The Chemical Principle of the Biuret Test
The Biuret test is a colorimetric assay used to detect the presence of proteins and polypeptides in a sample. The test is named after the chemical compound biuret, which gives a similar positive reaction, although the compound itself is not used in the reagent. The core of the test relies on a reaction between cupric ions ($Cu^{2+}$) from copper(II) sulfate and the peptide bonds that link amino acids together.
In an alkaline environment, which is typically provided by sodium hydroxide ($NaOH$), the cupric ions form a violet-colored chelate complex with the unshared electron pairs of the nitrogen atoms in the peptide bonds. This complex absorbs light at a wavelength of approximately 540 nm, causing the solution to appear purple or violet. For a positive reaction to occur, a molecule must contain at least two peptide bonds. Single amino acids and dipeptides do not produce a positive result.
Why Egg White Gives a Positive Biuret Test
Egg white, also known as albumen, is rich in various proteins, most notably ovalbumin, which constitutes over 50% of its protein content. It also contains ovotransferrin, ovomucoid, and ovomucin, among others. Since these proteins are long chains of amino acids linked by numerous peptide bonds, they are an ideal candidate for a positive Biuret test result.
When the Biuret reagent is added to an egg white solution, the copper ions readily react with the abundant peptide bonds in the albumin and other proteins. This reaction leads to the characteristic purple color change, confirming the presence of these large protein molecules. The test is so reliable for egg white that it is consistently used as a positive control in educational and research laboratories.
The Role of Reagents
- Copper(II) Sulfate ($CuSO_4$): Provides the cupric ($Cu^{2+}$) ions that form the colored complex with the peptide bonds.
- Sodium Hydroxide ($NaOH$): Creates the necessary alkaline environment for the reaction to take place. Without a high pH, the reaction will not proceed correctly.
- Potassium Sodium Tartrate: This optional but common component acts as a chelating agent to stabilize the copper(II) ions in the alkaline solution, preventing their precipitation.
How to Perform the Biuret Test on Egg White
Performing the Biuret test on egg white is a simple and common high school biology or chemistry experiment. The following steps outline a standard procedure.
Materials Needed
- Test tubes and a test tube rack
- A dropper or pipette
- Egg white solution (prepared by mixing egg white with distilled water)
- Distilled water (as a negative control)
- Biuret Reagent (a solution of copper sulfate and sodium hydroxide)
Procedure
- Preparation: Label two clean test tubes, one for 'Egg White' and one for 'Water' (negative control).
- Add Samples: Add 2 mL of the egg white solution to the labeled egg white tube. Add 2 mL of distilled water to the other tube.
- Add Reagent: Add 2 mL of Biuret reagent to each test tube. Mix gently by shaking or swirling.
- Wait: Allow the mixtures to stand for 3 to 5 minutes to give the reaction time to complete.
- Observe: Note the color change in each test tube and compare them.
| Sample | Observation | Inference |
|---|---|---|
| Egg White Solution | The solution changes from blue to purple or violet. | Presence of protein confirmed. |
| Distilled Water | The solution remains blue. | Absence of protein confirmed. |
Conclusion: A Clear Positive
The question "Is egg white positive in the Biuret test?" can be definitively answered with a "yes." The Biuret test provides a clear visual indicator—the color change from blue to violet—that proves the presence of proteins in egg white. This reaction occurs because the chemical makeup of egg white is rich in long-chain protein molecules that contain the necessary peptide bonds to react with the copper ions in the Biuret reagent. This simple yet powerful test demonstrates a fundamental principle of protein chemistry, confirming the high protein content that makes egg white such a valuable nutritional source.
For further detail on the chemical process, consult sources like Vedantu's explanation on the Biuret test.
