14 THE CARBOHYDRATES. 



25. Prepare pure glucose from cane-sugar by the following 

 method: 



Acidify 100 cubic centimeters of 90-per-cent. alcohol with 

 4 cubic centimeters of concentrated hydrochloric acid, warm the 

 liquid upon the water-bath to 45, and add gradually 30 grammes 

 of finely-powdered cane-sugar, stirring until it has dissolved. The 

 temperature should not rise above 50. After two hours at 50 

 the sucrose has been inverted. Then let it stand in a cool place. 

 The glucose commences to crystallize out in about a week, but 

 crystallization may be hastened by adding to the cold solution a 

 few crystals of glucose and by frequent stirring. After the glucose 

 has crystallized from the solution filter, best with the aid of a 

 filter-pump; wash free from the acid by 90-per-cent. alcohol, then 

 by absolute alcohol; finally dry the crystals. It may be purified 

 by dissolving in pure methyl alcohol by the aid of heat and 

 allowing it to again crystallize out. 



26. Prove that cupric hydrate (made by the addi- 

 tion of a few drops of copper sulphate to a sodium hydrate 

 solution) is soluble in a solution of glucose, giving a deep- 

 blue liquid. Also show that this blue solution of copper 

 and sugar is decomposed by heating, and yellow or red 

 precipitate of cuprous oxid is produced. This is Trom- 

 mels test for grape-sugar. To perform the test, mix about 

 equal volumes of sodium hydrate or potassium hydrate and 

 the glucose solution, then drop in copper sulphate solution 

 until a permanent precipitate begins to form or until the 

 mixture is deep blue ; finally, heat the solution. 



27. Show that cupric hydrate is also soluble in a 

 solution of Rochelle salt or glycerin in water if an alkaline 

 hydrate is present, but that these solutions are not decom- 

 posed by boiling. Add to each of them a little grape-sugar 

 and heat. Cuprous oxid is formed in both cases. The 

 former is called Fehling's test, the latter Haines's test for 

 glucose. 



