Paper Chromatography

One of the first-ever practical experiments you probably carried out in chemistry at school is a simple example of paper chromatography: separating a coloured ink. You draw a pencil line across the bottom of a sheet of paper, place a dot of ink on the line, and place the paper upright in a beaker of solvent. The solvent travels up the paper, carrying the ink with it, and the ink separates out into all of its different coloured components.

This is a simple but effective way of separating mixtures. You'll learn exactly how it works here.

What determines how quickly a substance travels up the paper? This is all to do with relative affinity.

Components with a greater affinity to the mobile phase will move faster up the plate than those with a greater affinity to the stationary phase. They are more soluble in the solvent and travel a greater distance in a given time period.

Let's look more closely at the mobile and stationary phases in paper chromatography to work out why some components have a greater affinity to one or the other.

Remember how the stationary phase is a matrix of cellulose and water? This means that it is polar and can experience permanent dipole-dipole forces. The water molecules can also form hydrogen bonds with suitable substances. In contrast, the mobile phase in paper chromatography is typically nonpolar. It can only form weak van der Waals forces. From this we can deduce the following:

• If any of the components within the sample mixture are polar, or contain chemical groups that can form hydrogen bonds, they will bond more strongly to the polar cellulose-water structure than to the nonpolar solvent. This means that they have a greater affinity to the stationary phase and will travel more slowly up the paper. These components give lower Rf values.
• However, nonpolar components will bond more strongly to the nonpolar solvent than to the polar paper. They are more soluble. They, therefore, have a greater affinity to the mobile phase and will travel more quickly up the paper. These components give higher Rf values.

Method for paper chromatography

That's enough of the technical details - how do you actually carry out paper chromatography?

1. Draw a pencil line along the bottom of a sheet of chromatography paper.
2. Place a spot of the mixture you want to analyse on the middle of the line.
3. Place the sheet of paper in a beaker filled with a shallow layer of your solvent of choice. Make sure the level of the solvent is below the pencil line.
4. Place a lid on the beaker and leave the solvent to travel up the paper, carrying the components of the mixture with it.
5. When the solvent level almost reaches the top of the paper, remove the paper from the beaker and mark the position of the solvent with another pencil mark. Your chromatogram is now ready to be analysed.

At the end of the experiment, the setup should look a little something like this:

The end of a paper chromatography experiment. Anna Brewer, StudySmarter Originals

The dot of ink has travelled up the paper and separated into several spots. Each spot represents a different component found in the original mixture. Each component moves up the paper at a different speed, depending on its relative affinity to the stationary phase and its relative affinity to the mobile phase.

We can now use these results to calculate Rf values for each spot.

Calculating Rf values

Earlier in the article, we mentioned Rf values. These are values that show the ratio between the distance travelled by each component and the total distance travelled by the solvent.

Let's look at calculating Rf values for the chromatogram we showed above.

1. Measure the distance between the base pencil line and one of the coloured spots on the chromatogram. This is the distance travelled by the component that produced that spot.
2. Measure the distance between the base pencil line and the pencil line you used to mark the solvent front. This is the distance travelled by the solvent.
3. Divide the distance travelled by the component by the distance travelled by the solvent. This gives you your Rf value.
4. Repeat for all of the coloured spots.

$\mathrm{Rf}\mathrm{value}=\frac{\mathrm{distance}\mathrm{travelled}\mathrm{by}\mathrm{solute}}{\mathrm{distance}\mathrm{travelled}\mathrm{by}\mathrm{solvent}}$

Calculating Rf values for a paper chromatogram. Note that the solvent has evaporated - this is why we drew a pencil line to marks its position. Anna Brewer, StudySmarter Originals

Let's calculate the Rf value for the green spot. The green spot has travelled 3.0 cm whilst the solvent front has travelled 9.8 cm. Divide 3.0 by 9.8 to get your answer:

$3.0÷9.8=0.306$

We tend to round Rf values to two decimal places. this gives us an overall answer of 0.31

Analysing chromatograms

Advantages of paper chromatography

Paper chromatography is a relatively simple technique. However, it does have its advantages.

• It is cheap and easy to run, with a simpler setup than other types of chromatography.
• It only uses small amounts of the sample mixture.
• It can analyse organic and inorganic compounds.

However, compared to other chromatography techniques such as gas chromatography and thin-layer chromatography, paper chromatography is less accurate. This is one of its main disadvantages.

Uses of paper chromatography

Paper chromatography is more than just a way of making pretty coloured patterns. It has a variety of different uses, many of which it shares with other chromatography techniques. These include:

• Separating mixtures. For example, in the early 20th century, paper chromatography was widely used to separate plant extracts.
• Obtaining pure compounds and removing impurities.
• Analysing drugs.
• Testing wastewater.