Running a paper chromatogram, the two phases, Rf values measured from the start line, repeats and identifying substances.
What you will learn
- Describe how to run a paper chromatogram and give a reason for each step
- Explain separation using the mobile and stationary phases
- Calculate Rf values and use them to identify substances
Running the practical
- Draw a start line in pencil about 1.5 cm from the bottom (ink would run).
- Put small spots of the unknown and of known substances on the line.
- Stand the paper in a little solvent: the solvent must be below the line. Put a lid on.
- Take the paper out before the solvent reaches the top, mark the solvent front and let it dry.
Colourless substances such as amino acids need a locating agent sprayed on the dry paper to make the spots show.
Why each step matters:
- Pencil does not dissolve in the solvent, so the line stays put and you can still find it to measure Rf after spraying.
- The solvent starts below the line so the spots do not simply dissolve into the solvent in the beaker.
- The lid stops the solvent evaporating, so the air inside stays full of solvent vapour and the solvent keeps rising evenly.
- The paper is dried before a locating agent is sprayed, so the solvent has gone and the spots stay where they stopped.
The rest of this lesson and its 9 practice questions are locked.
In this lesson
- Running the practical
- Two phases
- Identifying a substance
- Try it: Measure from the start line
9 practice questions, marked as you go; about 25 minutes in all.
Key terms
Pencil, Below, Locating agent, Stationary phase, Mobile phase, From the start line
In this unit: Chemical analysis
Purity and formulations, chromatography and Rf values, tests for gases, and for separate Chemistry the tests for ions and flame emission spectroscopy.
- Pure substances and formulations
- Required practical: chromatography
- Tests for gases
- Required practical: identifying ions
- Instrumental methods