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Monday, January 9, 2012

Paper Chromatography



Paper Chromatography labs are great for both middle and high school students.

What is paper chromatography?  It is a method of separating the components of a mixture.  During the procedure, the mixture will be separated into its individual components, allowing the individual parts to be identified.  Chromatography is used to separate and identify all sorts of substances in police work. Drugs ranging from narcotics to aspirin can be identified in urine and blood samples, often with the aid of chromatography.


How does it work?  Paper chromatography is a technique that involves placing a small dot or line of sample solution onto a strip of chromatography paper. The paper is then placed in a jar containing a shallow layer of solvent and sealed. As the solvent rises through the paper, it meets the sample mixture which starts to travel up the paper with the solvent. 


Several factors explain why the different parts of the mixture separate out as they do:

  1. Solubility:  If the components of the mixture are soluble in the solvent being used, the mixture will be carried up the paper strip as the solvent travels.  If the material is soluble, the mixture will dissolve as the solvent front moves through it.  If the material is a mixture of substances, some of these substances will likely be more or less soluble than others.  The more soluble substances will move faster and to a greater distance than those that are less soluble.
  2. Molecular Weight:  Those substances of lighter molecular weight will move higher up the paper than those substances having a higher molecular weight.
  3. The chromatography paper is made of cellulose, a polar substance, and the compounds within the mixture travel farther if they are non-polar. More polar substances bond with the cellulose paper more quickly, and therefore do not travel as far.
How can this be used in middle and high school experiments?  One of the most popular lab activities to use in middle schools is to separate mixtures of ink. The ink in a black Sharpie marker is actually a mixture of several different colors.  The ink is simply applied to the chromatograph paper and the tip of the paper is placed in a solvent.  Many common inks are water soluble and spread apart into the component dyes using water as a solvent. If the ink you are testing does not spread out using water, it may be “permanent” ink. In such cases, you will have to use a different solvent such as rubbing alcohol.  Below are some photos of one of our recent labs in a physical science class.





What materials are needed to do this?  This experiment can be done with very simple materials.  
  1. Chromatography paper
  2. Some sort of container (beaker or test tube) 
  3. Solvent
  4. A mixture that can be separated
What solvent should I use?  The solvent used depends upon the solubility of the mixture you are trying to separate.  I have had success with water, isopropyl alcohol, ethyl alcohol, acetone, and petroleum ether.  Before doing the lab with your students, experiment with various solvents to see which works best for you.

What other mixtures can be separated in this way?  My favorite use of paper chromatography is to separate the pigments found in leaves.  A green pigmented leaf can easily be separated to show that it contains chlorophyll-a, chlorophyll-b, xanthophyll and carotene.  Since our school is surrounded by trees and shrubs of all types, I have students run several chromatograms and compare the results.  During the winter, using fresh spinach from the grocery store will yield excellent results.

The pictures seen below show the separation of leaf pigments completed by a couple of my students:


I currently have two labs in my TeachersPayTeachers.com store that involve paper chromatography:
























3 comments:

Anonymous said...

Hi, why is paper chromatography used in schools and not over methods(TLC,Gas,Column)? Thank you for your help.

Caitlin Smith said...

As it is simpler and very visual! It's a good precursor to TLC, Gas, Column at a higher level :)

Anonymous said...

I'm doing paper chromatography for a science fair project, thanks for the help!! (Going on my bibliography)