218 PHYSIOLOGICAL CHEMISTRY 



using. A contains copper sulphate; B contains sodium hydrate 

 and sodium potassium tartrate (Rochelle salt). The solutions 

 are kept separate, as after mixing, a reduction will take place in 

 the course of time, due to the action of the tartrate. 



Mix equal portions (about 5 c.c.) of A and B. A temporary 

 whitish precipitate of cupric hydroxide forms, but dissolves 

 when the liquids are well mixed, the solution becoming deep blue. 

 The cupric hydrate forms a soluble compound with the tartrate 

 present, the tartrate thus serving to hold the cupric hydrate in 

 solution. Heat to boiling. No change occurs, since the cupric 

 hydrate held by the tartrate does not decompose. If the tartrate 

 were not present, the cupric hydrate would be converted into 

 black cupric oxide on boiling. To the hot liquid add a few drops 

 of dilute dextrose solution and boil. If no change is observed, 

 add more dextrose and boil again. Repeat until a reaction is 

 obtained. A precipitate forms which may be yellow at first 

 (cuprous hydroxide). On further boiling this is converted into 

 red or brownish red cuprous oxide. This is one of the most 

 widely used tests for reducing sugars. 



iii. Barfoed's Test is similar in principle to Fehling's. The 

 solution contains copper acetate and acetic acid, the copper being 

 reduced by monosaccharides as in the Fehling test. A disac- 

 charide usually gives the Barfoed reaction only on prolonged 

 boiling, which hydrolyzes the disaccharide to simple sugars. If 

 properly applied, Barfoed's test may be used to distinguish be- 

 tween mono- and disaccharides but a concentrated maltose solu- 

 tion will give a quicker reduction than a dilute glucose solution, 

 hence unless experimental conditions are carefully controlled, 

 erroneous conclusions may result. Place about 5 c.c. of Bar- 

 foed's solution in a test tube and heat to boiling. Add diluted 

 dextrose solution (1 to 5) a few drops at a time, heating after 

 each addition. A red precipitate of cuprous oxide forms. 



iv. Haines' Test is similar to Fehling's, except that glycerine 

 is used in place of Rochelle salt and potassium hydroxide in place 

 of sodium hydroxide. 



