THE CARBOHYDRATES 191 



* (6) Action of Concentrated Hydrochloric Acid. If a solution of 

 glucose be boiled for some time with an equal volume of concentrated 

 hydrochloric acid, the solution becomes brown and " humus" sub- 

 stances, which are black, separate out The chief product of the action 

 of hydrochloric acid on glucose is laevulinic acid or acetylpropionic 

 acid, CH 3 . CO . CH 2 . CH 2 . COOH. 



(7) Reduction of Metallic Oxides in Alkaline Solution. 



* (a) Silver. On adding a solution of glucose to some ammoniacal 

 silver nitrate solution (prepared by adding dilute ammonia to silver 

 nitrate until the precipitate first formed is just redissolved) and warning 

 in the water-bath, a mirror of metallic silver gradually forms. 



(b) Copper. 



* (i) Trammer's Test. On making a solution of glucose alkaline 

 with sodium hydroxide and adding copper sulphate, drop by drop, 

 shaking after each addition, the solution becomes deep blue. The 

 addition of excess of copper sulphate causes the precipitation of cupric 

 hydrate, i.e. it is no longer dissolved by the glucose solution. The 

 addition of a few small crystals of Rochelle salt will redissolve the 

 precipitate (see Fehl ing's test). On heating the clear blue solution 

 nearly to boiling a yellowish-red precipitate of cuprous oxide is formed 



* (ii) Fehling's Test. On adding some glucose solution to equal 

 quantities of Fehling's solution (a) CuSO 4 , () NaOH + NaK Tart, 

 and heating to boiling, cuprous oxide is precipitated. 



It should be noted that ammonium salts modify the reaction ; the 

 cuprous oxide is not precipitated, but the blue colour of the solution 

 becomes less intense and may disappear. 



* (iii) Benedict's Test. As glucose is destroyed by the action of 

 sodium hydroxide the reaction is more sensitive if sodium carbonate be 

 employed in its place. If sodium citrate be substituted for Rochelle 

 salt a permanent solution (Benedict's qualitative reagent, p. 613) is 

 obtained. 



On adding 5 to 10 drops of glucose solution to about 5 c.c. of 

 the reagent and boiling vigorously for 2 or 3 minutes, it becomes 

 turbid with a red, yellow, or green precipitate which fills up the 

 solution depending on the amount of glucose. If the amount of 

 glucose be very small a precipitate is only observed on allowing the 

 solution to cool. 



The test is sensitive to -08 per cent, of glucose. 



* (iv) Barfoe&s Test. Glucose also reduces cupric hydrate in acetic 

 acid solution. If some glucose solution be added, drop by drop, to 

 some Barfoed's reagent which is kept boiling during the addition, red 

 cuprous oxide is precipitated. 



