208 THE CARBOHYDRATES. 



According to BENEDICT * this test is more delicate if sodium carbonate is 

 used instead of sodium hydroxide in the preparation of FEHLING'S solution. 



BOTTGER-ALMEN'S test is based on the property dextrose possesses 

 of reducing bismuth oxide in alkaline solution. The reagent best adapted 

 for this purpose is obtained, according to NYLANDER'SS modification of 

 ALMEN'S original test, by dissolving 4 grams of Rochelle salt in 100 parts 

 of 10 per cent caustic-soda solution and adding 2 grams of bismuth 

 subnitrate and digesting on the water-bath until as much of the bismuth 

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

 -^o 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 dextrose 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 dextrose solution 

 gives a precipitate consisting of fine yellow crystalline needles which are 

 nearly 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 198). This com- 

 pounds melts when pure at 204-205 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. 3 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. Accord- 

 ing to NEUBERG 4 this behavior can be used in the purification of the 

 osazone. The diphenylhydrazone and the methyl phenylhydrazone 

 are also of interest. 



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

 completely precipitated by a solution of ammoniac al basic lead acetate. 

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

 reaction 5 ) . 



If a watery solution of dextrose is treated with benzoylchloride and 

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



1 Journ. of biol. Chem., 3. 



2 Zeitschr. f . physiol. Chem., 8. 



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



4 Ber. d. d. chem. Gesellsch., 32, 3384. 



5 Zeitschr. f . Biologie, 20. 



