Question 10
In the Biuret test for protein, what is added to the solution?
The right answer is the one marked above.
An excess of sodium hydroxide is mixed in first, then a few drops of copper sulphate. The solution turns rose-purple or dark purple.
Try it on a solution of ground lentils or on egg white. Note where the colour comes from: copper sulphate reacting in the presence of protein — which is why the same pair of reagents is used to show that a food contains nitrogen.