Why does starch give a negative fehling's test?
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this test the presence of aldehydes but not ketones is detected by reduction of the deep blue solution of copper(II) to a red precipitate of insoluble copper oxide. The test is commonly used for reducing sugars but is known to be NOT specific for aldehydes. For example, fructose gives a positive test with Fehling's solution as does acetoin.
Two solutions are required:
Fehling's "A" uses 7 g CuSO4.5H2O dissolved in distilled water containing 2 drops of dilute sulfuric acid.
Fehling's "B" uses 35g of potassium tartrate and 12g of NaOH in 100 ml of distilled water.
These two solutions should be stoppered and stored until needed.
For the test:
Mix 15 ml of solution-"A" with 15 ml of solution-"B"
Add 2 ml of this mixture to an empty test tube.
Add 3 drops of the compound to be tested to the tube.
Place the tube in a water-bath at 60° C.
A positive test is indicated by a green suspension and a red precipitate.
The test is sensitive enough that even 1 mg of glucose will produce the characteristic red colour of the compound.
The structure of copper(II) D-tartrate has been determined to be:
Two solutions are required:
Fehling's "A" uses 7 g CuSO4.5H2O dissolved in distilled water containing 2 drops of dilute sulfuric acid.
Fehling's "B" uses 35g of potassium tartrate and 12g of NaOH in 100 ml of distilled water.
These two solutions should be stoppered and stored until needed.
For the test:
Mix 15 ml of solution-"A" with 15 ml of solution-"B"
Add 2 ml of this mixture to an empty test tube.
Add 3 drops of the compound to be tested to the tube.
Place the tube in a water-bath at 60° C.
A positive test is indicated by a green suspension and a red precipitate.
The test is sensitive enough that even 1 mg of glucose will produce the characteristic red colour of the compound.
The structure of copper(II) D-tartrate has been determined to be:
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