The precipitation is accomplished as follows:
One part by weight of the sugar, two parts of phenylhydrazin hydrochlorate, and three parts of sodium acetate are dissolved in twenty parts of water and gradually heated on the water-bath.
The osazone slowly separates in a crystalline form and it is freed from the mother liquor by filtration, and purified by solution in alcohol and recrystallization. The crystals are composed of yellow needles, which are difficultly soluble in water and more easily in hot alcohol. The crystals are not decomposed by a dilute acid but are destroyed by the action of strong acids.
Dextrosazone.—The crystals melt at from 204° to 205°, reduce fehling liquor, and dissolved in glacial acetic acid are slightly left rotating.
Levulosazone.—This body has the same properties as the dextrose compound.
Maltosazone.—This substance melts at 206° with decomposition. It is left rotating. Its structure is represented by the formula C₂₄H₃₂N₄O₉.
Galactosazone.—This substance, C₁₈H₂₂N₄O₄, has the same centesimal composition as the corresponding bodies produced from dextrose and levulose. It is distinguished from these compounds, however, by its low melting point, viz., 193°.
The above comprise all the phenylosazones which are important from the present point of view. Sucrose, by inversion, furnishes a mixture of dextros- and levulosazones when treated with phenylhydrazin, while starch and dextrin yield the dextros- or maltosazone when hydrolyzed. Lactose yields a mixture of dextros- and galactosazones when hydrolyzed and treated as above described.
The reactions with phenylhydrazin are approximately quantitive and it is possible that methods of exact determination may be based on them in the near future.[138]
173. Other Qualitive Tests for Sugars.—The analyst may sometimes desire a more extended test of qualitive reactions than those given above. The changes of color noticed on heating with alkalies may often be of advantage in discriminating between different sugars. The formation of definite compounds with the earthy and other mineral bases may also be used for qualitive determinations. One of the most delicate qualitive tests is found in the production of furfurol and this will be described in the following paragraphs.