Study Chirality with a Homemade Polarimeter
AbstractSome molecules can be either left- or right-"handed." The left- and right-handed molecules have the same number and type of atoms, and their chemical structures look identical, but they are actually mirror images of each other. Many naturally occurring molecules have this property, called chirality. Chiral molecules can interact with polarized light in an interesting way—they rotate the plane of polarization. This chemistry science fair project describes how to make a homemade polarimeter that will allow you to investigate the ability of glucose, a chiral molecule, to rotate the plane of polarized light.
David Whyte, PhD, Science Buddies
The objective of this chemistry science fair project is to study the ability of a glucose solution to rotate polarized light.
Some molecules have "handedness." Like your hands, left- and right-handed molecules look similar, but are actually mirror images of each other. Many naturally occurring molecules have this property, called chirality. The handedness of a molecule can make a big difference in its chemistry. The molecule carvone, for example, smells like spearmint in its right-handed form, but is nearly odorless in its left-handed form. The receptors in your nose that bind carvone can distinguish between the different forms of the molecule. Handedness is also important in drugs; for instance, penicillin—an important and widely used antibiotic—is able to inhibit bacterial growth in its right-handed form, but is completely inactive in its left-handed form.
One of the interesting traits of left- and right-handed molecules is that they can rotate polarized light. If you have ever worn polarized sunglasses, you have "seen" polarized light. Polarized sunglasses do not simply block light, like colored glass does. They actually filter the light based on the light's angle of polarization. Imagine a light wave moving toward you from the Sun. If you could magnify it to the point that you could actually see it, you would notice that it is moving up and down within a plane. In other words, the light wave has a plane of polarization. The light from the Sun consists of many light rays, each with a different angle of polarization. If you were able to see many light rays, you would notice that they come in every possible angle of polarization. Because sunlight contains light with all possible angle of polarization, it is un-polarized. Polarized filters work by allowing through only light with certain angles of polarization. If you pass sunlight through a polarized filter that only allows light through that has a vertical plane of polarization, you will reduce the amount of light (just what you want in sunglasses) and the light that passes through is now vertically polarized. If you pass sunlight through a polarized filter that only allows light through that has a horizontal plane of polarization, you will have the exact same amount of light blocked as for the vertically polarized filter, but of course the light is now horizontally polarized. What happens if you pass sunlight through both filters? No light gets through, since the polarized light from the first filter is all blocked by the second filter.
The ability of a left- or right-handed molecule to change the angle of polarization can be detected by placing a solution containing the molecule between two polarized filters. The light beam's plane of polarization is rotated as it passes through the solution. Because the polarization angle light is no longer at right angles to the second filter, it is no longer totally blocked by the second filter. Solutions that rotate the plane of polarized light are said to be optically active. Chemists use the term chiral to refer to molecules whose structure can be left- or right-handed. Glucose is a chiral molecule, whereas water is not.
A solution of chiral molecules can consist of all left-handed molecules, all-right-handed molecules, or a mixture of both kinds. There are examples of drugs where one form is beneficial and the other form is actually toxic. In this case, the drug manufacturer must take careful steps during the drug's synthesis, or purification, to make sure the final product contains only the beneficial form.
Common table sugar, sucrose, is optically active: it rotates polarized light to the right. Sucrose is made up of two smaller sugar molecules, glucose and fructose. Both glucose and fructose are also optically active. Glucose, which is the kind of sugar found in corn syrup, rotates polarized light to the right. Fructose rotates polarized light to the left. Sucrose can be broken down into glucose and fructose by treating it with certain chemicals or enzymes. What happens to the plane of polarization when a solution of sucrose is converted into a solution of fructose and glucose? Will the resulting mixture still be optically active, or will the glucose and fructose cancel each other out? It turns out that the fructose has a somewhat larger polarizing effect, so the resulting solution is polarized to the left. The solution consisting of glucose and fructose is called invert sugar, referring to the fact that the direction of polarization has been changed (right for sucrose and left for invert sugar, which is a combination of glucose and fructose).
In this chemistry science fair project, you will make a homemade polarimeter. A polarimeter is a scientific instrument that precisely measures the angle of polarization and the brightness of light. Investigate the ability of glucose to rotate the plane of polarized light, using a flat-panel computer screen as the source of polarized light. The light from flat panels is polarized at a 45-degree angle. This angle of polarization was chosen so that you can still see the screen if you happen to be wearing polarized sunglasses. The second polarized filter will be polarized sunglasses. The test solution will be glucose, or corn syrup.
Terms and Concepts
- Polarized light
- Angle of polarization
- Plane of polarization
- Optical activity
- Angle of rotation
- Equation for specific angle of rotation
- Based on your research, what is the equation for the specific rotation of a chemical compound?
- How does the angle of rotation vary with concentration and path length? How about wavelength of the light?
- Two molecules that are identical, except for their handedness, are called enantiomers. What are some common drugs for which one enantiomer is active and the other one is inactive?
- Can the angle of polarization be used to determine the concentration of an optically active molecule?
- University of Michigan Space Grant Consortium. (n.d.). Polarized Light. Retrieved October 25, 2008.
- Exploratorium. (n.d.). Polarized Sunglasses. Retrieved October 25, 2008.
- Harvard University. (2008). Sugar Syrups. Retrieved October 25, 2008.
- The University of Texas at El Paso. (2007, October 12). Polarizers?. Retrieved October 25, 2008.
- National Institute of Standards and Technology Museum. (n.d.). Bates Saccharimeter: Instrument for measuring sucrose from its ability to rotate plane polarized light. Retrieved October 25, 2008.
Materials and Equipment
- Laptop computer, or flat-screen monitor
- Polarized sunglasses. A polarized filter made for your camera is preferable, but is more expensive.
- Small tin cans, such as 4-oz. fruit cans (4)
- Can opener
- Epoxy glue or caulk
- Small disposable stick to mix epoxy
- Disposable surface, such as a paper plate (1)
- Pane of glass, 8 x 10 inches
- Pane of glass, 5 x 7 inches
- Digital camera that will take movies
- A platform that rotates horizontally, also called a "lazy Susan"
- Large piece of cardboard, 3 x 3 feet
- 360-degree protractor
- Light corn syrup (glucose solution), such as Karo, available at most grocery stores
- Mixing bowls (4)
- Masking tape
- Permanent marker
- Liquid measuring cup
- Mixing spoon
- Tablespoon measure
- Clear tape
- Lab notebook
- Graph paper
Before you start the procedure, put on the polarized sunglasses and look at a laptop screen. Now rotate your head slightly. Do you see the change in brightness? If you place an optically active object or solution in between the polarized light (coming from the laptop screen) and the polarized filter (the polarized sunglasses), you can detect a change in the brightness, due to rotation of the plane of polarization.
Making the Solution Holders
- Use the tape to cover the edges of the 5- x 7-inch glass pane to avoid cuts or scratches. The glass pane provides an optically neutral base for the solutions. Avoid plastic, since it is optically active.
- Wash and dry the tin cans.
- Use a can opener to cut out the tops and bottoms of the cans.
Mix the epoxy cement on a disposable surface, such as a paper plate.
- Squeeze out equal amounts of epoxy and mix them with a small disposable stick.
Figure 1. Parts for making a 4-chamber solution holder: small tin cans, pane of glass, epoxy cement, and mixing stick for the epoxy.
- Spread the mixed epoxy onto the bottom rim of one of the cans.
Attach the can with the epoxy cement near a corner of the glass pane.
- Repeat with the other cans. The cans should all be touching, so don't place them too far toward the corners.
- Allow the epoxy to harden.
- Test the seal around the bottom of the can by pouring a little water into each one. You don't want them to leak onto the computer screen!
- If there is a leak, fill it in with fresh epoxy and retest the seal.
Setting Up the Workspace
- Open Microsoft Paint or PowerPoint, or some other graphics program. Create a large (roughly 8-inch diameter) circle and fill it with yellow color (or other color of your choice). This will give you a uniform background.
- Darken the room you are in. This will make it easier to see the contrast between light and dark.
- Attach the camera to the tripod. The camera should be pointing vertically downward.
Cover the lens of the camera with the polarized filter, such as the lens from a pair of polarized sunglasses.
- Exactly how you do this will depend on what type of filter and camera you have. Be creative!
Put the cardboard on the ground under the camera.
- The cardboard will be used to mark the angles at which the light is blocked in the test solutions.
- Put the lazy Susan on the cardboard under the camera. This will allow you to control the angle of the polarized light.
Put the laptop on the lazy Susan (rotating base), with the screen on the surface, facing upward.
- The polarized light from the screen will pass upward through the glass pane, the solution in the can, the polarized filter (sunglasses), and then into the camera. See Figure 2.
Figure 2. This is the experimental setup to view and record rotation of the light's plane of polarization through optically active solutions. You can observe the solutions without a camera by just wearing sunglasses. However, the camera attached to a tripod gives a stable point from which to measure the rotation of the plane of polarization. The camera also allows you to record the changing light levels as the platform rotates.
- Tape the edges of the 8- x 10-inch glass pane.
- Carefully place the 8- x 10-inch pane of glass onto the computer screen to protect it from getting scratched.
Making the Solutions
Make solutions of glucose in each of your four mixing bowls. Label your bowls with masking tape and a permanent marker so you know which solution is in each bowl. The glucose molecules will rotate the plane of polarization. The amount of rotation depends on two factors: the concentration of the glucose, and the length of the path of the light through the glucose. Higher concentration or a longer path will give a higher degree of rotation.
- Control in bowl #1: 1 cup of water
- 25 percent light corn syrup in bowl #2: 1/4 cup light corn syrup and 3/4 cup water
- 50 percent light corn syrup in bowl #3: 1/2 cup light corn syrup and 1/2 cup water
- 100 percent light corn syrup in bowl #4: 1 cup light corn syrup
Fill the cans with the following:
- Can #1: 3 tbsp. of water
- Can #2: 3 tbsp. of 25 percent light corn syrup
- Can #3: 3 tbsp. of 50 percent light corn syrup solution
- Can #4: 3 tbsp. of pure light corn syrup
- Place the 5- x 7-inch pane of glass, with the solutions, onto the 8- x 10-inch pane of glass above the computer screen.
- Look through the camera.
- Align the camera so that the solutions are directly underneath.
- Align the lazy Susan so that the axis of rotation is directly below the camera.
Testing the Solutions
You are now ready to test the solutions for optical activity. Look through the camera. Slowly rotate the solutions on the lazy Susan. As you rotate the computer screen with the solutions, you should see the brightness level of the screen and the solutions changing.
- Note that the computer screen gets darkest (minimum transmission of light) at two points, each 180 degrees apart. These are the angles at which the polarization for the screen and the filter are at right angles.
- Note that the solutions get darkest at different angles than the screen. The difference in the angles at which the computer screen and the solution get darkest is the angle of rotation.
- The level of light change does not depend on where the solutions are on the computer screen—this is why you can use a number of chambers at one time.
- Make a circle in the cardboard, around the edge of the computer and solutions. You can hold a pencil against the side of the lazy Susan as you rotate it to make the circle.
- Slowly rotate the computer with the solutions resting on the screen.
- On the cardboard, mark the two points at which the screen gets darkest, as well as the two points at which each solution gets darkest.
- Repeat steps 3–4 three more times to get accurate angles.
Analyzing Your Data
- Remove the computer from the cardboard.
- Use a ruler to draw lines connecting the two correlating points across the circle: the points at which the screen was darkest with the water, and the two points at which the screen was darkest for each of the other solutions.
- Measure the angles between the lines for the screen and each of the glucose solutions. Record the measurements in your lab notebook.
- Graph the angle of rotation versus the concentration of light corn syrup solution.
Ask an Expert
- Try adding different amounts of pure light corn syrup to each chamber. For example, 2, 4, and 8 tbsp. Put 4 tbsp. of water in a chamber as a control. This will change the length of the path through the glucose solution. Graph the amount of light corn syrup vs. the angle of rotation.
- Purchase some fructose from a grocery store or bakery supply shop and repeat the experiment. It will rotate the plane to the left. Try mixing glucose and fructose.
- Look online for other chiral molecules to experiment with that are commonly available, such as amino acids or over-the-counter drugs.
- Test various juices and fruit drinks for optical activity.
- Use the polarimeter to track the consumption of glucose by a yeast culture.
- Make a "high-throughput" holder that has 10 chambers for different solutions. Record the solutions as you rotate the base. Devise a way to measure the angle and light intensity using the digital images.
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