54 



PLANT COMPOUNDS 



Fehling's solution 1 and this reduction of Fehling's solution 

 is a characteristic reaction for dextrose. Dextrose also 



Fig. 11. — Crystals of anhydrous dextrose. 



C. A. Smith. 



Magnified. Drawing by 



unites with calcium hydroxide and barium hydroxide to 

 form compounds which might be called "dextrates" or 

 "dextroxides," C 6 Hn0 6 .CaOH and C 6 Hn0 6 .BaOH. 2 They 



1 Fehling's solution is made by mixing equal parts of a solution of copper 

 sulphate with a solution of sodium potassium tartrate (Rochelle salts) 

 in sodium hydroxide. The sodium hydroxide forms copper hydroxide which 

 dissolves to a deep blue color in the sodium potassium tartrate. The copper 

 hydroxide in solution is the reacting compound. It is reduced according 

 to the following equation: 



2 Cu(OH) 2 - Cu 2 (OH)2 + H 2 + O. 



On boiling the solution the yellow cuprous hydroxide, Cu2 (OH) 2, changes 

 to the brick red cuprous oxide and water, CU2O+H2O. The amount of 

 red precipitate is a measure of the amount of dextrose in solution. 



2 Their formation can be best illustrated graphically. The exact location 

 of the hydroxyl group which combines with the base is not known, but the 

 one chosen will at least illustrate the reaction: 



H 



I 

 H— C— O— H 



I 

 H— C— O— H 



I 

 H— C— O— H 



H— C— O— H 



I 

 H— C— O 



I 

 H— C=0 



H + H— O 



>Ca 



H— O' 



H 

 H— C— O— H 

 H— C— O— H + 



H— C— O— H 



I 

 H— O— C— H 



I 

 H— C— O— Ca— O— H 



H— C=0 



H2O 



