What is Fehling Solution? (Definition, Preparation & Test Results)
Fehling Solution is a classic laboratory reagent widely used in chemistry to identify reducing sugars and aldehyde groups. Understanding Fehling's solution helps students master key concepts in both practical experiments and theoretical chemistry, particularly in analytical and organic chemistry chapters.
What is Fehling Solution in Chemistry?
A Fehling Solution refers to a deep blue chemical reagent specifically used for detecting reducing sugars and distinguishing between aldehyde and ketone functional groups. First introduced by German chemist Hermann von Fehling, this solution plays an important role in qualitative analysis, especially in organic, carbohydrate, and redox chemistry. This topic is closely linked to analytical chemistry, laboratory techniques, and qualitative organic analysis.
Molecular Formula and Composition
Fehling solution does not have a single formula, as it is a mixture of two separate solutions:
| Component | Composition / Formula | Role |
|---|---|---|
| Fehling’s Solution A | Copper(II) sulfate pentahydrate (CuSO4·5H2O) | Supplies Cu2+ (deep blue color) |
| Fehling’s Solution B | Potassium sodium tartrate (Rochelle salt) + Sodium hydroxide (NaOH) | Creates alkaline medium and complexes Cu2+ |
To perform a test, equal parts of Fehling A and B are mixed. The resulting solution contains a complex of copper(II) ions and tartrate in strong alkaline conditions, enabling the detection of reducing groups.
Preparation and Synthesis Methods
Fehling solution is always freshly prepared in the lab before use to remain effective. Here’s how you make both parts:
- Fehling’s Solution A: Dissolve 34.65 g of CuSO4·5H2O in distilled water and dilute to 500 mL.
- Fehling’s Solution B: Dissolve 173 g of potassium sodium tartrate and 50 g of NaOH in distilled water; make to 500 mL.
- For testing, mix equal volumes of solutions A and B to get the working Fehling solution.
Mixing these just before use prevents copper precipitation and maintains analytical accuracy. This method is commonly used in practical exams and laboratory work.
Physical Properties of Fehling Solution
The prepared Fehling solution is a transparent or deep blue liquid, alkaline in nature due to NaOH, and has a strong, slightly irritating odor. The blue color is due to the copper(II)-tartrate complex. If left standing after mixing, a brick-red Cu2O precipitate may form, so the solution must be used fresh.
Chemical Properties and Reactions
Fehling solution acts as an oxidizing agent. It reacts with reducing sugars and most aldehydes by reducing blue Cu2+ ions to brick-red Cu2O (copper(I) oxide), while the aldehyde group is oxidized to a carboxylate ion. The general reaction is:
R–CHO + 2 Cu2+ + 5 OH– → R–COO– + Cu2O↓ + 3 H2O
Ketones and most aromatic aldehydes do not react because they lack a suitable hydrogen for oxidation under these conditions.
Frequent Related Errors
- Confusing Fehling solution with Benedict’s solution or Tollens' reagent.
- Believing all sugars will react—non-reducing sugars like sucrose do NOT give a positive test.
- Re-using old, pre-mixed Fehling solution, which results in false negatives.
Uses of Fehling Solution in Real Life
Fehling solution is widely used for detecting glucose in urine (screening for diabetes), in food chemistry for estimating reducing sugar content, and in academic or school labs for identifying organic compounds. It is also used to distinguish between aldehydes and ketones and test certain carbohydrates in industry and research.
Relation with Other Chemistry Concepts
Fehling solution links with Redox reactions, qualitative organic analysis, and the study of carbohydrates. Comparing its reactions to those of Tollens’ reagent and Benedict’s solution helps students understand subtle differences in the detection of aldehydes and reducing sugars.
Step-by-Step Reaction Example
1. Start with the reaction setup.Mix equal volumes of freshly prepared Fehling A and B solutions in a clean test tube.
2. Add the test sample (e.g., glucose solution) to this mixture.
3. Place the tube in a boiling water bath and observe.
4. If the sample is a reducing sugar or an aldehyde, you’ll see a brick-red precipitate, confirming a positive Fehling’s test.
5. The overall chemical reaction is as follows:
C6H12O6 (glucose) + 2 Cu2+ + 5 OH– → C6H11O7– + Cu2O ↓ + 3 H2O
Lab or Experimental Tips
Always prepare Fehling solution fresh before use for accurate results. Remember: blue to brick-red color change means “positive.” Vedantu educators teach this color cue to avoid confusion between positive and negative tests during live sessions.
Try This Yourself
- Write the chemical equations for Fehling’s test with glucose and acetone.
- List two substances that give negative results in Fehling’s test.
- Explain the difference between Fehling and Benedict reagent with examples.
Final Wrap-Up
We explored Fehling Solution—its composition, preparation, and importance in detecting reducing sugars and aldehydes. Remember its stepwise lab procedure and the significance of its color change for exams and practicals. For deeper explanations, experiment videos, and more chemistry guidance, join classes and access detailed material with Vedantu’s online resources.
- Compare testing methods with Benedict’s Test and Tollens’ Reagent for a complete understanding.
- Brush up on Carbohydrates and their classification.
- Explore more about Redox Reactions and Qualitative Chemical Analysis to see how Fehling’s fits into your chemistry toolkit.
FAQs on Fehling Solution – Definition, Test Procedure & Uses Explained
1. What is Fehling's solution and what is it used for?
Fehling's solution is a chemical reagent used to detect the presence of reducing sugars and aldehyde functional groups in a chemical sample. It's a classic qualitative test in organic chemistry, widely used to identify various compounds.
2. What are Fehling's Solution A and B, and how are they prepared?
Fehling's solution is actually a mixture of two separate solutions: Fehling's A and Fehling's B. Fehling's A is an aqueous solution of copper(II) sulfate (CuSO4), while Fehling's B contains potassium sodium tartrate (Rochelle salt) and a strong alkali, usually sodium hydroxide (NaOH). These two solutions are mixed in equal volumes *immediately before* use because the mixed solution is unstable and decomposes over time.
3. How do I perform a Fehling's test?
To perform a Fehling's test, mix equal volumes of Fehling's A and B to create the active reagent. Then, gently heat the solution with the substance you're testing. A positive result is indicated by the formation of a brick-red precipitate of copper(I) oxide (Cu2O).
4. What causes the color change in a positive Fehling's test?
The color change from blue to brick-red is due to a redox reaction. The aldehyde group in the reducing sugar reduces the blue copper(II) ions (Cu2+) in Fehling's solution to copper(I) ions (Cu+), which then precipitate as insoluble copper(I) oxide (Cu2O), a brick-red solid.
5. Which compounds give a positive Fehling's test?
Reducing sugars such as glucose, fructose, and maltose give a positive Fehling's test. Many aldehydes also react positively, while most ketones do not, except for some specific alpha-hydroxy ketones. Aromatic aldehydes generally do not react.
6. What is the difference between Fehling's and Benedict's solutions?
Both Fehling's and Benedict's solutions are used to detect reducing sugars, but they differ in their composition. Benedict's solution uses sodium citrate instead of potassium sodium tartrate. Benedict's is often considered less sensitive but more stable.
7. What safety precautions should I take when using Fehling's solution?
Fehling's solution contains caustic soda (NaOH), which is corrosive. Always wear appropriate safety goggles and gloves. Avoid skin contact and ingestion. Dispose of the solution according to your laboratory's guidelines.
8. Why is Fehling's solution prepared freshly before use?
The mixed Fehling's solution is unstable and decomposes over time. The copper(II) ions may precipitate or react with atmospheric oxygen, leading to inaccurate results. Preparing it fresh ensures accurate testing.
9. Can Fehling's solution detect ketones?
Generally, no. Most ketones do not react with Fehling's solution, because they are not easily oxidized. This lack of reactivity helps to distinguish them from aldehydes. However, some exceptions exist, primarily alpha-hydroxy ketones.
10. What are some real-world applications of Fehling's test?
Fehling's test is used in various applications, including: detecting glucose in urine (for diabetes screening), identifying reducing sugars in food and beverages, and as a tool in organic chemistry experiments to distinguish aldehydes from ketones.
11. What is the chemical equation for the reaction between an aldehyde and Fehling's solution?
The simplified equation is: RCHO + 2Cu2+ + 5OH- → RCOO- + Cu2O(s) + 3H2O. The aldehyde (RCHO) is oxidized to a carboxylate ion (RCOO-), and the copper(II) ions (Cu2+) are reduced to copper(I) oxide (Cu2O), the brick-red precipitate.
