Investigating Proteins
Biology: AQA A Level, topic 1.4.2
The Biuret Test
The biuret test is used to test for the presence of a protein in a sample.
1) Add sample
- Add sample to distilled water and biuret solution (grind in water if the sample is solid).
2) Shake solution
- Shake then leave upright for 5 minutes and observe the colour.
3) Analyse results
- The Biuret test changes colour from blue to violet when proteins are present.
- The result will only be positive if the amino acids are joined together by peptide bonds, as they are in proteins.
- The result will NOT be positive if the solution contains only free amino acids.
Test yourself on Investigating Proteins
Steps Involved in the Biuret Test
What colour change takes place if polypeptides are present?
What are the stages in the biuret test?