GLUCOSE. 215 



salt is dissolved as possible. If a glucose solution is treated with about 

 xV vol., or with a larger quantity of the solution when large quantities 

 of sugar are present, and boiled for a few minutes, the solution becomes 

 first yellow, then yellowish-brown, and finally nearly black, and after a 

 time a black deposit of bismuth (?) settles. 



The property that glucose has of reducing an alkaline solution of 

 mercury on boiling is the basis of KNAPP'S reaction with alkaline mercuric 

 cyanide, and of SACHSSE'S reaction with an alkaline potassium-mercuric 

 iodide solution. 



On heating with PHENYLHYDRAZINE ACETATE a glucose solution 

 gives a precipitate consisting of fine yellow crystalline needles which are 

 almost insoluble in water, but soluble in boiling alcohol, and which separate 

 again, on treating the alcoholic solution with water. The crystalline 

 precipitate consists of phenylglucosazone (see page 203). This com- 

 pound melts when pure at 205 C. It must be borne in mind that 

 the melting-point of this and other osazones is somewhat variable, depend- 

 ing upon the rapidity of the heating, the diameter of the tube and the 

 thickness of the sides of the tube. 1 The osazone dissolves readily in 

 pyridine (0.25 gram in 1 gram), and precipitates again from this solu- 

 tion as crystals on the addition of benzene, ligroin, or ether. According 

 to NEUBERG 2 this behavior can be used in the purification of the osazone. 

 The diphenylhydrazone and the methyl phenylhydrazone are also of 

 interest. 



Glucose is not precipitated by a lead-acetate solution, but is almost 

 completely precipitated by a solution of ammoniacal basic lead acetate. 

 On warming, the precipitate becomes flesh-color or rose-red (RUBNER'S 

 reaction 3 ) . 



If a watery solution of glucose is treated with benzoylchloride and 

 an excess of caustic soda, and shaken until the odor of benzoylchloride 

 has disappeared, a precipitate of benzoic-acid ester of glucose will be 

 produced which is insoluble in water or alkali (BAUMANN 4 ). 



If J-l cc. of a dilute watery solution of glucose is treated with a few 

 drops of a 10 per cent alcoholic solution (free from acetone) of a-naphthol, 

 on the slow addition of 1-2 cc. of concentrated sulphuric acid a beautiful 

 reddish-violet ring forms at the juncture of the liquids, or on shaking, the 

 entire mixture becomes a beautiful reddish-violet color (MOLISCH 5 ) . 



1 See E. Fischer, Ber. d. d. chem. Gesellsch., 41. 



2 Ber. d. d. chim. Gesellsch., 32, 3384. 



3 Zeitschr. f . Biologic, 20. 



4 Ber. d. deutsch. chem. Gesellsch., 19; also Kueny, Zeitschr. f. physiol. Chem., 14, 

 and Skraup, Wien. Sitzungsber., 98 (1888). 



6 Molisch, Monatshefte f. Chem., 7, and Centralbl. f. d. med. Wissensch., 1887, 

 pp. 34 and 49. 



