Paper Chromatography: Basic Version

Objective

The objective of this project is to use paper chromatography to analyze ink components in permanent black markers.

Introduction

Matter makes up everything in the universe. Our body, the stars, computers, and coffee mugs are all made of matter. There are three different types of matter: solid, liquid, and gas. A solid is something that is normally hard (your bones, the floor under your feet, etc.), but it can also be powdery, like sugar or flour. Solids are substances that are rigid and have definite shapes. Liquids flow and assume the shape of their container; they are also difficult to compress (a powder can take the same shape as its container, but it is a collection of solids that are very small). Examples of liquids are milk, orange juice, water, and vegetable oil. Gases are around you all the time, but you may not be able to see them. The air we breathe is made up of a mixture of gases. The steam from boiling water is water's gaseous form. Gases can occupy all the parts of a container (they expand to fill their containers), and they are easily compressed.

Matter is often a mixture of different substances. A heterogeneous mixture is when the mixture is made up of parts that are dissimilar (sand is a heterogeneous mixture). Homogeneous mixtures (also called solutions) are uniform in structure (milk is a homogeneous mixture). A sugar cube floating in water is a heterogeneous mixture, whereas sugar dissolved in water is a homogeneous mixture. You will determine whether the ink contained in a marker is a heterogeneous or homogeneous mixture, or just one compound.

In a mixture, the substance dissolved in another substance is called the solute. The substance doing the dissolving is called the solvent. If you dissolve sugar in water, the sugar is the solute and the water is the solvent.

For this project, you will be making a small spot with an ink marker onto a strip of paper. The bottom of this strip will then be placed in a dish of water, and the water will soak up into the paper.

The water (solvent) is the mobile phase of the chromatography system, whereas the paper is the stationary phase. These two phases are the basic principles of chromatography. Chromatography works by something called capillary action. The attraction of the water to the paper (adhesion force) is larger than the attraction of the water to itself (cohesion force), hence the water moves up the paper. The ink will also be attracted to the paper, to itself, and to the water differently, and thus a different component will move a different distance depending upon the strength of attraction to each of these objects. As an analogy, let's pretend you are at a family reunion. You enjoy giving people hugs and talking with your relatives, but your cousin does not. As you make your way to the door to leave, you give a hug to every one of your relatives, and your cousin just says "bye." So, your cousin will make it to the door more quickly than you will. You are more attracted to your relatives, just as some chemical samples may be more attracted to the paper than the solvent, and thus will not move up the solid phase as quickly. Your cousin is more attracted to the idea of leaving, which is like the solvent (the mobile phase).

To measure how far each component travels, we calculate the retention factor (R_f value) of the sample. The R_f value is the ratio between how far the component travels and the distance the solvent travels from a common starting point (the origin). If one of the sample components moves 2.5 cm up the paper and the solvent moves 5.0 cm, then the R_f value is 0.5. You can use R_f values to identify different components as long as the solvent, temperature, pH, and type of paper remain the same. In the image below, the light blue shading represents the solvent and the dark blue spot is the chemical sample.

When measuring the distance the sample traveled, you should measure from the origin (where the middle of the spot originally was) and then to the center of the spot in its new location.

To calculate the R_f value, we use the equation:

In our example, this would be:

Note that an R_f value has no units because the units of distance cancel.

Chromatography is used in many different industries and labs. The police and other investigators use chromatography to identify clues at a crime scene like blood, ink, or drugs. More accurate chromatography in combination with expensive equipment is used to make sure a food company's processes are working correctly and they are creating the right product. This type of chromatography works the same way as regular chromatography, but a scanner system in conjunction with a computer can be used to identify the different chemicals and their amounts. Chemists use chromatography in labs to track the progress of a reaction. By looking at the sample spots on the chromatography plate, they can easily find out when the products start to form and when the reactants have been used up (i.e., when the reaction is complete). Chemists and biologists also use chromatography to identify the compounds present in a sample, such as plants.

Terms, Concepts and Questions to Start Background Research

• adhesion, cohesion forces
• capillary action
• stationary phase, mobile phase
• hydrophilic, hydrophobic
• R_f value
• paper chromatography
• solvent
• solution

Questions

• Why do different compounds travel different distances on the piece of paper?
• How is an R_f value useful?
• What is chromatography used for?

Bibliography

• Basic chemistry concepts:
  http://www.chem4kids.com
• Overview of liquids and capillary action:
  http://encarta.msn.com/encnet/refpages/RefArticle.aspx?refid=761571486&sec=18#s18
• Chromatography:
  http://www.doggedresearch.com/chromo/chromatography.htm

Materials and Equipment

• water
• at least 15 identically sized strips of paper (5 for each pen)
  Note: chromatography paper or laboratory filter paper is preferable, but you can use a paper towel. The problem with paper towels is that they may be too absorptive and smear the ink. For more information on which papers work and which don't, see: Papers and Solvents for Paper Chromatography
• ruler
• pencils
• at least three different types of black markers (including one permanent marker), or at least three different colors of marker (including one permanent marker)
• a wide-mouth jar for the solvent

Experimental Procedure

Note: To make sure you can compare your results, as many of your materials as possible should remain constant. This means that the temperature, type of water used, size of paper strips, where the ink is placed onto the paper etc. should remain the same throughout the experiment.

1. Cut paper strips about one by four inches in area (they must all be the same size).
2. Take one of the paper strips and use a ruler and pencil to draw a line across it horizontally two cm from the bottom. This is the origin line (see illustration, below).
   
   
3. Pour a small amount of water into your glass (there should be barely enough for the paper strip to hang inside of the jar and just touch the water).
4. Using one of the markers, place a small dot of ink onto the line (see illustration, above).
5. Use the pencil to label the strip, so that you know which marker it represents.
6. Tape the paper to a pencil and hang it into the jar of solvent so that the bottom edge is just barely touching (see illustration, below).
   
   
7. Let the water rise up the strip until it is almost at the top.
8. Remove the strip from the jar and mark how far the solvent rose with a pencil.
9. Analyze the ink component(s):
   Measure the distance the solvent and each ink component traveled from the starting position, then calculate the R_f value for each component (some of the ink components might not have moved at all!).
10. Repeat this experiment for each brand or color of marker five times.

Questions

• Did the different inks separate differently? By looking at the R_f values, can you tell if any of the ink components from the different markers are the same?
• If the ink components separated differently for each marker, why did this happen (think about the strength of attractions)?

Variations

• You could try using other solvents than water (or mixture of solvents) and see if the inks are separated differently (rubbing alcohol, vinegar, nail polish remover, and turpentine would be good to try). Which solvent separates the ink the best? Why? (Remember to account for attractions between chemicals.)
• After buying a black light, you could try to see if any more components are visible on the paper. Some chemicals are invisible under white light, but can be seen with a black light. If you decide to do this variation, you should analyze the paper strips the same day or the next day, otherwise the spots will fade! Why are some chemicals visible under one type of light but not another? Read more here:
  http://www.colorado.edu/physics/2000/waves_particles/index.html
  http://www.glenbrook.k12.il.us/gbssci/Phys/Class/light/lighttoc.html
• For more advanced chromatography projects, see:
  Paper Chromatography: Advanced Version 1
  Paper Chromatography: Advanced Version 2

• For more science project ideas in this area of science, see Chemistry Project Ideas

Credits

Author: Amber Hess
Editor: Andrew Olson, Science Buddies

Last edit date: 2008-04-18 22:00:00