THE CARBOHYDRATES 01 



due to the formation of caramel and levulinic acid; a faint odour of 

 caramel is also produced. 



2. Trommer's test. A mixture of weak copper sulphate and 

 potassium hydrate yields, on heating with the sugar solution, a red 

 precipitate of cuprous oxide. At first cupric hydrate is formed by 

 the interaction of the alkali and the copper sulphate; this hydrate is 

 then reduced to cuprous hydrate, with the formation of acid. The 

 cuprous hydrate on further heating loses water, forming cuprous 

 oxide. 



Copper Caustic Cupric Potassium 

 sulphate potash hydrate sulphate 



2. 2Cu(OH) 2 + RCHO = CiL,(OH) 2 + R.COOH + H.,0 



Cupric Aldehyde Cuprous Acid and water 

 hydrate hydrate 



3. Ca,(OH) 



The above only takes place when the amount of copper solution is 

 neither too little nor too much. Excess of copper salt leads to the 

 formation of brown cupric hydrate, which spoils the test. 



3. Fehling's test. For this reason the excess of copper salt is 

 generally held in solution by the addition of Rochelle salt (sodium 

 potassium tartrate) in definite proportion to the alkali and copper 

 salt. This is known as Fehling's solution.* When freshly prepared 

 it is unaltered by boiling. It is reduced by all the sugars except 

 cane-sugar. Unfortunately, it is also reduced by other bodies, such 

 as uric acid, creatinin, glycuronic acid, etc. 



4. Nylander's test. This disadvantage does not pertain so much to 

 the salts of bismuth. The solution most generally employed is Nylander's 

 solution. f Upon boiling for a few minutes with a sugar, the solution 

 goes yellowish-brown to black, and finally deposits a precipitate of 

 metallic bismuth. Glycuronic acid gives this test; uric acid and 

 creatinin do not. 



5. Barfoed's test. An acid solution of cupric acetate gives a red 

 precipitate with the monosaccharides. 



6. Phenylhydrazine test. This test consists in heating about 10 c.c. 

 of the sugar solution in a test-tube on a water-bath with phenyl- 

 hydrazine hydrochloride and sodium acetate as much as will cover 

 a threepenny-piece of the former, and a sixpenny-piece of the latter. 

 After about thirty minutes a yellow crystalline precipitate settles out 

 which looks like yellow sheaves under the microscope. 



The osazones yielded by dextrose and levulose are the same; that 

 of galactose is the same in appearance, but differs in melting-point 

 (see table, p. 04). 



Of the disaccharides, cane-sugar does not yield an osazone; until 



* Fehling's solution = Copper sulphate 34-36 grammes, NaOH 50-60, Rochelle 

 salt 173 grammes, per litre of water. 



t Nylander's solution = Bismuth subnitrate 20 grammes, Rochcllc salt 40 grammes, 

 dissolved in 1,000 c.c. of 8 per cent. NaOH. 



