256 PHYSIOLOGICAL CHEMISTRY 



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 'a 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. 



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. 



4. The Phenylhydrazine Test depends on the formation of 

 osazones, relatively insoluble compounds of phenylhydrazine and 

 sugars. 



In a test tube mix 5 drops phenylhydrazine, 10 drops glacial 

 acetic acid and 15 drops saturated sodium chloride. To the re- 

 sulting solid add about 3 c.c. of dextrose solution and boil for 

 2-3 minutes. Allow to stand. Examine crystals under the micro- 

 scope and draw. The osazones form yellow needles which often 

 group together in rosettes, sheaves, or fans. Many of the sugars 

 may be identified by the crystal form or the melting point of 

 their osazones. If the crystals are slow in forming, place the 

 test tube in a beaker of boiling water for 20 minutes, then allow 

 to cool. 



5. Molisch's Test. To about 5 c.c. of dextrose solution add 2 

 drops of Molisch's reagent (15% alcoholic <x naphthol). In- 

 cline the test tube and pour concentrated sulphuric acid care- 

 fully down the side to form a layer at the bottom of the tube. 

 Notice the reddish violet ring at the junction of the two liquids. 

 This test is extremely delicate, but is given by various substances 

 other than carbohydrates. A negative result is good evidence 



