106 THE CARBOHYDRATES. [CH. V. 



NOTES. i. The amount of copper necessary depends on the percentage 

 of sugar present. If only a small amount of sugar be present a mere disappear- 

 ance of the blue colour is all that may happen, or possibly the fluid may assume 

 a faint yellowish-red tint. If excess of copper be added, the 'reduction is 

 obscured by the blue cupric hydrate in solution, or the black precipitate of 

 cupric oxide that is formed on heating this in the alkaline solution. It is 

 always best to add the copper sulphate a few drops at a time, boiling between 

 each addition. 



2. The reaction is a type of several that have been introduced for the 

 detection of glucose, all of which depend on the fact that in alkaline solution it 

 has reducing properties when boiled. For this reason, glucose, and all sugars 

 that have this property are sometimes spoken of as " reducing sugars." 



3. The property that glucose and other sugars have of dissolving cupric 

 hydrate is common to a large number of organic compounds, such as glycerol, 

 Rochelle salt and sodium citrate. 



97. Boil about 3 cc. of Fehling's solution (see Note i) in a 

 test-tube. No change occurs. Add about 3 cc. of the glucose 

 solution and boil again. A red precipitate of cuprous oxide is 

 formed. (Fehling's test.) 



NOTES. i. Fehling's fluid is prepared as follows : 



(a) Dissolve 103-92 grams, of pure copper sulphate in warm distilled 

 water and dilute to one litre. 



(b) Dissolve 320 grams, of potassium sodium tartrate (Rochelle salt) in 

 warm water, add a little carbolic acid to prevent the growth of fungi, dilute to 

 exactly a litre and filter. 



(c) Dissolve 150 grams, of sodium hydroxide in distilled water and dilute 

 to i litre. 



For use take exactly equal quantities of a, b, and c, and mix. Though the 

 individual constituents keep indefinitely, the fluid when prepared suffers 

 decomposition, so that a reduction occurs on boiling. For this reason the fluid 

 should be prepared just before use, and must always be tested by boiling before 

 being used. 



The fluid is of such a strength that the copper sulphate in 10 cc. is just 

 reduced by 0-05 grams, of dextrose. 



2. The addition of the Rochelle salt is for the purpose of dissolving the 

 cupric hydroxide that would otherwise be precipitated by mixing (a) and (c). 



3. The test is much more delicate and certain than Trommer's test, and 

 should always be used in preference to it. 



4. If the fluid that is being tested is acid, it should be neutralised. 



5. Ammonium salts considerably interfere with Fehling's test owing 

 to the ammonia liberated dissolving the cuprous oxide to a colourless com- 

 pound. If they are present a little extra alkali should be added, and the 

 mixture boiled for two or three minutes to allow of the evolution of the 

 ammonia. 



6. In testing for small amounts of glucose it is advisable to avoid an 

 excess of Fehling's solution, owing to the excess of alkali tending to destroy the 

 glucose before the latter can exert its reducing reaction on the copper. The 

 neutral solution should be made faintly blue with Fehling's solution, and then 

 boiled. 



