AQA · A-Level · Chemistry
Course version: 7405 · Version from Sept 2016
Amino acids can be located on a chromatogram using developing agents such…
Full specification wording
Proteins (A-level only): Amino acids can be located on a chromatogram using developing agents such as ninhydrin or ultraviolet light and identified by their R values.
Amino acids are colourless, so a finished plate looks blank and the spots have to be made visible.
Checked against the AQA 7405 specification (opens in a new tab) on .
- Point 6 of 7
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Amino acids are colourless, so a finished plate looks blank and the spots have to be made visible. Spraying with ninhydrin turns each spot purple; iodine also works, and a plate made with a fluorescent additive shows the spots as dark patches under an ultraviolet lamp. Once the spots are visible, the positions are measured. The Rf value is the distance travelled by the spot divided by the distance travelled by the solvent front, both measured from the pencil baseline and the spot measured to its centre. Rf always lies between 0 and 1. It is only fixed for a given solvent and stationary phase, so a known sample is run alongside on the same plate for comparison.
What earns the marks
- Naming ninhydrin, or iodine, as the developing agent.
- The reason it is needed, that the amino acids are colourless.
- The Rf calculated as the distance moved by the spot divided by the distance moved by the solvent front.
Common mistakes and how to put them right
Measuring the solvent front from the bottom edge of the plate rather than from the baseline.
Measuring to the leading edge of a spot instead of its centre.
Quoting an Rf value greater than 1, which means the two distances have been divided the wrong way round.
Treating an Rf value as a fixed property of a substance regardless of the solvent used.
Exam tips
- State the final colour only as extra detail; on its own 'it turns them purple' does not explain why a developing agent is needed.
- Show the two distances in your working before you divide, so a method mark survives an arithmetic slip.
- The specification mentions ultraviolet light, but when AQA asks you to NAME a developing agent the mark scheme ignores UV: write ninhydrin (or iodine).
Try the lesson
The start of the Proteins (A-level only): Amino acids can be located on a chromatogram using developing agents such as ninhydrin or ultraviolet light and identified by their R values. lesson, made from checked teaching notes for this specification point.
Have a go first
A student runs a paper chromatogram of a mixture of amino acids. The dried chromatogram is colourless, so the student sprays it with ninhydrin. What is the purpose of spraying the chromatogram with ninhydrin?
Listen
Amino acids are colourless, so a finished plate looks blank and the spots have to be made visible. Spraying with ninhydrin turns each spot purple; iodine also works, and a plate made with a fluorescent additive shows the spots as dark patches under an ultraviolet lamp. Once the spots are visible, the positions are measured. The Rf value is the distance travelled by the spot divided by the distance travelled by the solvent front, both measured from the pencil baseline and the spot measured to its centre. Rf always lies between 0 and 1. It is only fixed for a given solvent and stationary phase, so a known sample is run alongside on the same plate for comparison. A strong answer earns marks for these points. naming ninhydrin, or iodine, as the developing agent. the reason it is needed, that the amino acids are colourless. the Rf calculated as the distance moved by the spot divided by the distance moved by the solvent front. Exam tip: State the final colour only as extra detail; on its own 'it turns them purple' does not explain why a developing agent is needed.
Key words
Say what each word means before you turn the card.
ninhydrin
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Rf value
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comparison with standards
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See every definition
- ninhydrin
- A developing agent that can be sprayed on a plate to make the spots of amino acids visible.
- Rf value
- The distance travelled by the spot divided by the distance travelled by the solvent front, both measured from the pencil baseline with the spot measured to its centre.
- comparison with standards
- Running a known sample alongside on the same plate, because an Rf value is only fixed for a given solvent and stationary phase.
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