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What is the Biuret test? How do I interpret the results?

For the identification of different biological molecules, different tests are performed. For example, for identifying reducing sugars, Benedict’s test can be performed, for identifying fats, emulsion test can be done and for identifying starch, iodine test can be performed. Similarly, if we want to identify if a sample has proteins, then we can perform a biuret's test.

Proteins are made up of amino acids which have different charges on them. The amino acids are held together in the form of a polypeptide with the help of peptide bonds. These peptide bonds can react with the Biuret reagent, to give out results. Not only proteins, but other compounds which have a peptide bond (CO-NH) will give a result with this reagent.

A Biuret reagent is made up of condensation of urea molecules when they are heated. It also has sodium hydroxide, potassium sodium tartrate and hydrated copper (II) sulfate. When the Cu+2 ions are present, they provide a blue- coloured solution. When this reagent comes in contact with peptide bonds, copper ions form a complex with it, and hence, form a purple-coloured solution. If there are more peptide bonds, hence the colour intensity would be greater.


1.     Take 2 ml of a sample in a test tube.

2.     Add 2ml of Biuret reagent in the test tube.

3.     Shake well and wait for 5 minutes.

4.     Observe for colour changes; if a purple colour is given, it means proteins are present in the sample. If the colour remains blue, this means no proteins are there in the sample.




        Purple/ violet colour

      Positive result

    Proteins are present 

        Remains blue colour

Negative result

    Proteins are absent

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