_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:[125] 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.[126] 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.[127] The phenylhydrazin solution being unstable, is to be prepared at the time of use.