Fehling’s solution test

FEHLING S SOLUTION. Test reagent consisting of two solutions, one of copper sulfate, the other of alkaline tartrate, which are mixed just before use. 

Preparation and Properties of Dextrin (section 377).— Heat about 5 grams of starch for one-half hour at 220°-225° in an oil-bath or air-bath. Pour the product into a mortar, add 2 cc. of water, and notice the adhesive quality of the mixture. Add 25 cc. of water and grind with a pestle. If there is a residue of starch which has not been converted into dextrin, filter through a folded filter. Use 2 cc. of the solution to determine whether the product reduces Fehling s solution. Test 2 cc. of the solution with a drop of iodine solution and note the color. 

A Cu -taitrate complex, Fehling s solution, tests for arsenite ... 

Dissolve 0.2 g. of sucrose in 5 cc. of water, add 2 drops of cone. HCl and heat the solution in the steam-bath for five minutes. Neutralize the free acid with alkali and apply the Fehling s Solution test. Sucrose hydrolyzes far more readily than do most polysaccliaroses. 

Add 2 -3 drops of phenylhydrazine to about 2 ml. of Fehling s solution in a test-tube and shake the mixture vigorously nitrogen is evolved and reddish-brown cuprous oxide is precipitated. The reaction proceeds rapidly on gentle warming, more slowly in the cold. 

Oxidation, (a) Unsaturation test. Dissolve about o-i g. of cinnamic acid or of a soluble cinnamate in about 5 ml. of 10% NajCOg solution. To the cold solution add 1% aqueous KMn04 drop by drop. Immediate decolorisation denotes unsaturation. (Note. Many easily oxidisable substances, e.g.y formic acid, acetaldehyde, etc.y also rapidly decolorise alkaline permanganate. Cinnamates, however, do not reduce Fehling s solution.)... 

Formation of silver mirror or precipitate of silver indicates reducing agent. (This is often a more sensitive test than I (a) above, and some compounds reduce ammoniacal silver nitrate but are without effect on Fehling s solution.) Given by aldehydes and chloral hydrate formates, lactates and tartrates reducing sugars benzoquinone many amines uric acid. 

Place 10 ml. of 1% starch solution (prepared as described above) in a boiling-tube, add 2 ml. of 1% sodium chloride solution and place the tube in a water-bath maintained at 38-40 . Place about 5 ml. of water in a series of test-tubes and to each add a few drops of 1% iodine solution. Now add 4 ml. of the diluted saliva solution to the starch solution, mix well and note the time. At intervals of about 30 seconds transfer 2 drops of the reacting mixture, by means of a dropping tube, to one of the test-tubes, mix and note the colour. As in the previous experiment, the colour, which is blue at first, changes to blue-violet, red-violet, red-brown, pale brown, and finally disappears at this stage the solution will reduce Fehling s solution. If the reaction proceeds too quickly for the colour changes to be observed, the saliva solution should be diluted. 

Mix each solution well and place the boiling- tubes in a water-bath maintained at 50 . After 10 minutes, transfer i ml. of each of the reaction mixtures to separate test-tubes, add 2 ml. of Fehling s solution to each and boil. A marked reduction is obtained in A, no reduction is obtained in B, and no reduction or only very slight reduction is obtained in C. This shows that the activity of the invertase is destroyed both by heat and alkali. 

Aromatic aldehydes react with the dimedone reagent (Section 111,70,2). All aromatic aldehydes (i) reduce ammoniacal silver nitrate solution and (ii) restore the colour of SchifiF s reagent many react with sodium bisulphite solution. They do not, in general, reduce Fehling s solution or Benedict s solution. Unlike aliphatic aldehydes, they usually undergo the Cannizzaro reaction (see Section IV,123) under the influence of sodium hydroxide solution. For full experimental details of the above tests, see under Ali-phalic Aldehydes, Section 111,70. They are easily oxidised by dilute alkaline permanganate solution at the ordinary temperature after removal of the manganese dioxide by sulphur dioxide or by sodium bisulphite, the acid can be obtained by acidification of the solution. 

Reduction of a nitro compound to a hydroxylamine. Dissolve 0-5 g. of the compound in 10 ml. of 50 per cent, alcohol, add 0-5 g. of solid ammonium chloride and about 0 5 g. of zinc powder. Heat to boiling and allow the ensuing chemical reaction to proceed for 5 minutes. Filter from the excess of zinc powder and test the filtrate with ToUen s reagent see Section III,70,(i). An immediate black or grey precipitate or a silver mirror indicates the presence of a hydroxylamine formed by the reduction of the nitro compound. Alternatively, warm the filtrate with Fehling s solution a hydroxylamine will precipitate red cuprous oxide. (A blank test should be performed with the original compound.)... 

Copper has wide use as an agricultural poison and as an algicide in water purification. Copper compounds, such as Fehling s solution, are widely used in analytical chemistry tests for sugar. 

Fehling s solution, a tartrate complex of copper(II) sulfate, has also been used as a test for reducing sugars. 

Reaction.—Boil a small quantity for a few minutes with dilute hydrochloric acid, and test with Fehling s solution. The oxime is decomposed into acetone and hydroxylamine,... 

Experiment 1.—Dilute a few drops of formaldehyde or acetaldehyde with a few c.c. of water, add a small amount of ammoniacal silver solution, and divide the mixture between two test tubes. Into one test tube run a few drops of sodium hydroxide solution an immediate separation of metallic silver takes place. From the other solution after standing for some time in the cold, or more quickly on warming, the silver separates. Thus the oxidising action of ammoniacal silver solution is very considerably increased by sodium hydroxide (Tollens). Also test the reducing action of the aldehydes on Fehling s solution. 

Test the behaviour of phenylhydrazine towards Fehling s solution and towards ammoniacal silver solution. 

Drug/Lab test interactions False-positive urine glucose reactions may occur with penicillin therapy if Clinitest, Benedict s Solution, or Fehling s Solution are used. It is recommended that enzymatic glucose oxidase tests (such as Clinistix or Tes-Tape) be used. Positive Coombs tests have occurred. High urine concentrations of some penicillins may produce false-positive protein reactions (pseudoproteinuria) with the P.870... 

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