56 PHYSIOLOGICAL CHEMISTRY 



The Fehling test will detect 0.1% glucose in a solution. 



Benedict's Test. The fundamental principle of this test is 

 similar to that of Fehling 's test. In place of sodium hydroxide, 

 sodium carbonate is used, and in place of sodium potassium tar- 

 trate, sodium or potassium citrate. This test has points of 

 superiority over Fehling 's test. In the first place the reagent 

 can be made up in one solutoin instead of two, since the citrate 

 does not tend to reduce the copper, even on long standing. 

 Since sodium carbonate is not so strong an alkali as the hydrox- 

 ide, there is less danger of a trace of sugar being destroyed. 

 Benedict's test will detect a lower concentration of sugar than 

 will Fehling 's, and has replaced it in many laboratories. 



Barfoed's Test. This test also depends upon the reduction of 

 a copper salt (copper acetate) by the sugar. It is performed, 

 however, in acid solution. Barfoed's reagent contains copper 

 acetate and acetic acid. The reducing action of the carbohy- 

 drates is very much less in acid than in alkaline solution. All 

 of the reducing carbohydrates will reduce Barfoed's reagent if 

 boiled a sufficient length of time, producing red cuprous oxide 

 as in the Fehling test. The monosaccharides, however, reduce 

 Barfoed's solution faster than do the disaccharides in equal 

 concentration, so that if properly used, this test may serve to 

 distinguish between monosaccharides and disaccharides. But a 

 concentrated maltose solution will reduce Barfoed's reagent 

 more rapidly than a weak glucose solution, so that this fact 

 should be borne in mind or erroneous conclusions may result. 



Almen-Nylander Test. Nylander's reagent -contains bismuth 

 subnitrate, sodium potassium tartrate and potassium hydroxide. 

 The part played by each constituent is similar to that in Feh- 

 ling 's solution. The bismuth subnitrate is reduced to black, 

 metallic bismuth. The equations follow: 



Bi ( OH ) 2 N0 3 +KOH->Bi ( OH ) 3 +KN0 3 



2 Bi(OH) 8 +Sugar-+Bi 2 +3H 2 0+(Sugar+30) 



Certain substances which interfere with Fehling 's test, have 

 no disturbing influence on the Nylander reaction (uric acid, 



