Polarimetry Experiments

1. Cut the PVC pipe to desired length (30″ PVC holds about 200 mL of solution).
2. Glue one of the pieces of polarized film to the bottom of the PVC pipe using the epoxy glue. Allow to dry completely.

   

3. Print a protractor dial template and use a copy machine to fit the dial such that its length is the same as the circumference of the tube.

   

4. Each line represents 22.5° when wrapped around the tube. Label each line successively starting at 0°, 22.5°, 45°, 67.5°…..etc. until 360°. You should laminate the template to prevent it from getting wet.
5. Tape the protractor template to the body of the tube about 3/4” below the top.

   

6. Glue the other piece of polarized film to top of the copper fitting (the threaded end) using the epoxy glue. This will serve as the “lens” for the polarimeter.

   

7. Make a black line on the copper fitting using a sharpie marker to read the angle of rotation. Place the dial (copper fitting) into the top of the PVC tube and be sure it rotates freely in the tube.

   

8. Clamp the PVC pipe to a ring stand such that the light source is at the bottom shining up into the pipe. CAUTION: Cover the light source with a cloth so you are not staring directly at the light source.

   

Light is an electromagnetic wave. Light from the sun or from a light bulb is said to be unpolarized because the electric and magnetic fields in the wave oscillate at right angles to each other and randomly in any direction in space. If the light passes through a polarized film, say through the lens of polarized sunglasses, only light with a specific orientation is allowed to pass. This is because conductive polymer chains in polarized lenses are all aligned in one direction.

Chiral substances are carbon based compounds which contain four different substituents (a substituent is something bonded to the carbon). A molecule of such a substance will have another version of itself with the same chemical formula but that is nonsuperimposable. Such molecules are called enantiomers of each other. For example, a pair of gloves are nonsuperimposable…in other words there is no way to rotate the right and left hand gloves such that they can be laid on top of each other and look the same.

A pair of socks, on the other hand, are superimposable because you can rotate the socks such that they can be paired together. We say that gloves have a “right or left-handedness,” whereas a pair of socks do not.

Chiral molecules likewise have a “right or left-handedness.” A molecule such as bromo-chloro-fluoromethane has two nonsuperimposable versions of each other and is chiral as shown below.

Chiral molecules have two enantiomers of each other which rotate plane polarized light at the same angle but in the opposite direction. We can therefore learn something about the structure and chirality of a molecule by observing how it interacts with plane polarized light. For example, dextrose was observed to rotate polarized light to the right or clockwise known as “dextrorotatory” thus the name dextrose. Levulose rotated the light to the left or counterclockwise known as “levorotatory” thus the name levulose. The molecules rotate light differently because as photons of light contact the molecule there are time delays between absorption and emission which causes the refraction of the light. The amount of refraction or bending also depends on light wavelength.

In fact most sugar molecules are chiral. If you look at the structure of a glucose molecule, you will notice the number 1 carbon (anomeric carbon) has four different substituents bonded to it and is therefore chiral. Such a carbon is known as a stereogenic center. Are there any other stereogenic centers in glucose?

The angle at which a molecule rotates polarized light is called its specific rotation or optical rotation, 𝜶 , and is an intrinsic property of the substance just like density or melting point. In sugars, each stereogenic center rotates light differently so the specific rotation of the molecule is the sum total. In fact, it is possible for the optical rotations of stereogenic centers to “undo” each other and have a chiral molecule which is optically inactive.

Specific rotation does depend on temperature and the wavelength of polarized light used and is usually reported at 20℃ and the bright line of sodium at wavelength 589 nm. The actual angle of rotation of polarized light also depends on the path length through which the light passes, the concentration of the substance, and the specific rotation of the substance. This is mathematically written as Biot’s Law:

In other words, the more concentrated the substance the more the light will rotate and the greater the observed optical rotation. Similarly, the longer path the light travels through the substance the more the light will rotate and the greater the observed optical rotation. Lastly, some molecules just rotate light more than others based on their structures.

The technique which is used to measure the optical rotation of substances is called polarimetry and the instrument used is called a polarimeter. A polarimeter basically consists of a light source, two polarizing filters, a chamber to put a sample, and a detector to measure the specific rotation.

Polarimetry is an important technique which can be used to identify substances (since the specific rotation is unique to the substance, like density), determine the concentration or purity of a sample using Biot’s law, or to measure the progress of a chemical reaction where the optical rotation changes over time. In the food industry, polarimetry can be used to determine the concentration and purity of sugars. In the drug industry, polarimetry can be used to determine the purity of amino acids, antibiotics, steroids, and vitamins because many of the molecules contained in these drugs are chiral.

A sort of worst case scenario happened with the morning sickness drug thalidomide in the 1960’s. One enantiomer of the drug was responsible for causing birth defects while the other enantiomer was inert until metabolized in the body. The usefulness of many drugs is dependent on using the correct enantiomer.

Using the polarimeter you built, you can now conduct many experiments in polarimetry.

Record the following in your lab notebook.

In this experiment, students will observe how light intensity changes as the polarized film on the polarimeter is rotated.

1. Prepare 200 mL of a 30% sucrose solution and pour into the polarimeter tube.
2. Clamp a Vernier Light sensor* so it is above the dial assembly.
3. Record the light intensity and color of solution at even angle increments through 360°. I chose to record data every 10°.
4. Prepare a graph of light intensity vs. angle.
5. Describe any relationships observed between light intensity and angle as well as color of solution.

Record the following in your lab notebook.

Each enantiomer of a chiral substance will rotate plane polarized light the same amount but in opposite directions. In this experiment, students will observe how two different enantiomers rotate polarized light.

1. Choose two different optical enantiomers of a substance. For my experiment, I chose D(-) Tartaric Acid and L(+) Tartaric Acid, however the specific rotation is small so there is not a large change in angle. A substance with a higher specific rotation may work better.
2. Prepare solutions of increasing concentrations for one of the enantiomers and pour into the polarimeter tube. Be sure to rinse the polarimeter tube with distilled water between solutions. Test each of these solutions in the polarimeter and measure the angles of minimum light intensity. For the remainder of the experiments, we will call this angle the observed optical rotation. This light intensity change can also be accompanied by color change from red to blue or vice versa especially for concentrated sugar solutions (Fig. 18). There will actually be two of these angles of minimum light intensity 180° apart. This is because the polarizing filters only allow light with the right orientation to pass through and block out light at angles to the film (see Fig. 10). You should get in the habit of recording both angles for each observation.
3. Repeat procedure for step 2 for the other enantiomer.
4. Prepare a graph of angle of optical rotation vs. concentration for each enantiomer.
5. Explain any similarities and differences between the two graphs.

Record the following in your lab notebook.

According to Biot’s Law, the observed optical rotation ⍺ depends on the path length through which light must pass through the sample. In this experiment, students will observe how observed optical rotation and path length are related.

1. Prepare 200 mL of a 30% sucrose solution.
2. Fill the polarimeter tube with the sucrose solution such that the path length increases by 0.5 dm in length for each observation. Record the angles of minimum light intensity. These will be the angles of optical rotation.
3. Prepare a graph of angle of optical rotation vs. length. Since there will be two angles for each path length observation, choose one set to graph.
4. Perform a line of best fit.
5. Paste the graph in your lab notebook and describe the relationship between observed optical rotation and length. How well does the data follow Biot’s Law? What does the slope of the graph represent? Give units for the slope.

Record the following in your lab notebook.

In this experiment, students will determine the concentration of an unknown sugar solution prepared by the teacher. Students will prepare a calibration curve using observed optical rotation vs. concentration for a series of standard solutions. By measuring the observed optical rotation of the unknown and the line of best fit on the calibration curves, students can calculate the concentration of the unknown solution. This is similar to the Beer’s Law experiment often done in introductory chemistry.

1. Prepare five 200 mL of standard sugar solutions ranging from 0.10 g/mL up to about 0.30 g/mL (10-30% solutions).
2. The teacher will prepare an unknown sugar solution.
3. Test each of these solutions in the polarimeter and measure the angles of minimum light intensity.
4. Prepare a plot of observed angle of rotation vs. concentration in g/mL.
5. Perform a linear fit and determine the equation of best fit.
6. Obtain a sugar solution of unknown concentration from your instructor. Measure the angles of minimum light intensity and use the calibration plot to calculate the concentration of the unknown.
7. Obtain the value of the unknown concentration from the instructor and calculate a percent error.
8. Comment on sources of error and how they affect results.

Example: Angle of rotation for the unknown was found to be 315°. Calculate the concentration of the unknown.

Equation of best fit: y = 204x + 268

Note: the equation should have units of mL (milliliters)

Record the following in your lab notebook.

In this experiment, students will prepare standard solutions of various sugar solutions. Using Biot’s Law, the specific rotation of the sugar can be calculated. The teacher will then give the students an unknown sugar solution and ask the students to identify it.

1. Prepare five 200 mL of standard sugar solutions ranging from 0.10 g/mL up to about 0.30 g/mL (10-30% solutions). I used D(+) galactose, dextrose, D(-) fructose, and D(-) maltose.
2. The teacher will prepare an unknown sugar solution.
3. Pour each of these solutions in the polarimeter and measure the angles of minimum light intensity.
4. Prepare a plot of observed angle of rotation vs. concentration in g/mL.
5. Measure the length of the polarimeter tube. This length represents the distance light must travel through the sample and is called the path length.
6. When making a linear plot of observed angle of rotation vs. concentration the slope of the line represents the product of the specific rotation of the sugar and the path length in dm according to Biot’s law. Since the path length is known from step 5, the specific rotation can be calculated and the sugar identified.
7. Look up the literature value of the specific rotation and calculate a percent error. Comment on sources of error and how they affect results.

In the 1970’s, US manufacturers began using high fructose corn syrup to sweeten products previously sweetened by sugar (sucrose) due to the cheaper price of using corn instead of sugar. High fructose corn syrup is actually a misnomer given its composition in drinks such as soda is normally 55% fructose, 42% glucose, and 3% other ingredients. Over time there has grown a sort of backlash against the use of high fructose corn syrup (HFCS) with claims that the rise in the use of HFCS coincides with the rise in obesity levels. In response, sodas such as Blue Sky advertise all natural sugar with no added HFCS and can be found in the organic food sections in supermarkets. In addition, some people say they prefer the taste of foods sweetened with sucrose instead of HFCS. For example, some people seek out “Mexican soda” because sodas such as Coke are sweetened with cane sugar instead of HFCS in Mexico. “Kosher” soda sold in the US during the Jewish Passover is also sweetened with sucrose instead of HFCS.

Question. But how about you? Do you prefer the taste of soda sweetened with sugar or high fructose corn syrup or can you tell a difference? How can we test the labels on a soda to verify the sweetener used is the one advertised? In this experiment we will use Sprite flavored with sugar and compare it to Sprite flavored with high fructose corn syrup using an instrument called a polarimeter.

Taste test. The teacher will put out samples of sugar flavored Sprite and HFCS flavored Sprite in two cups. This will be conducted as a blind taste test so the teacher will not reveal which soda each cup contains until the end of the experiment. Record any observations of taste as well as any visual observations (i.e. can you see any difference between the two types of sodas?) below. At this point, keep all observations to yourself as to not bias or influence classmates.

Decide which soda you prefer or if you have no preference record below. Report the decision to your teacher. The teacher will keep a tally of the results to share with the class at the end of the experiment.

1. Prepare 100 mL of the following solutions.
   1. 11.3% sucrose solution (use cane sugar as the source of sucrose)
   2. HFCS-55 solution (55% fructose, 42% glucose)
2. Clamp the polarimeter tube so that it is on top of the light source.
3. Pour distilled water into the polarimeter. Since water is not chiral it will serve as the control.
4. Turn on the light source. Place the dial on top of the polarimeter. Rotate the dial and view the light coming up through the polarimeter tube. Record your observations.
   1. Students should notice the intensity and/or color of the light chaning as the dial is rotated.
5. Which color appears to be the least intense? Most intense? Why might this be?
   1. Red is least intense, yellow is most intense. The optical rotation of light depends on its wavelength with red being rotated the least amount.
6. Record the angle/angles at which the light coming through the polarimeter tube appears to be the most intense. What is the relationship between these two angles?
   1. Angles will vary but students should notice the angles are about 180° apart.
7. Record the angle/angles at which the light coming through the polarimeter tube appears to be the least intense. What is the relationship between these two angles and the angles of most intensity?
   1. Angles will vary but will also be 180° apart on 90° apart from the most intense angles.
8. Empty the distilled water from the polarimeter tube and pour in a sucrose solution.
9. Record the angle/angles at which the light coming through the polarimeter tube appears to be the least intense. How do these angles compare to the distilled water?
   1. Students should notice a different angle of rotation. I got 70° and 245°.
10. Empty out the sucrose solution and rinse out the polarimeter tube and pour in the HFCS-55 solution.
11. Record the angle/angles at which the light coming through the polarimeter tube appears to be the least intense. How does this compare to the sucrose? The distilled water?
    1. Students should notice another angle of rotation. I got 150° and 330°.
12. Empty out the HFCS-55 solution and rinse out the polarimeter tube. Pour in the sample of Sprite you preferred. You will notice the Sprite is “flat” as the teacher degassed it. The CO2 bubbles do not affect the optical rotation but can make it difficult to see into the polarimeter. *Teachers can degas the Sprite by placing it on a magnetic stirrer and gently heating it until the carbonation is mostly gone.
13. Record the angle/angles at which the light coming through the polarimeter tube appears to be the least intense. Do these angles match the sucrose or HFCS-55 solutions better?
    1. The Sprite sweetened with HFCS matched the HFCS-55 solution exactly. The Sprite sweetened with sucrose was different than pure sucrose and HFCS-55 at angles 100° and 285°. This is addressed in experiment #7.
14. Based on your measurements, is the Sprite tested flavored with sucrose or flavored with high fructose corn syrup?
    1. Answers will vary depending on the teacher’s choice.
15. Compare your results with those of your classmates and the results of the taste test.
    1. Answers will vary depending on the teacher’s choice.
16. Look up the molecular structure of glucose and fructose. Explain why these molecules are chiral.

    

17. Use Biot’s Law to calculate the specific rotation for sucrose. How does this compare to the literature value?
18. Why would you have trouble using Biot’s law for the HFCS-55 solution? What might you do to calculate the specific rotation for HFCS-55?
    1. Biot’s Law is for a pure substance with one specific rotation. HFCS is a mixture of two substances glucose and fructose each with its own optical rotation. The optical rotation of a mixture depends on the specific rotation and mole fraction of each component in the mixture.
19. Explain sources of experimental errors and how they affect results.
    1. -parallax error reading the dial
       -light is broad spectrum not just 589 nm of sodium
       -temperature may not be at 20℃
       -polarimeter tube not rinsed out thoroughly
       -scratches on polarized film
       -solution not filled up to the same path length in the polarimeter tube each time
       -orientation of the polarimeter tube in bumped
       -solution contaminated
       -sucrose, glucose, or fructose may contain impurities
20. Do some research. What are some of the controversies surrounding the use of high fructose corn syrup? What is your opinion? Explain.

It has been claimed HFCS can lead to the following health problems ….
• Weight Gain.
• Cancer. …
• Increased Cholesterol Levels. …
• Diabetes. …
• High Blood Pressure. …
• Heart Disease. …
• Leaky Gut Syndrome. …
• Increased Mercury Intake.

The general consensus is the health problems caused by HFCS are no different or worse than sucrose.

*According to Wikipedia, a 2011 study further backed up the idea that people enjoy sucrose (table sugar) more than HFCS. The study, conducted by Michigan State University, included a 99-member panel that evaluated yogurt sweetened with sucrose (table sugar), HFCS, and different varieties of honey for likeness. The results showed that, overall, the panel enjoyed the yogurt with sucrose (table sugar) added more than those that contained HFCS or honey.
http://onlinelibrary.wiley.com/doi/10.1111/j.1471-0307.2011.00694.x/abstract

A problem we ran into was the Mexican Sprite with sugar in it did not exactly match the optical rotation of the cane sugar A little investigation on-line revealed even soda sweetened with cane sugar may actually contain HFCS once the consumer drinks it.

http://www.acsh.org/news/2016/04/08/why-cokes-cane-sugar-soda-may-seem-just-like-the-high-fructose-kind

This is because in the acidic conditions found in soda, the sucrose breaks down into glucose and fructose with almost 90% of the sucrose breaking down within 100 days. The decomposition can occur more rapidly if the soda is exposed to higher temperatures.

To determine the fructose content of the Mexican Sprite and how much of the original sucrose had broken down, a series of standard solutions of HFCS were prepared and a plot of angle of rotation vs. percent fructose was prepared.

1. Prepare 200 mL of the solutions shown in Fig. 30.
2. Pour each of these solutions in the polarimeter and measure the angles of minimum light intensity.
3. Prepare a plot of angle of rotation vs. percent fructose.
4. Pour the Mexican Sprite into the polarimeter and measure its optical rotation. Use the calibration curve to calculate the percent fructose.
5. Estimate the how long it has been since the soda was bottled.
6. Do you think it false advertising for sodas to claim they are sweetened with 100% pure sugar? Explain your thinking.

The optical rotation of the Mexican Sprite was observed to be 100°. Using the calibration curve this corresponds to a percent fructose of 28%.

This means the Sprite was actually 28% fructose, 28% glucose, and only 44% sucrose. In other words, 56% of the sucrose had already decomposed meaning the soda was bottled roughly two months prior.

High Fructose Corn Syrup What it is and What it Ain’t

Modified version of An Inexpensive Homemade Polarimeter

Build a Homemade Polarimeter

Sugar Identification Using Polarimetry

Understanding Polarimetry with Vernier

Experiments with Polarized Light

The Story of Mexican Coke is a lot More Complex than Hipsters Would Like to Admit

Why Coke’s Cane Sugar May Seem Just Like the High Fructose Kind