Question

What are the limitations for the Biuret test for proteins (where a positive result is seen if the solution turns purple)?

Answer 1

Answer:

Biuret test: also called as “Piotrowski’s test”, is performed to detect the existence of peptide bonds in the protein. Peptide’s presence turns the sample violet/purple in color after reacting with copper ions. The proportion of protein can be judged by the intensity of the purple color.

There are few limitations to this test:

• Biuret test indicates only the peptide bond’s presence but not the actual count of the bonds in the protein sample unless a comparison is made between the tested sample and the known standard sample.
• Color of other chemicals or molecules present in the sample can fluctuate the actual purple color of the protein reaction and thus can give inappropriate results.
• Biuret test will not detect amino acids present in the protein sample. It only identifies the peptide bonds.
• In case of sample having very small peptide bond concentration, the biuret test fails to obtain the desired color.
• If sample is not in proper dilute condition or it is blurred and below the standard value of 5 milligram per ml, the test fails to give desired output.