=147. Precipitation of Sugars with Phenylhydrazin=.—The combination of phenylhydrazin with aldehyds and ketones was first studied by Fischer, and the near relationship of these bodies to sugar soon led to the investigation of the compounds formed thereby with this reagent.[112] Reducing sugars form with phenylhydrazin insoluble crystalline bodies, to which the name osazones has been given. The reaction which takes place is a double one and is represented by the following formulas: Dextrose. Phenylhydrazin. Dextrose-phenylhydrazone. C₆H₁₂O₆ + C₆H₅NH.NH₂ = C₆H₁₂O₅.N.NHC₆H₅ + H₂O and C₆H₁₂O₅.N.NHC₆H₅ + C₆H₅NH.NH₂ = Phenyldextrosazone. C₆H₁₀O₄(N.NHC₆H₅)₂ + 2H₂O. The dextrosazone is commonly called glucosazone. The osazones formed with the commonly occurring reducing sugars are crystalline, stable, insoluble bodies which can be easily separated from any attending impurities and identified by their melting points. Glucosazone melts at 205°, lactosazone at 200° and maltosazone at 206°. The osazones are precipitated in the following way: The reducing sugar, in about ten per cent solution, is treated with an excess of the acetate of phenylhydrazin in acetic acid and warmed to from 75° to 85°. In a short time the separation is complete and the yellow precipitate formed is washed, dried and weighed. The sugar can be recovered from the osazone by decomposing it with strong hydrochloric acid by means of which the phenylhydrazin is displaced and a body, osone, is formed, which by treatment with zinc dust and acetic acid, is reduced to the original sugar. The reactions which take place are represented by the following equations:[113] Glucososone. C₆H₁₀O₄(N.NH.C₆H₅)₂ + 2H₂O = C₆H₁₀O₆ + 2C₆H₅N₂H₂ Dextrose (Glucose). C₆H₁₀O₆ + H₂ = C₆H₁₂O₆. For the complete precipitation of dextrose as osazone Lintner and Kröber show that the solution of dextrose should not contain more than one gram in 100 cubic centimeters. Twenty cubic centimeters containing 0.2 gram dextrose should be used for the precipitation.[114] To this solution should be added one gram of phenylhydrazin and one gram of fifty per cent acetic acid. The solution is then to be warmed for about two hours and the precipitate washed with from sixty to eighty cubic centimeters of hot water and dried for three hours at 105°. One part of the osazone is equivalent to one part of dextrose when maltose and dextrin are absent. When these are present the proportion is one part of osazone to 1.04 of dextrose. Where levulose is precipitated instead of dextrose 1.43 parts of the osazone are equal to one part of the sugar. Sucrose is scarcely at all precipitated as osazone until inverted. After inversion and precipitation as above, 1.33 parts osazone are equal to one part of sucrose.