Paper chromatography
In this topic you will:
- use paper chromatography to separate dissolved substances
- interpret chromatograms
- use scientific language accurately.
Key words
- chromatogram
- paper chromatography
- permanent
- solvent front
Colours in ink
Black coloured ink looks as if it is just one colour – black. In fact, it is a mixture of different coloured inks. You can separate out the coloured inks by using a technique called paper chromatography.
Special paper, a bit like filter paper, is used.
Look at the photograph (right). A small drop of black ink has been placed on the paper. The water in the beaker has soaked up into the paper. As the water moves up the paper, the different coloured inks that make black ink separate out.
The resulting image on the paper is called a chromatogram.
The coloured inks separate because the water dissolves them. Water is the solvent. As the water moves up the paper, it carries the ink particles with it. The different kinds of ink particles are carried different distances before they are left behind on the paper. This is because not all the ink particles have the same solubility. The more soluble the ink, the further its particles are carried.
In the photograph below, you can see the different coloured inks that make up the ink in three different coloured pens – green, black, and red.
Some ink is not soluble in water, such as the ink in permanent marker pens. To separate out the colours in these inks, you would need to use a different solvent, such as alcohol.
Think Like a Scientist
Separating the Colours in Ink
You will need: chromatography paper (or filter paper) • beaker • water • pencil and ruler • glass rod or wooden spill • ink or a pen • various other inks and/or food dyes • pipette (if using liquid ink)
Step 1. Take a strip of chromatography paper. Draw a pencil line about 1 cm from the end.
Step 2. Place a spot of ink on the pencil line. The spot should be as small as possible.
Step 3. Dry the spot and then add a little more ink.
Step 4. Place about 2 cm depth of water in a beaker.
Step 5. Hang the paper over a glass rod, pencil or wooden spill so that the end with the ink spot is just in the water. Make sure the ink spot stays above the level of the water.
Step 6. Watch what happens as the water moves up the strip of paper.
Step 7. Remove the strip of paper before the water reaches the top. You need to be careful as the wet paper can tear easily.
Step 8. Allow the strip to dry and then stick it in your book. This is your chromatogram.
You can try this with all sorts of coloured liquids. Different inks and food dyes, especially from sweets or fruit syrup, are very good. You could also try this with permanent marker pens that have ink that is not soluble in water.
Questions
Show Answer
Pencil is insoluble in water, so it does not interfere with the chromatography results.
Show Answer
If the ink is submerged, it will dissolve directly into the water and not separate properly along the paper.
Show Answer
If the water reaches the top, it may distort or wash away the separated dyes, ruining the chromatogram.
Show Answer
The ink separated into several colored spots at different heights, showing that the ink is made of a mixture of dyes with different solubilities.
Important Concept
Chromatography is a method used to separate substances based on their solubility in a solvent. Substances that dissolve more readily travel further up the paper.
When comparing different substances, a scientist may use a large piece of chromatography paper and place spots of different items alongside each other. The scientist will allow the solvent to move up through all the samples at the same time. To do this, the paper needs to be placed in a large chromatography tank.
The chromatogram shown here has been produced using this technique. It shows the colours in a number of different felt tip pens.
Scientists use chromatography to study the dyes used in food. Some food dyes contain only one substance; they are a pure substance. Other dyes contain a mixture of substances.
It is important to know exactly what is being used when our food is processed – we need to know if any substances could be a health risk; for example, they could be toxic or cause allergic reactions.
Common Mistake
Not all food dyes are pure substances. Assuming they are can lead to incorrect conclusions during testing. Chromatography helps reveal whether a dye contains multiple components.
Questions
Show Answer
Blue and yellow, because each shows a single spot.
Show Answer
Brown, because it separates into multiple coloured spots.
Show Answer
Blue, because it travelled the furthest up the paper.
Show Answer
Red (in the brown dye), because it travelled the shortest distance.
Public health scientists may also use chromatography to check that the colourings being used in products such as hair dye or the ink in pens are not harmful.
To do this they compare chromatograms taken from a solution of the food, hair dye or ink with chromatograms taken from the colourings that are permitted.
Show Answer
Yes — spot C in Sunny Red does not match any permitted dye.
Show Answer
To confirm the result and check for consistency or error in the first test.
Show Answer
Substance D, because it travelled the furthest up the paper.
Think Like a Scientist
Separating the Colours in Ink
You will need: chromatography paper (or filter paper) • beaker • water • pencil and ruler • glass rod or wooden spill • ink or a pen • various other inks and/or food dyes • pipette (if using liquid ink)
Step 1. Take a strip of chromatography paper. Draw a pencil line about 1 cm from the end.
Step 2. Place a spot of ink on the pencil line. The spot should be as small as possible.
Step 3. Dry the spot and then add a little more ink.
Step 4. Place about 2 cm depth of water in a beaker.
Step 5. Hang the paper over a glass rod, pencil or wooden spill so that the end with the ink spot is just in the water. Make sure the ink spot stays above the level of the water.
Step 6. Watch what happens as the water moves up the strip of paper.
Step 7. Remove the strip of paper before the water reaches the top. You need to be careful as the wet paper can tear easily.
Step 8. Allow the strip to dry and then stick it in your book. This is your chromatogram.
You can try this with all sorts of coloured liquids. Different inks and food dyes, especially from sweets or fruit syrup, are very good. You could also try this with permanent marker pens that have ink that is not soluble in water.
Questions
Show Answer
Pencil is insoluble in water, so it does not interfere with the chromatography results.
Show Answer
If the ink is submerged, it will dissolve directly into the water and not separate properly along the paper.
Show Answer
If the water reaches the top, it may distort or wash away the separated dyes, ruining the chromatogram.
Show Answer
The ink separated into several colored spots at different heights, showing that the ink is made of a mixture of dyes with different solubilities.