Fehling’s test is possible in presence of certain organic functional group with the help of following reacting agent as mentioned below −
Carbohydrates with free C=O (carbonyl) group i.e. aldehyde or ketones can possibly behave as a reducing sugars.
Fehling’s solution is mixture of $\mathrm{CuSO_4\:,\:NaOH}$, (strong alkali), and potassium sodium tartarate which is deep blue in colour.
On heating the test sample with Fehling’s solution, oxidation occurs by bistartarocuparate(II) complex which oxidises aldehydes into carboxylic acid.
Reduction occurs of copper(II) ion complex and forms red or yellow insoluble precipitate of $\mathrm{Cu_2O}$ [cuprous(I) oxide].
By oxidation of ketones can form short chain of acids.
When $\mathrm{CuSO_4}$ and NaOH react in the solution, tartrate ions inhibit insoluble $\mathrm{Cu(OH)_2}$ formation by producing bistartarocuprate(II) complex.
These complexes inhibit the black cupric oxide formation and discharge cupric ions for reduction process.
Black cupric oxide precipitate forms on heating Fehling’s solution without reducing sugars.
In laboratories, Fehling’s test is done by using freshly prepared solution known as Fehling’s solutions. Basically Fehling’s solution are available in two forms i.e. Fehling’s A solution and Fehling’s B solution.
Fehling’s A solution is composed of $\mathrm{CuSO_4}$ i.e. copper(II) sulphate which is in liquid form gives deep blue colour.
Fehling’s B solution is a mixture of Rochelle salt i.e. potassium sodium tartarate and sodium hydroxide (NaOH) which is a strong alkali. This is a transparent liquid solution.
Both Fehling A and Fehling B solutions are prepared separately in fresh form and afterwards it is stored. Later on, both of Fehling’s A and B solutions are mixed to prepare a absolute product of Fehling’s solution. While mixing this solution both Fehling A and B solutions are taken in same volume and hence, a deep blue colour solution is formed. The deep blue colour of the solution is due to the formation of $\mathrm{Cu^{2+}}$ ion of tartrate complex. Here, in the solution the tetra anions of tartrate behave as chelating agents.
Fehling’s test plays a vital role to distinguish the aldehydes and ketones compounds. If the test result is positive then it confirms the presence of aldehyde and if the test result is negative it indicates absence of aldehyde may be presence of ketone or any other functional group which does not respond to the Fehling’s test.
Figure 1: Indicator of Fehling test
Fehling’s test is done with the following procedure −
Figure 2: Fehling’s test
In an empty dry test tube take the test sample.if there is any formation of red precipitate.
In another test tube take distilled water as a blank.if there is any formation of red precipitate.
Then add the Fehling’s solution in both the test tubes containing sample and blank.if there is any formation of red precipitate.
Place the test tubes in water bath container on a burner to heat.if there is any formation of red precipitate.
Observe the colour change in the test tubes if there is any formation of red precipitate.
After the experiment given above show some changes in the test tubes. The formation of brick red colour then it shows positive result and if there is no change in colour or it has only blue colour then it shows negative result.
If both the Fehling A and B solutions is mixed and heated. Fehling B solution behaves as a chelating agent as it is predominant in the solution. If the test sample solution contains aldehyde or sugar compound or any chelating particles, the colour changes to brick red on addition of Fehling’s solution. If the brick red precipitate occurs in the sample solution, it shows that aldehyde or reduced sugar group is predominant in it. Thus, the prime purpose to execute Fehling’s test is to determine the reduced sugar. For better results and accuracy in Fehling’s test, it is necessary to prepare fresh Fehling’s solutions every time before the further procedure of test. To the extent of Fehling’s test for the samples like fructose, lactose, glucose, etc. always shows positive sugar test.
$$\mathrm{RCHO + 2Cu^{2+} + 5OH^- →RCOO- + Cu_2O + 3H_2O}$$
The above reaction occurs between aldehyde and copper(II) ions, hence there is formation of RCOO- (carboxylic ion) ion.
$$\mathrm{RCHO + 2Cu(C_4H_4O_6)2^{2-} + 5OH^-\:\rightarrow\:RCOO^- + Cu_2O + 4(C_4H_4O_6)^{2-} + 3H_2O}$$
Here, the reaction show colour change after addition of the tartrate complex to the previous reaction.
The above reactions are redox reactions, the copper(II) ions are reduced to copper(I) oxide on completion of redox reaction and hence, there is formation of brick red colour precipitation which is not soluble (insoluble) in water. The sodium (Na) salt of carboxylic acid remains as it is in that solution. Therefore, the brick red precipitate formation indicates the positive result of Fehling’s test.
Fehling’s test has various applications and used for many purposes, some of the applications are as follows −
Common use of Fehling’s test is to detect the presence of carbonyl group containing compounds i.e. aldehydes or ketones. Aldehydes are oxidised and shows positive test results while ketones (except α-hydroxy ketones) shows negative test results.
Fehling’s test also primarily is used to determine the simple sugars (monosaccharides) including other reducing sugars. Thus, if monosaccharides are present in both aldoses and ketoses shows positive results. Though ketoses get converts into aldoses due to presence of base in that reagent.
Diabetic patients undergo glucose urine test which is done by using Fehling’s test to identify the person is diabetic or not.
It is also used in the starch break down which converted into a syrup containing glucose and maltodextrin.
Fehling’s test also is used to identify formic acids with positive test result.
Fehling’s test is discovered by a German scientist H. C. Von Fehling. Fehling’s test is basically used to identify the carbonyl group containing compounds like aldehydes, ketones, carbohydrates, reduced sugars, etc. There are two types of Fehling solutions i.e. A and B. Fehling solutions are composed of only copper (II) sulphate, NaOH, and sodium-potassium tartrates.
If there is presence of any sugar, aldehyde or ketone group in the test sample then the solution colour changes from dark blue to brick red indicates positive result and if no colour change means absence of such functional group in the sample which results in no significant colour change and shows negative result.
Q1. Who discovered Fehling’s test?
Ans. It is discovered by H. C. Von Fehling.
Q2. What is Fehling A solution?
Ans. The solution of copper (II) sulphate is known as Fehling A solution.
Q3. What components present in Fehling B solution?
Ans. Strong alkali i.e. sodium hydroxide and sodium potassium tartrate is present in Fehling B solution.
Q4. What results forms if the sample test compound present aldehyde?
Ans. It shows positive test when the deep blue colour changes to brick red precipitate confirms the presence of aldehyde.
Q5. Give three uses of Fehling’s test.
Ans.
It is used to distinguish between aldehyde and ketone groups.
It is used in glucose urine test for diabetic patients.
It is used to determine carbohydrates (simple sugar or monosaccharides).
Q6. What happens when Fehling A get mixed with Fehling B solution?
When Fehling A solution is mixed with Fehling B solution, the complex named bistartarocuprate (II) complex is formed and finally, Fehling solution is formed.
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