108 THE CARBOHYDRATES. [CH. V. 



solution diluted I in 5. A reduction may or may not be obtained, 

 depending on the sensitiveness of the reagent (Barfoed's test, 

 Hinkel and Sherman's modification). 



NOTES. -i. The reagent is prepared by dissolving 45 grams, of neutral 

 crystallised cupric acetate in 900 cc. of distilled water and filtering if necessary. 

 To the filtrate add 1-2 cc. of 50 per cent, acetic acid and dilute to i litre. 



2. A portion must show no change when heated in a boiling water bath 

 for 10 minutes. 



3. 0-0005 gram, glucose generally gives the test, whereas 0-02 gram, lactose 

 or maltose, or 0-03 gram, sucrose fail to give the test. 



102. Measure 2 cc. of a i per cent, solution of glucose into a 

 test-tube. Add 3 drops of pure glycerol. Measure 20 drops of a 

 20 per cent, solution of pure crystalline copper sulphate into the tube 

 by means of a dropping pipette (see fig. 5). Add 2 cc. of 20 per 

 cent, sodium hydroxide. Boil the mixture and keep it boiling for 

 half a minute, shaking the tube during the boiling to prevent loss 

 by spurting. The addition of a couple of glass beads helps smooth 

 boiling. Filter through a small paper or allow the tube to stand 

 in a rack till the cuprous oxide has settled. Repeat the experi- 

 ment, using 21, 22, etc., drops of the copper sulphate if the filtrate 

 was yellow ; 19, 18, etc., if it was blue, until a point is found at 

 which an extra drop of copper causes a change in the nitrate from 

 yellow to a faint blue. 



NOTE. The experiment gives one a very rough method of determining 

 the concentration of a solution of glucose, which can be applied for finding the 

 approximate dilution necessary when an accurate estimation has to be made. 

 The reason for the addition of glycerol is explained in Ex. 96, note 3. 



103. Measure 2 cc. of the i per cent, solution into a test-tube, 

 add 0-5 cc. of pure concentrated hydrochloric acid, and boil gently for 

 2 minutes. Cool under the tap. Add the number of drops of copper 

 sulphate necessary to give a faint blue, as found in the preceding 

 exercise, and three drops of glycerol. Neutralise by the addition 

 of 20 per cent, sodium hydroxide, the neutral point being shown by 

 the appearance of a grey precipitate. Now add 2 cc. of 20 per cent, 

 sodium hydroxide, boil for i minute, and allow to stand. An 

 increase in the reducing power is not obtained. 



NOTE. Compare the results with those from maltose and lactose, Exs. 

 120 and 126. 



104. To 5 cc. of the solution in a test-tube add a large "knife 

 point " (half a gram.) of anhydrous sodium carbonate. Shake, and 



