CARBOHYDRATES 2 9 



from a pipette, one drop of the solution to be tested, and keep the fluid at the 

 boiling point for sixty seconds, without, however, permitting active boiling. 



If no change is produced add a second drop of the solution, and heat once 

 more. If any notable quantity of sugar is present, the fluid will be observed to 

 change from pure blue to violet, then to purple and red, and in extreme cases 

 will fade to a pale yellow. If there is only a trace of sugar, the color will merely 

 change to one of the intermediate shades. 



Care should be exercised to prevent agitation or boiling of the liquid during 

 this test. Contact with oxygen of the air from boiling or agitation prevents the 

 discharge of the blue color. 



(g) Barfoed's Test. Place about 5 c.c. of Barfoed's solution 1 hi a test-tube 

 and heat to boiling. Add glucose solution slowly, a few drops at a time, heating 

 after each addition. Reduction is indicated by the formation of a red precipitate 

 of cuprous oxide. If the precipitate does not form after boiling one-half minute 2 

 allow the tube to stand a few minutes and examine. 



According to Welker 3 chlorides interfere s pronouncedly with the 

 reaction causing the formation of a green precipitate. 



Barfoed's test is not a specific test for glucose as is frequently stated, 

 but simply serves to detect monosaccharides. Disaccharides will also 

 respond to the test, under proper conditions of acidity. 4 Also if the 

 sugar solution is boiled sufficiently long, in contact with the reagent, to 

 hydrolyze the disaccharide through the action of the acetic acid present 

 in the Barfoed's solution a positive test results. 5 Barfoed's is a copper 

 reduction test, but differs from Fehling's and other reduction tests in 

 that the reduction is brought about in an acid solution. It is unsuited 

 for the detection of sugar in urine. 



(h) Picric Acid Test. To 5 c.c. of the sugar solution add 2-3 c.c. of saturated 

 picric acid solution and about i c.c. of 10 per cent KOH. Warm. Note the 

 development of a mahogany red color in the presence of glucose due to 

 reduction of the picric acid with the formation of picramic acid : 



OH(NO 2 ) 3 -> CeH 2 OH NH 2 (NO 2 ) 2 



Picric Acid Picramic Acid 



This test has been made the basis of a method for the colorimetric determina- 

 tion of sugar in blood. See Chapter XVI. 



7. Alcoholic Fermentation. Prepare 500 c.c. of a concentrated (10 per cent) 

 solution of glucose, add a small amount of egg albumin or commercial peptone 

 and introduce the mixture into a liter flask. Add yeast, and by means of a bent 



1 Barfoed's solution is prepared as follows: Dissolve 9 grams of neutral crystallized 

 cppper^acetate in 100 c.c. of water and add 1.2 c.c. of 50 per cent acetic acid. This solu- 

 tion should be freshly made. 



2 Blake: Jour. Am. Chem. Soc., 38, 1245, 1916. 



3 Welker: Jour. Am. Chem. Soc., 37, 2227, 1915. 

 4 Mathews and McGuigan: Am. Jour. PhysioL, 19, 175, 1907. 

 6 Hinkle and Sherman: Jour. Am. Chem. Soc., 29, 1744, 1907 



