ORGANIC CHEMISTRY 5 



carbon atoms renders the ketone sugars capable of reducing 

 metallic hydroxides. 



Fehling's Test* Instead of using copper sulphate and 

 alkali, a mixture is made containing copper sulphate, a tartrate 

 and alkali. This, in spite of the absence of sugar, gives a 

 deep blue solution. A little of this is added to the sugar 

 solution and the mixture is boiled. As in Trommer's test 

 the sugar is oxidised and the cupric hydroxide is reduced and 

 dehydrated to the cuprous oxide so that a red precipitate 

 occurs. 



Pavy's Test.] consists of an ammoniacal copper solution. 

 Ammonia not only holds the cupric hydroxide in solution but 

 it also dissolves the cuprous hydroxide to a colourless solution ; 

 therefore on heating with sugar the solution is decolourised 

 and no red precipitate occurs. 



Other metals are also reduced by sugar solutions. An 

 ammoniacal silver solution can be reduced to form a silver 

 mirror. Nylander's reagent consists of bismuth hydroxide 

 held in solution by tartrate and on heating with sugar a 

 black precipitate of metallic bismuth occurs.} 



The alkaline copper solutions are frequently used for the 

 quantitative estimation of reducing sugars ; for the details 

 the reader must be referred to books on practical chemistry. 



These tests are given by all reducing sugars, that is those 

 which possess a free aldehyde or a ketone associated with an 

 hydroxyl group. This includes all monosaccharides and some 

 disaccharides. Those carbohydrates which do not reduce 

 metallic hydroxides can be converted into reducing substances 

 by hydrolysis with acid. Alkali is not used for hydrolysis, as 

 the sugar is oxidised in the presence of alkali to form dark 

 brown coloured products. The formation of these coloured 

 products in the presence of alkali is the basis of Moore's 

 Test.\\ 



Barfoed's Test. The monosaccharides differ from the 

 reducing disaccharides in that they are more powerful reducing 

 substances, and under standard conditions will reduce copper 



* H. Fehling, Annalen der Chemie, 1849, vol. 72, p. 106. 



f F. W. Pavy, Proc. Roy. Soc., 1879, vol. 28, p. 260, and 1880, vol. 

 29, p. 272. 



J Bottger, Journ. f. prakt. Chem., 1857, vol. 70, p. 432 ; E. Nylander, 

 Zeit. /., physiol. Chem., 1883, vol. 8, p. 175. 



S. R. Benedict, Journ. Biol. Chem., 1907, vol. 3, p. 101, describes 

 a useful modification. See also P. A. Schaffer and A. F. Harstmann, 

 Journ. Biol. Chem. 1921, vol. 45, p. 365. 



|| J. Moore, Lancet, 1844, vol. i, p. 751. 



