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Lesson: Chapter 1 — The Chemical Basis of Life

1.2. Proteins 6 of 6

The biuret test to identify proteins

TEST The biuret test

We know that dhal and eggs contain protein. But how do we confirm it in the laboratory? The biuret test is used for this.

Method

  1. Take a solution made by crushing dhal and mixing it with water, or a solution containing egg white, into a test tube.
  2. Add a larger amount of sodium hydroxide (NaOH) solution to it and mix.
  3. Then add a few drops of copper sulphate (CuSO4) solution.

Observation

Pale blue → Pink-purple → Deep purple

The pale blue colour of the copper sulphate changes, and the solution turns pink-purple or deep purple. This is evidence that protein is present.

Comparing three tests

Let us bring together the tests we have learned so far for identifying biological molecules and compare them.

What it identifies Reagent used Positive result
Starch Iodine solution Dark blue tending to purple
Monosaccharides Heating with Benedict's solution Brick red precipitate
Proteins Sodium hydroxide (NaOH) + copper sulphate (CuSO4) Purple colour

How does this connect?

What the biuret reagent really reacts with is the nitrogen-containing bonds that hold a protein's amino acids to each other. So this test can also be used to show that nitrogen is present in a piece of living material.

Sources