Fehling’s test is a simple chemical test used to differentiate reducing sugars from non-reducing sugars based on the ability to reduce copper (II) ions to copper (I) oxide. Reducing sugars, aldoses, and some ketoses gives a positive reaction to Fehling’s test producing a brick-red copper (I) oxide precipitate (Cu2O).
Non-reducing sugars generally do not react with Fehling’s solution unless they are hydrolyzed first. Therefore this test is useful in identifying reducing sugars.
Fehling’s test is a chemical test used to identify reducing sugars. This reaction is based on reducing sugar's ability to reduce copper (II) ions present in the Fehling’s reagent to copper (I) oxide when heated.
Fehling’s reagent contains copper (II) sulfate, sodium potassium tartrate and sodium hydroxide. When reducing sugar is heated with Cu2+ from the Fehling's reagent is reduced to Cu+. In the strong alkaline environment of Fehling’s solution, Cu+ combines with hydroxide ions to form a very unstable hydroxide based intermediate that immediately dehydrates into copper (I) oxide.
Fehling’s solution A: Dissolve 7 grams of cupric sulphate pentahydrate in 100 milliliter of distilled water.
Fehling’s solution B: Dissolve 34.6 grams of potassium sodium tartrate (Rochelle salt) and 12 grams of KOH in distilled water and make the volume upto 100 ml.
Fehling’s reagent: For usable Fehling’s reagent both Fehling’s A and Fehling’s B are mixed in equal volume together before the test.
Sample Solution: 5% is a suitable concentration for this test. Eg: for 5% glucose (W/V) sample solution take 5 grams of glucose and dissolve in 100 ml of distilled water.
1. Take 1 ml of given 5% (W/V) sample solution in a clean dry test tube.
2. Take 1 ml of pure distilled water in another test tube as a control.
3. Add 2-3 drops of Fehling’s reagent to all the tubes and mix it well.
4. Observe the color changes in the test tubes and not down the color changes.
The development of brick-red precipitate indicates the positive result and indicates the presence of reducing sugars.
The absence of brick-red precipitate or appearance of deep blue (Fehling’s solution color) indicates a negative result and absence of reducing sugar.
Fehling’s test is used in the laboratory to detect the difference between aldehyde and ketone functional groups. Ketones do not react to this test, except α-hydroxy ketones eg: fructose.
Fehling’s test is used as a preliminary test for determining monosaccharides and other reducing sugars.
In the clinical biochemistry field, Fehling’s test is used to detect glucose in urine samples as a part of detecting diabetes mellitus.
Fehling’s test can give a positive result for certain ketoses such as fructose, so they are not a final test for identifying only for aldoses.
Several sugars such as glucose, fructose, lactose, maltose all can produce a positive test. So this test is not a confirmation test for finding any specific sugar.
Compounds such as ascorbic acid and few other high concentrations of reducing agents can reduce Cu2+ and produce a false positive reaction.
Non-reducing sugars generally do not react with Fehling’s solution unless they are hydrolyzed first. Therefore this test is useful in identifying reducing sugars.
Principle of Fehling’s Test
Fehling’s test is a chemical test used to identify reducing sugars. This reaction is based on reducing sugar's ability to reduce copper (II) ions present in the Fehling’s reagent to copper (I) oxide when heated.
Fehling’s reagent contains copper (II) sulfate, sodium potassium tartrate and sodium hydroxide. When reducing sugar is heated with Cu2+ from the Fehling's reagent is reduced to Cu+. In the strong alkaline environment of Fehling’s solution, Cu+ combines with hydroxide ions to form a very unstable hydroxide based intermediate that immediately dehydrates into copper (I) oxide.
Preparation of Fehling’s Reagent
Fehling’s solution A: Dissolve 7 grams of cupric sulphate pentahydrate in 100 milliliter of distilled water.
Fehling’s solution B: Dissolve 34.6 grams of potassium sodium tartrate (Rochelle salt) and 12 grams of KOH in distilled water and make the volume upto 100 ml.
Fehling’s reagent: For usable Fehling’s reagent both Fehling’s A and Fehling’s B are mixed in equal volume together before the test.
Sample Solution: 5% is a suitable concentration for this test. Eg: for 5% glucose (W/V) sample solution take 5 grams of glucose and dissolve in 100 ml of distilled water.
Procedure of Fehling’s Test
1. Take 1 ml of given 5% (W/V) sample solution in a clean dry test tube.
2. Take 1 ml of pure distilled water in another test tube as a control.
3. Add 2-3 drops of Fehling’s reagent to all the tubes and mix it well.
4. Observe the color changes in the test tubes and not down the color changes.
Results of Fehling’s Test
The development of brick-red precipitate indicates the positive result and indicates the presence of reducing sugars.
The absence of brick-red precipitate or appearance of deep blue (Fehling’s solution color) indicates a negative result and absence of reducing sugar.
Applications of Fehling’s Test
Fehling’s test is used in the laboratory to detect the difference between aldehyde and ketone functional groups. Ketones do not react to this test, except α-hydroxy ketones eg: fructose.
Fehling’s test is used as a preliminary test for determining monosaccharides and other reducing sugars.
In the clinical biochemistry field, Fehling’s test is used to detect glucose in urine samples as a part of detecting diabetes mellitus.
Limitations of Fehling’s Test
Fehling’s test can give a positive result for certain ketoses such as fructose, so they are not a final test for identifying only for aldoses.
Several sugars such as glucose, fructose, lactose, maltose all can produce a positive test. So this test is not a confirmation test for finding any specific sugar.
Compounds such as ascorbic acid and few other high concentrations of reducing agents can reduce Cu2+ and produce a false positive reaction.
