What Is Paper Chromatography and How Does it Work?
What Is Chromatography?
Chromatography is a method for separating the parts of a mixture of either a gas or liquid solution containing different chemicals. For example, pen inks are often made up of different colours. The different bonding properties of each molecule type is exploited. Chromatography is used in both qualitative and quantitative analysis of both organic and inorganic samples. This technique uses two types of substances:
- Mobile phase: a gas or liquid that transports the solution being tested through the other substance (water, rubbing alcohol are examples).
- Stationary phase: the liquid or solid through which the tested substance is carried (coffee filter paper, paper towel are examples).
It is necessary for the different chemicals in the solution fo have different properties such as molecule size or a different ability to dissolve in a solvent. The stationary phase will absorb or slow down different components of the tested solution to different degrees creating layers as the components of the solution are separated. Chromatography was invented by the Russian botanist, Mikhail Tsvet. Chemists use this process to identify unknown substances by separating them into the different molecules that make them up.
Chromatography Using Water Soluble MarkersClick thumbnail to view full-size
How Does Paper Chromatography Work?
Chromatography can be demonstrated using a simple demonstration.
What you need:
- water soluble pens or markers of different brands or colours
- strips of paper towel
- rubbing alcohol
- nail polish remover
- straw or pencil or pen
What you do:
- Draw a line across 6 paper towel strips or coffee filter strips about 1 inch from the bottom.
- Tape the paper towel strip from the end opposite to the pen line onto the middle of the straw so the strip will hang when the straw is placed crosswise onto the cup.
- There should be enough water in the cup so that the paper towel strip touches the water but the pen line is NOT submerged.
- The strip should not touch the bottom of the cup.
- If using alcohol or nail polish remover as all or part of your solvent, place a cap of aluminum foil over the glass to keep the vapours in the glass. It will improve the results.
- Observe what happens.
- If the pen line remains unchanged in water, try with the same pen type on another strip of paper towel but substitute the alcohol or nail polish remover.
- Try mixing one half water and one half alcohol as your solvent and see how the colours separate.
What's Happening in Paper Chromatography?
Ink is a solution containing a number of different molecules. These different molecules have different characteristics such as size and solubility. Solubility is a molecule's ability to dissolve in a particular solvent such as alcohol, water or nail polish remover. Because of their different characteristics, each molecule travels at a different speed when pulled along the piece of paper towelling by the solvent. The lightest particles, which are not necessarily the lightest coloured particles, move more quickly and a greater distance than the heavier particles. Thus, all of the pigments that make up an ink sample are separated out.
Chromatograph of Black Ink
Applications of Paper Chromatography
Chromatography is used in chmistry in a number of applications:
- Unknown substances left at a crime scene can be identified by separating the molecules that make them up. Matching this unknown chromatogram to chromatograms of known substances can help identify the unknown substance providing a clue to the crime.
- The ink used in a note left at a crime scene can be identified through paper chromatography. The chromatogram showing the components of the ink used in the note can be compared to chromatograms of known inks hopefully identifying the type and brand of ink used providing a clue to the crime.
- It is used in the sequencing of DNA and RNA.
- Paper chromatography is used as a qualitative analytical chemistry technique for identifying and separating colored mixtures like pigments.
- It is used in scientific studies to identify unknown organic and inorganic compounds from a mixture.
- In order to obtain a pure sample each component of the separated mixture, the separated spots on the finished and dried chromatogram can be cut out and re-dissolved.
Uses of Paper Chromatography. Tutor Vista.com. 2010.
Chromatography. Asistm Forensic Investigations.
Cook, Trevor. Experiments with States of Matter. New York: Power Kids Press, 2009.