Paper chromatography is a simple and useful technique used to separate and identify components in a mixture. In this post, we will learn the principle behind paper chromatography, how to set up the experiment, and how to calculate the Rf value.
What is Chromatography and Chromatogram
Chromatography is a physical method used to separate mixtures of substances that dissolve in the same solvent.
One type of chromatography is paper chromatography. Another important type is gas chromatography. We will focus on paper chromatography in this post.
It can be used to:
- Separate different components in a mixture
- Identify substances
- Check the purity of a substance (a pure substance gives only one spot on the chromatogram)
So, what is chromatogram? Chromatogram is the result of chromatography, showing us the separated components of a sample.
How to Set Up Paper Chromatography
We draw a line at the bottom of the chromatography paper, about 1 cm from the bottom of this strip will do. Do note, it is important that we use pencil instead of pen for this line because we do not want the ink from the pen to dissolve in the solvent and interfere with the result.
Materials needed
- Chromatography paper
- Pencil
- Sample (mixture or substance)
- Suitable solvent (e.g. ethanol or water)
- Beaker or chromatography tank
Procedure
- Draw a straight line near the bottom of the chromatography paper using a pencil (never use pen as the ink may dissolve and interfere with results).
- Place a small drop of the sample on the pencil line and allow it to dry.
- Pour a suitable solvent into a beaker or chromatography tank.
There are different solvents available, such as ethanol. The solvent will then get soaked up by the paper, and it will travel up the paper and dissolve the sample. The sample will travel up the chromatography paper together with the solvent. Different components in the sample will move at different speeds, and so become separated. - Place the chromatography paper into the solvent so that the pencil line is above the solvent level.
- Cover the beaker/tank and leave it undisturbed until the solvent has travelled near the top of the paper.
- Remove the paper and mark the solvent front immediately. Allow it to dry.
Interpretation of results
At the end of the experiment, you would achieve a chromatography paper with the separated components. We call this the chromatogram.
- If the original sample was a mixture, you will see multiple spots.
- If the original sample was a pure substance, only one spot should appear.
Calculating the Rf Value
So what are these components present in the sample? We can determine that by calculating the Rf value, which is, the distance travelled by the substance divided by the distance travelled by the solvent. Different substances in a sample will have its own Rf values. We can then identify the substance by comparing its Rf value with Rf value of a known substance.
Questions
1. Will the Rf value be influenced by the duration of the chromatography?
No. If we give it more time for the solvent to travel up the chromatography paper, the distance travelled by the component will increase too. Hence, Rf value remains the same. This is so if the conditions of the chromatography are the same – the same solvent is used and the temperature of the room is the same.
Important Notes About Rf Value
- The Rf value is not affected by how long the chromatography runs, as long as the solvent and temperature remain the same.
- Rf values are only reliable when the experiment is carried out under identical conditions (same solvent, same temperature, same type of paper).
Summary
Paper chromatography is used to separate and identify components in a mixture.
- Use pencil to draw the baseline.
- The solvent carries the components up the paper at different speeds.
- A pure substance gives one spot; a mixture gives multiple spots.
- The Rf value can be calculated to help identify substances.
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