In a Biuret test , some protein was mixed with sodium hydroxide solution. Which of the following chemicals should be added to the mixture for a positive result?

Step 1: Analyze the question The Biuret test is the standard food test for proteins. You're asked which chemical is added after sodium hydroxide (NaOH) to get a positive result.

Step 2: The concept The Biuret test detects peptide bonds. The reagents are:

• Sodium hydroxide (or potassium hydroxide) to make the solution alkaline
• Dilute copper(II) sulphate solution, which supplies the Cu²⁺ ions

In an alkaline medium, Cu²⁺ ions form a coordination complex with the nitrogen atoms in peptide bonds. This gives a purple (violet) colour.

Step 3: Step-by-step procedure

1. Take a sample of the food (or protein solution) in a test tube.
2. Add an equal volume of sodium hydroxide solution and mix.
3. Add a few drops of dilute copper sulphate solution, drop by drop.
4. Observe the colour.

Results:

• Purple/violet means protein is present (positive).
• Stays blue means no protein (negative).

Step 4: Eliminate the wrong options

• A. Mercurous nitrate is used in Millon's test for proteins (a different test, which gives a red/brick-red colour on heating).
• C. Sodium carbonate is not a Biuret reagent.
• D. Silver nitrate is used for testing chlorides.

Answer: B. copper sulphate

Tip: Remember "Biuret = Blue to Purple". Biuret uses NaOH + CuSO₄, and the colour change goes from blue to purple/violet when protein is present. Don't confuse it with Millon's test, which uses mercury compounds.