Getting Connected: What do the Students Know?
Ask Students:
- Is the ink made up of several dyes or just one?
- How do the colors of the dyes that are used result in the color of the ink which we observe?
Sharing the Wealth of Knowledge
The Big Idea
Ask Students:
Did you know that chromatography is a technique used to analyze and purify chemical mixtures?
Explain To Students:
Besides being widely used in the chemical and pharmaceutical industries, chromatography is also applied to environmental monitoring, food processing, and forensic science applications. The separation of the mixture occurs because the components are not equally soluble in a solvent. The mixture that is to be separated is placed on a special medium. A solvent is allowed to travel across the medium. As the solvent moves across the medium, the components of the mixture separate. The components of the mixture that travel the farthest are the ones that are most soluble in the solvent and least attracted to the medium. The components of the mixture that travel the shortest distance are the ones that are the least soluble in the solvent and most attracted to the medium.
One familiar mixture is ink. Ink consists of many pigments that may be either soluble or insoluble in water. Ink that is insoluble in water is referred to as indelible ink. The separation of water-soluble ink (a polar mixture) is accomplished using water because it is a polar solvent. Indelible ink is a mixture which is significantly less polar than water and must be separated with a relatively nonpolar solvent. Rubbing alcohol is a mixture of 70 to 91% isopropyl alcohol (2-propanol) in water. This commonly available solvent is less polar than water.
The reason a fairly polar solvent such as 2-propanol is able to move the nonpolar indelible ink is that the substrate (cotton) is also fairly polar. Cotton is primarily cellulose, a polymer of sugar, and as such is covered with hydroxy groups (-OH), which makes it close in polarity to 2-propanol. When the indelible ink is given a choice of binding to the cotton or dissolving in the solvent, it doesn't have a strong preference either way and so is moved along by the solvent. When indelible ink is placed on a medium and the rubbing alcohol is allowed to travel across the medium, the ink will separate into its various components and produce a colorful chromatogram.
Demonstration 1: Paper Chromatography
Materials:
- Coffee filter
- Crayola washable markers
- Black Sharpie
- 25-ml 50% isopropyl alcohol
- Water
- Plastic pipet
- 50-ml plastic beakers
Procedures:
Tell Students: Today, I will demonstrate how to use paper chromatography to investigate whether all brands of black ink consist of the same components. We will be using water as the solvent for water-soluble markers and rubbing alcohol as the solvent with indelible markers.
Cut the coffee filter into 1 inch by 3 inch strips.You will need at least five strips.Having extras is a good idea.
Pour about ¼ inch of water into the plastic cup provided.
Conduct a blank chromatography test: take a filter paper strip and attach it to a pencil with a piece of masking tape.Suspend the strip into one of the plastic cups so that it just touches the water.
Observe what happens to the strip and discuss why the water appears to travel up the piece of paper.
Activity 1: T-Shirt Chromatography
Materials:
- Goggles
- White cotton T-shirt (prewashed and dried)
- 4 or 5 permanent felt-tip markers (assorted colors)
- 100 mL of 50% rubbing alcohol solution
- 250 mL beaker (for the rubbing alcohol)
- Plastic pipet
- Wide-mouth container (large can or jar)
SAFETY NOTE: Goggles at all times during the experiment. Rubbing alcohol is flammable. Make sure that there are no flames or sparks near the activity area. This activity uses isopropyl alcohol, which is harmful if put in eyes or swallowed. Please perform this activity with caution.
Procedures:
Tell Students:
Put on your goggles and apron.
Obtain 100 mL of rubbing alcohol that has been poured in a 250 mL beaker.
Place the part of the T-shirt that you wish to decorate first over the open top of the wide-mouth container. Pull the cloth taut and fasten it in place by stretching the rubber band around the can.
Use one of the markers to make a small circle of dots on the fabric. The circle should be about the size of a quarter and consist of about five dots.
Make dots of a different color in between the first dots to make a two-color design.
Fill the dropper with rubbing alcohol.
Drip the alcohol into the center of the ink-dot circle. Continue dripping the alcohol until it has spread to the edge of the container.
Allow the wet part of the T-shirt to dry.
On a new section of the T-shirt, repeat steps 3 - 8 above. Experiment by making the dots different sizes, the circles different sizes, or by using different colors of ink.
Allow the T-shirt to dry completely when you are finished making patterns.
Clean up your work area. Dispose of leftover materials according to your teacher's directions. Wash your hands.
At home, iron the T-shirt to help set the patterns. To help minimize fading, wash the T-shirt in cold water before wearing.
- Have children dump trash in trash cans.
- Have children replace non-trash items into kits. Wash as needed between classes.
