Fehling's test

Summary

The video explains the Fehling's test, a method for detecting aldehydes and reducing sugars using two solutions: Fehling's solution A (copper sulfate) and B (potassium sodium tartrate and sodium hydroxide). When mixed, they form a blue complex. The test distinguishes between glucose, a reducing sugar that reacts positively due to its aldehyde group, and sucrose, which does not react because of its glycosidic bond. The addition of citric acid can hydrolyze sucrose into glucose and fructose, enabling a positive result. The process highlights the reduction of copper ions and the formation of copper(I) oxide as a precipitate, illustrating fundamental concepts in carbohydrate chemistry.

Takeaways

• 🧪 The Fehling's test is used to detect aldehydes and reducing sugars.
• ⚠️ Copper sulfate, a key component of the test, is toxic to aquatic life and can irritate skin and eyes.
• 🧴 Sodium hydroxide can cause severe chemical burns.
• 🔬 The test involves two solutions: Fehling's solution 1 (copper sulfate) and Fehling's solution 2 (potassium sodium tartrate and sodium hydroxide).
• 💧 A 1:1 volumetric mixing ratio of the two solutions forms a deep blue complex.
• 🍬 Glucose, a monosaccharide, reacts positively in the Fehling's test due to its aldehyde group.
• 🍭 Sucrose, a disaccharide, does not react unless treated with citric acid, which allows it to break down into monosaccharides.
• 🔥 Samples are heated in a water bath to facilitate reactions during the test.
• 🌈 A positive result is indicated by a precipitate of copper(I) oxide, which may exhibit various colors.
• 🥛 Lactose is an example of a disaccharide that can also act as a reducing sugar due to the ability of its glucose component to open its ring structure.

Q & A

What are the two solutions used in the Fehling's test?

The two solutions are Fehling's solution 1, which contains 7 grams of copper sulfate in 100 milliliters of distilled water, and Fehling's solution 2, which is made of 35 grams of potassium sodium tartrate, 10 grams of sodium hydroxide, and 100 milliliters of distilled water.

What is the purpose of Fehling's test?

Fehling's test is used to detect reducing sugars and aldehydes by observing the formation of a precipitate of copper(I) oxide.

Why is it important to mix the two Fehling's solutions in a 1:1 ratio?

The 1:1 mixing ratio ensures that the copper ions react effectively with the tartrate ions, forming a deep blue complex that is crucial for the subsequent reactions with reducing sugars.

What type of sugar is glucose classified as?

Glucose is classified as a monosaccharide, which is a single sugar molecule.

Why did sucrose not give a positive result in Fehling's test?

Sucrose did not react because its glycosidic bond prevents the glucose ring from opening, which is necessary for the reduction reaction to occur.

What role does citric acid play in the reaction with sucrose?

Citric acid helps to hydrolyze sucrose under heated conditions, breaking the glycosidic bond and allowing the monosaccharides to be formed, which can then react positively in the Fehling's test.

What does a positive result in the Fehling's test look like?

A positive result is indicated by the formation of a precipitate of copper(I) oxide, which can exhibit different colors.

Can disaccharides be reducing sugars? Give an example.

Yes, some disaccharides can be reducing sugars. An example is lactose (milk sugar), which can reduce copper ions due to the presence of an aldehyde group.

What is the chemical process that occurs when an aldehyde reacts in Fehling's test?

In the reaction, copper ions are reduced from the +2 oxidation state to +1, forming copper(I) oxide, while the aldehyde is oxidized to a carboxylic acid.

What safety precautions should be taken when conducting the Fehling's test?

Safety precautions include wearing gloves and goggles to prevent irritation or burns from copper sulfate and sodium hydroxide, as they are toxic and can cause severe chemical burns.