Fehling’s Test
Fehling’s test is used for the detection of reducing sugars or to differentiate between water soluble carbohydrates or ketone functional groups. It is a deep blue alkaline solution used to identify the existence of aldehyde or any groups that contain aldehyde functional group -CHO in addition with the Tollen’s reagent in order to differentiate between the reducing and non-reducing agents. Fehling solution also finds its use in differentiating a ketone group and water-soluble carbohydrates. The test was developed by German Chemist Hermann von Fehling in 1849. In the test Fehling’s solution is used to get the result. In this article we will discuss the preparation of the Fehling solution, the Fehling test procedure and the reactions involved in this, the result of the Fehling test and its applications.
To carry out the Fehling test, the substance that is to be tested is heated with the fehling solution. If there is the presence of any aldehyde group, it is indicated by the formation of a brick-red precipitate.
Fehling’s Solution
The Fehling solution for the Fehling test is prepared by combining two separate solutions called Fehling’s Solution A and Fehling’s Solution B. Fehling solution A and Fehling solution B are mixed together in equal amounts before the test to get Fehling's solution. Aqueous solution of copper sulfate is called Fehling solution A which is blue in color. Fehling solution B is prepared by mixing aqueous potassium sodium tartrate (Rochelle salt) in a strong alkali (common alkali used is NaOH). It’s a colorless solution. Fehling’s solution A and B can be stored separately in the laboratory. Fehling’s solution which is prepared freshly by mixing Fehling’s solution A and B is deep blue in color due to the bis(tartrate) complex of Cu2+. The tartrate anions act as chelating agents.
Procedure of Fehling’s Test
Procedure of Fehling’s test is as follows –
Mix Fehling solution A and B in equal quantities to prepare fresh Fehling’s solution.
Take freshly prepared Fehling’s solution in a washed and dried test tube.
Take a sample in another washed and dried test tube.
Take distilled water in another test tube as control.
Now tubes are kept in a boiling water bath.
Observe and record if development of red colored precipitate takes place.
Reaction Of The Fehling Test
The Reaction occurring between the Copper (II) Ions and Aldehyde in the Fehling solution is as follows;
RCHO + 2 Cu2+ + 5 OH− → RCOO− + Cu2O + 3 H2O
With the addition of Tartrate, the Reaction is changed as follows;
RCHO + 2 Cu(C4H4O6)22− + 5 OH− → RCOO− + Cu2O + 4 C4H4O62− + 3 H2O
With the completion of the redox reaction, the copper II ions are reduced to copper I Oxide. This is indicated with the formation of the red precipitate and its insoluble nature in water. The Sodium salt of the given acid is left behind in the solution.
Result
After mixing the Fehling solution in the sample, if you observe red precipitate then it indicates the result is positive while if you don’t observe any red (or brownish red) precipitate then the result is negative. Positive results in the Fehling test indicate presence of glucose, fructose and lactose or presence of reducing sugar in the sample. The negative result of the Fehling test indicates the presence of non-reducing sugars such as sucrose, starch.
Application of Fehling’s Test or Fehling’s solution
It is used to test Monosaccharides. Fehling test is used as a general test to determine Monosaccharides and other reducing sugars. In case of the aldose monosaccharides, the result of the test is positive. This is mainly because of the oxidizable aldehyde group. In case of ketose Monosaccharides as well a positive result is witnessed. These get converted to aldoses with the help of the base present in the reagent.
It is used for detection of reducing sugar like glucose in the sample.
It can be used to distinguish aldehyde and ketone functional groups.
In the medical field It can be used to screen for glucose in urine in order to detect diabetes.
It can be used to detect diabetes.
It is also used in the process of the breakdown of the starch. Here it changes to glucose syrup and maltodextrins, and is used to measure and determine the amount of reducing sugar.
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FAQs on Fehling Solution
1. Why are aldehydes and ketones classified differently?
Although aldehydes and ketones are similar in many aspects and properties , they are still classified differently. The reason behind this is the lone hydrogen that is seen bonded to the carbon of the functional group in the aldehydes which makes the aldehydes very easy to oxidize and thus are known to be strong reducing agents. ketones do not possess this hydrogen, In Spite of being a very minor property, this change alone gives rise to the different chemical and physical properties of the aldehydes when compared to the ketones.
2. What is the difference between acetaldehyde and formaldehyde?
Although both acetaldehyde and formaldehyde are organic compounds, formaldehyde is much more complicated in comparison. This is because it takes up different forms. Apart from this, another component that differentiates acetaldehyde from formaldehyde is the fact that acetaldehyde gives out a much pleasant odor in comparison to formaldehyde. This is the reason that aldehydes widely find their use in the production of perfumes, pharmaceuticals and dyes. On the other hand, formaldehyde finds its use in numerous products like paints, glues, resins, antiseptics, preservatives etc.
3. Why are aldehydes more reactive towards nucleophilic reactions than ketones?
Aldehydes are more reactive than ketones towards nucleophilic reactions because of the given reasons;
Aldehydes are known to be less hindered than ketones.
The carbonyl carbon present in the aldehydes have more partial positive charge as compared to the ketones. This is the result of the electron donating nature associated with the alkyl group.
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4. What is the difference between Acetaldehyde and Benzaldehyde?
Both Benzaldehyde and Acetaldehyde are organic compounds, but Benzaldehyde is composed of a benzene ring that is bonded to an Aldehyde functional group. On the other hand, acetaldehyde is composed of a Methyl group attached to the aldehyde functional group. Another difference between the two is that benzaldehyde is known as an aromatic aldehyde, whereas acetaldehyde is known as an aliphatic aldehyde. When talking about the appearance and odour, benzaldehyde is a colourless liquid that gives out an almond like odour. Acetaldehyde is also a colourless liquid that gives out a pungent odour. Benzaldehyde on the one hand is insoluble in water, whereas, Acetaldehyde is miscible with water. The boiling point of Acetaldehyde is 20.2 degree C, whereas the melting point of benzaldehyde is near 178 degree C.
5. Why is fehling solution freshly prepared, and is it harmful?
It is necessary to prepare the Fehling solution freshly because the complex tartrate complex is known to be unstable which results in its decomposition readily.
Yes, fehling solution can be harmful if swallowed. It can cause irritation in the respiratory tract and can also result in some burns. Therefore, it is always advised to be careful while handling the fehling solution.
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