AQA · A-Level · Biology
Course version: 7402 · Version from Sept 2016
General properties of proteins: The biuret test for proteins.
The biuret test detects peptide bonds, so it is a test for peptides and proteins rather than for individual amino acids.
Checked against the AQA 7402 specification (opens in a new tab) on .
- Point 8 of 9
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
The biuret test detects peptide bonds, so it is a test for peptides and proteins rather than for individual amino acids. Work at room temperature, because no heating is needed and heating is a common error. Add an equal volume of sodium hydroxide solution to the sample to make it alkaline, then add a few drops of dilute copper(II) sulfate solution and mix gently; many laboratories supply the two combined as biuret reagent. A positive result is a colour change from blue to purple, lilac or violet, produced by copper(II) ions forming a coloured complex with the nitrogen atoms of peptide bonds. A negative result stays blue. Because the test depends on peptide bonds, a solution of free amino acids gives a negative result. The intensity of the purple increases with protein concentration, so with a colorimeter and a calibration curve made from known standards the test can be made quantitative.
What earns the marks
- Adding sodium hydroxide solution to make the sample alkaline.
- Then adding dilute copper(II) sulfate solution, or for using biuret reagent.
- A positive result being a colour change from blue to purple, lilac or violet.
- Stating that the solution stays blue if no protein is present.
- Using a colorimeter with a calibration curve of known concentrations to make the test quantitative.
Common mistakes and how to put them right
Heating the biuret test in a water bath, as for Benedict's.
Giving the positive result as brick-red, confusing it with the reducing sugar test.
Omitting the sodium hydroxide, so the reaction cannot occur in alkaline conditions.
Saying the test detects amino acids, when free amino acids have no peptide bonds and give a negative result.
Reporting the blue starting colour as the positive result.
Exam tips
- Remember what the test actually detects - peptide bonds - because that explains both the positive and the negative results.
- Name both reagents, sodium hydroxide and copper(II) sulfate, even if you also mention biuret reagent, and leave heating out of the method - biuret is done at room temperature.
- If asked to compare protein concentrations, describe a colorimeter and a calibration curve built from known standards.
Try the lesson
The start of the General properties of proteins: The biuret test for proteins. lesson, made from checked teaching notes for this specification point.
Have a go first
What colour change indicates a positive result in the biuret test for proteins?
Listen
The biuret test detects peptide bonds, so it is a test for peptides and proteins rather than for individual amino acids. Work at room temperature, because no heating is needed and heating is a common error. Add an equal volume of sodium hydroxide solution to the sample to make it alkaline, then add a few drops of dilute copper(II) sulfate solution and mix gently; many laboratories supply the two combined as biuret reagent. A positive result is a colour change from blue to purple, lilac or violet, produced by copper(II) ions forming a coloured complex with the nitrogen atoms of peptide bonds. A negative result stays blue. Because the test depends on peptide bonds, a solution of free amino acids gives a negative result. The intensity of the purple increases with protein concentration, so with a colorimeter and a calibration curve made from known standards the test can be made quantitative. A strong answer earns marks for these points. adding sodium hydroxide solution to make the sample alkaline. then adding dilute copper(II) sulfate solution, or for using biuret reagent. a positive result being a colour change from blue to purple, lilac or violet. stating that the solution stays blue if no protein is present. Exam tip: Remember what the test actually detects - peptide bonds - because that explains both the positive and the negative results.
Key words
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biuret test
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peptide bonds detected
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sodium hydroxide and copper sulfate
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blue to purple
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See every definition
- biuret test
- Detects peptide bonds, so it is a test for peptides and proteins rather than for individual amino acids.
- peptide bonds detected
- A positive result is produced by copper(II) ions forming a coloured complex with the nitrogen atoms of peptide bonds.
- sodium hydroxide and copper sulfate
- An equal volume of sodium hydroxide solution makes the sample alkaline, then a few drops of dilute copper(II) sulfate solution are added and mixed gently.
- blue to purple
- A positive result is a colour change from blue to purple, lilac or violet, while a negative result stays blue.
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