Odczyn Biernackiego

A Comprehensive Guide to the Odczyn Biernackiego Test

Introduction

The Odczyn Biernackiego, also known as the Biernacki reaction, is a chemical test used to differentiate between sucrose and reducing sugars like glucose and fructose.

Developed by Polish chemist Janusz Leon Biernacki in 1908, it is based on the reduction of Fehling's solution by reducing sugars, resulting in the formation of a brick-red precipitate of cuprous oxide.

Principle of the Test

Fehling's solution is a mixture of copper sulfate, sodium citrate, and sodium hydroxide. Reducing sugars, such as glucose and fructose, contain a free aldehyde or keto group that can react with the copper ions (Cu²⁺) in Fehling's solution. This reaction reduces the copper ions to cuprous ions (Cu⁺), which then precipitate out as cuprous oxide (Cu₂O), giving a brick-red color to the solution.

Sucrose, on the other hand, is a non-reducing sugar and does not contain a free aldehyde or keto group. Therefore, it does not react with Fehling's solution and does not produce a brick-red precipitate.

Reagents Required

• Fehling's solution A: 34.66 g of copper sulfate pentahydrate (CuSO₄·5H₂O) dissolved in 500 mL of distilled water
• Fehling's solution B: 173 g of sodium potassium tartrate (NaKC₄H₄O₆·4H₂O) and 50 g of sodium hydroxide (NaOH) dissolved in 500 mL of distilled water
• Sample solution: The unknown sugar solution to be tested

Procedure

1. Mix equal volumes of Fehling's solution A and Fehling's solution B in a test tube
2. Add a few drops of the sample solution to the mixture
3. Heat the mixture in a boiling water bath for several minutes
4. Observe the color of the solution

Interpretation of Results

• Positive result (reducing sugar present): Formation of a brick-red precipitate indicates the presence of a reducing sugar.
• Negative result (non-reducing sugar present): No precipitate or a faint blue color indicates the presence of a non-reducing sugar, such as sucrose.

Applications of the Odczyn Biernackiego Test

• Identification of reducing sugars in urine, which can indicate conditions such as diabetes
• Detection of reducing sugars in food products
• Quality control in the sugar industry

Limitations of the Test

While the Odczyn Biernackiego test is a useful tool for differentiating between reducing and non-reducing sugars, it has certain limitations.

Some non-reducing sugars, such as maltose, can give a weak positive reaction if the heating time is prolonged.

Additionally, the test is not quantitative and cannot be used to determine the concentration of sugars in a sample.

Conclusion

The Odczyn Biernackiego test is a simple and reliable method for distinguishing between reducing and non-reducing sugars. It is widely used in various applications, including clinical diagnostics and food testing.