling reagent. If phenylhydrazin be present, the color of the mixture will change from blue to green. By repeating the work, with varying quantities of phenylhydrazin, a point will soon be reached showing the end of the reaction in a manner entirely analogous to that observed in volumetric sugar analysis.

In practice the volumetric methods have given place to the more exact gravimetric methods described below.

=154. Gravimetric Methods.=—The distillation is carried on and the volume of the distillate completed to half a liter as described above. Chalmot and Tollens then proceed as follows: Ten cubic centimeters of a solution of phenylhydrazin acetate, containing in 100 cubic centimeters twelve grams of the phenylhydrazin and seven and a half grams of glacial acetic acid dissolved and filtered, are added to the distillate and the mixture stirred with an appropriate mechanism for half an hour. The furfurolhydrazone at the end of this time will have separated as small reddish-brown crystals. The mixture is then thrown onto an asbestos filter and the liquid separated with suction. The suction should be very gradually applied so as not to clog the felt. The precipitate adhering to the beaker is washed into the filter with 100 cubic centimeters of water. The precipitate is dried at about 60° and weighed. As a check the hydrazone may be dissolved in hot alcohol, the filter well washed, dried and again weighed. To obtain the weight of furfurol the weight of hydrazone found is multiplied by 0.516 and 0.025 added to compensate for the amount which was held in solution or removed by washing. Less than one per cent of pentose can not be determined by this method since that amount is equalled by the known losses during the manipulation.

Factor.—To convert the furfurol found into pentoses, the following factors are used:

Per cent furfurol Multiply for obtained from Multiply for Multiply for penta-glucoses five grams of arabinose by. xylose by. by. pentoses.