CARBOHYDRATES 255 



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. 



For the quantitative method, see chapter on urine. 



3. Benedict's Qualitative Reagent for Sugar. 



Dissolve 85 g. sodium citrate and 50 g. anhydrous sodic car- 

 bonate in 400 c.c. of water. Dissolve 8.5 copper sulphate in 

 50 c.c. of hot water. Pour the copper sulphate solution slowly, 

 and with stirring, into the alkaline citrate solution. Filter if 

 necessary. Label and preserve. 



Heat to boiling about 5 c.c. of Benedict's reagent in a test 

 tube together with a pebble or two, to prevent bumping. Add 

 about 8 drops of sugar solution (or urine) and boil for 2 min- 

 utes. If more than two- or three-tenths per cent of sugar is 

 present, the solution will be filled with a colloidal (greenish, 

 yellow or reddish) precipitate. With smaller amounts of sugar 

 the precipitate will usually appear only on cooling (the cooling 

 should not be hastened by immersion in cold water). 



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

 solution contains copper acetate and acetic acid, the copper being 



