Fehling Test: Definitions, Objectives, Solution, Principles, Process, Requirements, Result, Uses, and Limitations

Fehling Test Overview

One of the most commonly used tests for estimating or identifying reducing and non-reducing sugars is the Fehling test. It is frequently used to discriminate between functional groups of ketones and water-soluble carbohydrates using this test developed by the German scientist H.C. Von Fehling.

What is the Fehling Test?

The Fehling test is a prominent method for estimating or detecting reducing and non-reducing sugars. The test devised by H.C. Von Fehling, a German chemist, is also used to distinguish between ketone functional groups and water-soluble carbohydrates.

Fehling Test Definition in Detail

Detecting reducing sugars and differentiating between water-soluble carbohydrates and ketone functional groups is done using the Fehling test. To distinguish between reducing and non-reducing substances, a deep blue alkaline solution is employed to identify the presence of aldehyde or any groups containing the aldehyde functional group -CHO in addition to Tollen's reagent. To discriminate between a ketone group and water-soluble carbohydrates, use the Fehling solution. The test was created in 1849 by German chemist Hermann von Fehling. A Fehling solution is used in the test to ascertain the result. The preparation of the Fehling solution, the Fehling test method, and associated reactions. The Fehling test is carried out by heating the test material while using the Fehling solution. Brick-red precipitate development indicates the presence of an aldehyde group.

Objectives of the Fehling Test

The Fehling test has numerous goals, but two of its most popular applications are as follows-

1. To distinguish reducing sugar from non-reducing sugar.
2. To identify the different types of carbohydrates in a solution or to determine their existence.

Fehling Test Solution

The Fehling Solution is created by combining two different solutions. The first is Rochelle salt, produced strongly with sodium hydroxide, which is a colorless solution and is known as Fehling A solution. The second is copper sulfate, which is a deep blue aqueous solution and is known as Fehling B solution. The Fehling Solution is created by combining the Rochelle salt and copper sulfate from the A and B solutions. Additionally, each of the solutions—A and B—is produced independently and kept during the assessment. The active chemical in this reaction is the tartrate complex, which acts as an oxidizing agent.

Principles of Fehling Test

The following are some of the principles of the Fehling test-

1. Aldehyde or ketone-containing carbohydrates can function as reducing sugars because they have free or potentially free carbonyl groups.
2. Copper sulfate, potassium sodium tartrate, and a potent alkali—typically sodium hydroxide—combine to form Fehling solution, a dark blue solution.
3. When heated, the sample containing the bistartratocuprate (II) complex from the Fehling solution oxidizes the aldoses into the corresponding aldonic acids.
4. The copper (II) ions in the complex are converted throughout the process to an insoluble yellow or red precipitate or cuprous (I) oxide (Cu2O) ions.
5. On the other hand, ketone oxidation results in shorter acid chain lengths.
6. In the interaction of copper sulfate and sodium hydroxide in the solution, tartrate ions stop the development of insoluble Cu(OH)2 by creating a bistartratocuprate (II) complex.
7. This compound stops the growth of dark cupric oxide by progressively releasing cupric ions for reduction.
8. Absent reducing sugars, heating the Fehling solution causes a black cupric oxide precipitate to develop.

Fehling Test Process

The following are some of the steps to do the Fehling test-

1. Combine equal amounts of Fehling solutions A and B to create a Fehling solution.
2. Fill a test tube with the newly made Fehling solution, making sure it is clean and dry.
3. Another clean and dry test tube should be filled with a sample.
4. Put distilled water in a different test tube as a control.
5. Watch for and note the development of reddish precipitation.

Precaution: Fehling solution frequently has a caustic character. Wearing protective clothing, such as gloves and goggles, is therefore always preferable.

Requirements of Fehling Test

The following are some of the requirements of the Fehling Test-

1. 7 g of CuSO47H2O are dissolved in 100 ml of water to create Fehling solution A.
2. 24 g of KOHand 34.6 g of potassium sodium tartrate are mixed with 100 mL of water to create Fehlings solution B.
3. Just prior to usage, equal amounts of both solutions are combined to create the Fehling solution.
4. 5 percent each of glucose, sucrose, fructose, starch, and lactose make up the sample for testing.
5. Among the extra supplies are pipettes, test tubes, a test tube stand, and a water bath.

Result of Fehling Test

The following are the results of the Fehling test-

1. Positive Result: The formation of a reddish-brown precipitate in test tubes indicates the presence of reducing sugars.
2. Negative Result: A negative outcome and a deficiency in reducing sugars are indicated by the absence of a reddish precipitate or the presence of a deep blue hue.

Uses of Fehling Test

The Fehling tests have several applications, some of which are listed below-

1. As a general test for monosaccharides, the most basic kind of sugar, the Fehling test is also helpful.
2. Fehling solution is useful and frequently used when diabetes patients are checking the level of glucose in their urine.
3. It aids in distinguishing between the functional groups of ketone and aldehyde.
4. The starch is broken down into glucose syrup and maltodextrins in order to assess the quantity of reducing sugar, and the Fehling solution is helpful for this starch breakdown.

Other Common Uses of the Fehling Test

1. Aldehydes frequently oxidize and provide favorable effects. Alpha-hydroxy-ketones are the only ketone that reacts.
2. Aldose and ketose monosaccharides produce good results in the Fehling test, which is also used as a generic monosaccharide test.
3. The Fehling test is frequently used to establish if a carbonyl group belongs to an aldehyde or a ketone. 
4. In addition to these, the Fehling test is employed in the medical industry to find out whether urine contains glucose. 
5. Knowing whether or not the person has diabetes is helpful.

Limitations of the Fehling Test

1. Aromatic aldehydes cannot be found using the Fehling test.
2. Only in the presence of an alkaline environment does this reaction take place. 
3. The copper (II) ions would be stabilized and less likely to oxidize if the combination was acidic. The response would thus be unsuccessful. 
4. Alcohols and aldehydes would also provide a favorable outcome because they may both act as reducing agents.
5. Store in a container that is well closed and out of direct sunlight at or below 30°C. 
6. After opening, the product should be carefully stored in a dry, well-ventilated environment away from sources of ignition and temperature extremes. 
7. After usage, securely cap the container.

Quality Controls of Fehling Test

1. The solution's appearance is light blue.
2. Clarity: Particle-free clarity.
3. Results: When standard Fehling solutions No. 1 (R037) and No. 2 (R038) were combined for sugar estimation, the results fell within the range of established sugar standards.

Points to Remember

1. Fehling reagent: A deep blue solution is created by combining equal amounts of Fehling I and II.
2. In order to make Fehling I, you need to dissolve 7 g of hydrated copper (II) sulfate in 100 mL of separate water.
3. Making Fehling II involves mixing 100 mL of concentrated water with 35 g of potassium sodium tartrate and 10 g of sodium hydroxide.