Select your language

Suggested languages for you:
Log In Start studying!
StudySmarter - The all-in-one study app.
4.8 • +11k Ratings
More than 3 Million Downloads
Free
|
|

All-in-one learning app

  • Flashcards
  • NotesNotes
  • ExplanationsExplanations
  • Study Planner
  • Textbook solutions
Start studying

Paper Chromatography

Save Save
Print Print
Edit Edit
Sign up to use all features for free. Sign up now
Chemistry

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.

Paper chromatography is an analytical technique used to separate and analyse mixtures of soluble substances. It is a type of chromatography.

  • This article is all about the type of chromatography known as paper chromatography.
  • We will start by looking at the principles of chromatography before applying them specifically to paper chromatography.
  • We'll then run through the method, and you'll be able to calculate Rf values using an example to guide you through the process.
  • After that, we'll explore how you read chromatograms and interpret the results.
  • To finish, we'll look at the advantages and uses of paper chromatography.

Principles of paper chromatography

All types of chromatography follow the same basic principles.

  1. We use a solvent, called the mobile phase, to dissolve a sample of a soluble mixture. The solvent carries the mixture up a solid called the stationary phase.
  2. Some components of the mixture are carried up the solid by the solvent more quickly than others. We say that components that travel faster have a greater affinity to the mobile phase. This separates the mixture out into its component parts and produces a chromatogram.
  3. We then use the distances travelled by the components to work out Rf values. These help us identify the component.

If you haven't already done so, we'd recommend looking at Chromatography for a more detailed explanation of these ideas.

Let's now look at some of those ideas in terms of paper chromatography.

Stationary phase

The stationary phase is a static solid, liquid, or gel. In chromatography, the solvent carries the soluble mixture through the stationary phase.

In paper chromatography, the stationary phase is - as the name suggests - paper. However, it's a bit more complicated than that. Paper is made of cellulose, a polymer of glucose. Cellulose fibres bind to water vapour in the air, alongside any water that was around when the paper was made. You can actually think of the stationary phase as being a complex matrix made of water and paper, not just the paper itself.

Mobile phase

The mobile phase is the solvent used to carry the mixture analysed through the stationary phase.

The stationary phase - the paper - is placed in a solvent. This is our mobile phase. In paper chromatography, we typically use a nonpolar solvent. The solvent travels up the paper, carrying the different components within the mixture with it.

Retention factors

The components of the sample mixture travel up the stationary phase at different speeds. This means that in a given time period, the different components will all travel different distances. We'll look at why this is in just a second.

You'll see these components as spots on the chromatogram, which is the name for your paper once the experiment has finished. The ratio between the distance travelled by each component and the total distance travelled by the solvent gives us Rf values.

To calculate an Rf value, divide the distance travelled by the component - in other words, the distance from the starting pencil line to the coloured spot -by the distance travelled by the solvent.

Rf values are important because each component has a fixed Rf value under a specific set of conditions. If you repeated the experiment again, keeping the mobile phase, the stationary phase, and the temperature exactly the same, you would get the same Rf value for the same component. We can then compare Rf values to ones in a database to identify the components in the mixture.

Relative affinity

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

In chromatography, relative affinity describes how well a component is attracted to either the stationary or mobile phase. It determines how quickly the component moves through the stationary phase.

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.

Check out Intermolecular Forces for more on permanent dipole-dipole forces, hydrogen bonds, and van der Waals forces.

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.

What's the importance of, say, drawing the line in pencil? Here are some of the reasons behind particular steps in the method.

  • We draw the line in pencil because pencil is insoluble. This prevents it being carried up the paper with the mobile phase. If we were to use ink, for example, the ink would also dissolve in the solvent and travel up the paper, producing confusing results.
  • The solvent level must be below the spot of your mixture to prevent the spot fully dissolving in the solvent and being washed away.
  • You should also make sure that you handle the paper by its edges, to avoid getting fingerprints on it. This could dirty the paper and give misleading results.
  • We use a lid to keep the environment saturated with solvent and to prevent the solvent from evaporating. You could also line the sides of the beaker with filter paper soaked in the solvent to saturate it even more.

A typical setup for paper chromatography. Anna Brewer, StudySmarter Originals

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

Paper Chromatography setup end results StudySmarterThe 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.

Paper Chromatography Rf values StudySmarterCalculating 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:

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

Remember that the distance travelled by a substance all depends on its relative affinities to each of the stages. A substance with a greater affinity to the stationary phase will travel more slowly up the paper and will travel less far in a given time period. This means that it will have a lower Rf value. In contrast, a substance with a greater affinity to the mobile phase will travel more quickly up the paper and will have a higher Rf value.

Analysing chromatograms

Chromatograms show us two things.

  1. The number of different components in our starting mixture.
  2. The identity of each component in our starting mixture.

Number of different components

Remember, each spot represents a different component found in the original solute mixture. In our example above, we have three different spots on our chromatogram. We, therefore, know that we have three different substances present.

Identity of each component

There are two ways of identifying substances in a chromatogram. Firstly, when setting up the experiment, you could also place a small dot of a known substance, such as a particular amino acid or organic molecule, on the pencil line to the side of your solute dot. This known substance acts as a reference molecule. It will also be carried up the plate by the solvent, producing a visible spot. If any of the spots from your mixture match the known substance's spot, you know that substance is present in your mixture.

Sound a little confusing? Here's what it looks like in practice.

Paper Chromatography identifying components StudySmarterIdentifying components in paper chromatography. Anna Brewer, StudySmarter Originals


The red spot on the left is from a known substance. One of the spots produced by our mixture matches it exactly. We can therefore deduce that the mixture contains this particular substance.

But there is another way of identifying components. We also mentioned earlier that, provided you keep the conditions the same, a particular component will always produce the same Rf value. Let's say that a particular component has an Rf value of 0.4. If we look in a database, we should be able to find a substance that also produces an Rf value of 0.4 under the same conditions - the same mobile phase, stationary phase, and temperature. These two substances are one and the same.

Two-way paper chromatography uses two different solvents, one after the other, on the same sample. It is useful for separating out components with similar Rf values.

To carry out this technique, place a small spot of your mixture at one edge of the base pencil line. Place the paper in a beaker with your first solvent, removing it when the solvent front has almost reached the top of the paper. Mark the position of this first solvent front.

Your paper should look a little something like the diagram below.

Paper Chromatography two-way chromatography StudySmarterTwo-way chromatography. Anna Brewer, StudySmarter Originals

You'll notice that two components produce one merged spot - they haven't clearly separated. This is because they have similar relative affinities to the stationary and mobile phases and so have travelled at similar speeds up the paper.

Now, rotate your paper by 90° so that the separated spots now lie along the bottom of the paper. Choose a different solvent and repeat the experiment again. It is very unlikely that the two substances that produced the merged spot will also have similar affinities to the stationary and mobile phases in this solvent. Therefore, they will travel at different speeds up the paper and separate out into clear, distinct spots.

Paper Chromatography two-way chromatography StudySmarterTwo-way chromatography. Anna Brewer, StudySmarter Originals

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.

Paper Chromatography - Key takeaways

  • Paper chromatography is an analytical technique used to separate and analyse mixtures of soluble substances.
  • In paper chromatography, the stationary phase is a sheet of paper and the mobile phase is a solvent.
  • You can identify components in paper chromatography by calculating their Rf values and comparing them to those in a database.
  • Two-way chromatography is a variation of paper chromatography that uses two different solvents to separate out components with similar Rf values.
  • Paper chromatography is cheap, simple, and uses small sample sizes.

Paper Chromatography

Paper chromatography is an analytical technique used to separate and analyse mixtures of soluble substances.

In paper chromatography, a sheet of paper known as the stationary phase is placed in a solvent, known as the mobile phase. A small spot of a soluble mixture is placed on the paper. The solvent carries the mixture up the paper. Different components of the mixture have different relative affinities to the stationary and mobile phases and so travel up the paper at different speeds. This separates the components out.

Paper chromatography is used for separating mixtures, obtaining pure compounds, and analysing drugs.

Different components within a mixture have different affinities to the stationary phase - the paper, and the mobile phase - the solvent. This means that they travel up the paper at different speeds. Some will travel much further than others in a given time period. This separates the components.

An example of paper chromatography is separating the different dyes within an ink.

Final Paper Chromatography Quiz

Question

What is the mobile phase in paper chromatography?

Show answer

Answer

A solvent.

Show question

Question

What is the stationary phase in paper chromatography?

Show answer

Answer

Chromatography paper.

Show question

Question

What do you use to draw the base line in paper chromatography?

Show answer

Answer

Pencil.

Show question

Question

In paper chromatography, the starting solvent level should be _____.

Show answer

Answer

Below the pencil line containing the spot of mixture.

Show question

Question

In paper chromatography, components that travel faster up the paper have a greater affinity to the ______.

Show answer

Answer

Mobile phase.

Show question

Question

In paper chromatography, more soluble components have a _____ affinity to the mobile phase.

Show answer

Answer

Stronger.

Show question

Question

What are the units for Rf values?

Show answer

Answer

cm

Show question

Question

What are the advantages of paper chromatography?

Show answer

Answer

  • It is fast.
  • It is cheap.
  • It is simple.
  • It uses small amounts of the mixture.
  • It can analyse organic and inorganic compounds.

Show question

Question

What are the uses of paper chromatography?

Show answer

Answer

  • Separating mixtures.
  • Obtaining pure compounds.
  • Removing impurities.
  • Analysing drugs.

Show question

60%

of the users don't pass the Paper Chromatography quiz! Will you pass the quiz?

Start Quiz

Discover the right content for your subjects

No need to cheat if you have everything you need to succeed! Packed into one app!

Study Plan

Be perfectly prepared on time with an individual plan.

Quizzes

Test your knowledge with gamified quizzes.

Flashcards

Create and find flashcards in record time.

Notes

Create beautiful notes faster than ever before.

Study Sets

Have all your study materials in one place.

Documents

Upload unlimited documents and save them online.

Study Analytics

Identify your study strength and weaknesses.

Weekly Goals

Set individual study goals and earn points reaching them.

Smart Reminders

Stop procrastinating with our study reminders.

Rewards

Earn points, unlock badges and level up while studying.

Magic Marker

Create flashcards in notes completely automatically.

Smart Formatting

Create the most beautiful study materials using our templates.

Sign up to highlight and take notes. It’s 100% free.