Materials Research Science and Engineering CenterUniversity of Wisconsin-Madison

Research Experience for Teachers Summer 2017

Activity Title:  Which soda do you prefer?  Experiments in Polarimetry

Kevin Amundson

The Polarimetry Experiment Activities I developed during the summer 2017 RET program was first performed in my two general chemistry classes at Poynette High School.  Poynette High School is on a 4X4 block schedule so 1.00 credit of chemistry is offered in one semester with classes of 85 minutes each day. I repeated the experiment in the spring semester with my third general chemistry class.  We performed this activity after the mole and stoichiometry units had been covered.

My first chemistry class consists of 18 female and 3 male Caucasian students.  My second chemistry class consists of 9 female and 6 male Caucasian students. The third chemistry class in the spring semester consists of 6 male and 7 female Caucasian students.  In total the three classes have 24 sophomores, 20 juniors, and 4 seniors.

Polarimetry Experiments Lesson Plans

Day One      Concentration and Molarity

*Define the terms solute, solvent, solution, soluble, dissolve*Discuss the difference between density and concentration*Introduce the molarity equation

*Practice calcuations with molarity*Calculate how much solid in needed to prepare a solution of a given concentration*Demonstrate how to prepare solutions using a volumetric flask*Assign molarity calculation problems.Sample Molarity Problems

Day Two      Nature of Light

Pose the question: How could we determine the molarity of an unknown sugar solution?

*Discuss the wave nature of light including wavelength, amplitude, crest, trough, transverse vs. longitudinal, frequency, period, energy, and wave speed.*Explain why light is called an electromagnetic wave.*Discuss the regions of the electromagnetic spectrum from radio to gamma.

*Demonstrate the polarized nature of light.  I used the Vernier polarizer kit.*Assign background reading for the polarimetry experiment.

Day Three Polarimetry Experiments

*Discuss chiral molecules and how they can rotate plane polarized light.*Have students in groups of 3-4 prepare the five standard sucrose solutions.*Students use the polarimeters to measure the angle of rotation for each of the standard sucrose solutions.*Students prepare a calibration curve by plotting angle of rotation versus solution concentration.  They determine the line of best fit.*Students measure the angle of rotation of an unknown sucrose solution and calculate the concentration.*Students measure the angle of rotation of “real sugar Sprite” and determine the sugar content.  They compare results to the label on the Sprite Bottle.*Assign a lab report for the polarimetry experiment

Assessment

Students presented the results of their polarimetry experiment in a lab report format with objective, procedure, data, analysis, and conclusion sections which were assessed by the standards based grading lab rubric.

Standards Based Grading Lab Rubric

They were also assessed on the Solution Stoichiometry unit test.   

Solution Stoichiometry Exam with Rubrics

We used standards-based grading at Poynette High School.  The Solution Stoichiometry exam is broken up into two sections, the first section addresses Poynette 9-12 Science Standard 2 "uses mathematics to support explanations and draw conclusions” while the second section addresses Poynette 9-12 Science Standard 5 “evaluates hypotheses and data and draws conclusions based on evidence”.  Questions 6 and 7 on the exam related to polarimetry experiment and were used to assess Standard 5.    Students are scored on the rubric as “advanced, proficient, developing, beginning, or no evidence”. A graph showing composite scores is below:

The chart shows 15% advanced, 26% proficient, 29% developing, 14% beginning, and 16% no evidence.  At Poynette, students are able to retake alternate versions of the assessment. The data given is for the first assessment only.

Issues/Changes:  I chose to do experiment #4 “Determining the concentration of the unknown” and experiment #6 “Which soda do you prefer?” only due to time considerations.  My original activity has 7 possible experiments which could be performed if time allows or could be split up such that each lab group is doing a different experiment and then results are shared as a whole class at the end.  Due to some absences on my part, loss of days due to standardized testing, and snow days I was under time pressure to fit the experiments into the curriculum.

During the fall semester, we ran into considerable trouble with the glue not holding the polarized film in place on the polarimeter tubes.  The film would fall off and the solution leaked on the floor….it made a mess and obviously made it hard to get reliable data. We ended up using data from a few groups whose polarimeters did not leak to do our analysis.  I fixed this problem in the spring semester by using water-proof epoxy glue and also having students pour the solution into a glass eudiometer tube and then sliding that into the PVC tube so there was no pressure on the polarized film.

The other issue we encountered was inconsistent results.  The angle of rotation should change in a linear fashion as the concentration of the sugar solutions increases.  Many groups, however, ended up with data where the angle of rotation did not change with concentration or it changed wildly.  On the other hand, I did have some groups get data which was spot on. I think in the future I would do a better job having students norm results so there is better consistency from one polarimeter to the next.  I think students would also get better consistency if they used more than 5 standard solutions.

Overall, I think this activity has merit.  While we did have trouble getting consisten results, it does give students practice with the idea of using energy to learn something about matter.  It helps them learn how a calibration curves and standard solutions can be used to determine the concentration of an unknown. It helps them learn how the physical property of optical rotation is measured and used.  In short, it gives teachers an alternative to the Beer’s Law experiment traditionally used in a chemistry class with a low-cost alternative. While the Beer’s Law experiment requires a spectrophotometer which costs hundreds of dollars, this experiment requires a PVC tube and flashlight which can be purchased for around $20.  It also uses sugars which have minimal safety issues compared to iron or copper solutions. It is more relatable for the students since it discusses sugar and soda which students naturally have an interest in.

The details of the activities appears below.

Materials Research Science and Engineering CenterUniversity of Wisconsin-Madison

Research Experience for Teachers Summer 2017

Activity Title:  Which soda do you prefer?  Experiments in Polarimetry

Kevin AmundsonChemistry and Physics Teacher

Poynette High School608-635-4347 ext. 423

kamun@poynette.k12.wi.us

Activity Description:  Students build and use a home-made polarimeter to perform a variety of experiments including identifying and measuring the concentration of sweeteners in soda.

Audience (Grade Level and Subject): 10-12 chemistry class

Learning Objective(s):

Students will understand that light interacts with matter. Students will understand that light can be polarized and that plane polarized light

is rotated by chiral molecules.

Students will use polarimetry to determine the identity and/or concentration of an unknown solution.

Standards Addressed: HS-PS1-3; HS-PS4-4; HS-ETS1-2

Engineering Principles Addressed: Practice 1 Asking Questions and Defining ProblemsPractice 2 Developing and Using ModelsPractice 4 Analyzing and Interpreting DataPractice 5 Using Mathematics and Computational Thinking

Time (Setup, Activity, Cleanup): Building the polarimeter (30 minutes) 2 hour prep time to gather materials and equipment 7 Experiments (2, 90 minutes blocks or 4,  45 minute class periods)

Supplies: PVC pipe (¾” diameter, 30” length holds approximately 200 ml of solution) Light source (I used an LED Promier® Flashlight and Flood Light - Assorted

Colors Model Number: PDUAL-6-24  | Menards® SKU: 5757215 from Menards) $4.99

*The LED light source is extremely bright.  Do not stare directly at light source or shine in someone’s eye!!!

Fig. 1  Light Source Cloth to cover light source ¾” copper or PVC fitting Home Depot Model # C604  $1.40

Fig.2 Copper Fitting (PVC can be used as well) for lens Polarizing film sheets (cut two squares about 1”X1”) $9.90

https://www.amazon.com/Polarizing-Film-Sheet-Gadget-Electronics/dp/B004X3XFHU/ref=sr_1_1?ie=UTF8&qid=1499979716&sr=8-1&keywords=polarized+film+sheets

Ring stand Ring clamps Printable dial http://www.weldingdata.com/pipetemplatesample.htm Distilled water Meter stick Epoxy glue (for use on PVC pipe) Vernier light sensor (optional) $55

https://www.vernier.com/products/sensors/light-sensors/ls-bta/

Vernier polarimeter (optional) $499https://www.vernier.com/products/sensors/chem-pol/

Any Dangerous Materials? No, but safe lab practices such as safety goggles should always be observed!    https://www.flinnsci.com/

Cane sugar (grocery store) Glucose Fructose Other sugars such as galactose, mannose, etc. D(-) and L(+) Tartaric Acid

Building the Polarimeter

Fig. 3  Homemade Polarimeter

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.

Fig. 4  Glue polarized film to bottom of PVC pipe3. Print a protractor dial template below and use a copy machine to fit the dial such

that its length is the same as the circumference of the tube.    A template can be found at: How to Pipe Template http://www.weldingdata.com/pipetemplatesample.htm

Fig.5   Dial Template4. Each line represents 22.5° when wrapped around the tube.  Label each line

sucessively 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 ¾” below the top.

Fig. 6  Tape dial to top of tube6. 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

 Fig.7  Glue polarizing film to copper fitting for a “Lens” 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.

Fig. 8  Make black line on copper 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.

Fig 9  Light source shining up tube

Background:  Light and Chiral Substances

Light can be considered 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 (Fig.10). This is because the polymer chains in polarized lenses are all aligned in one direction.  

Fig.10  Light encoutners polarized filmhttp://wwwcdn.skyandtelescope.com/wp-content/uploads/Polarized-light-Kaidor-Wiki-CC-3.0ST.jpg

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 which 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 (Fig.11).  

Fig.11  Gloves are nonsuperimposable

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” where a pair of socks do not (Fig.12).

Fig.12  Socks are superimposable

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 in Fig.13.

Fig. 13  Two enantiomers of a chiral moleculehttp://chemistrybook2011.blogspot.com/2011/05/stereoisomer.html

It turns 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 “dextorotatory” 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 the glucose molecule in Fig.14 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?

Fig. 14  Glucose moleculehttps://upload.wikimedia.org/wikipedia/commons/6/64/Glucose_Haworth.png

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:

Fig. 15  Biot’s Law of Polarimetry

In other words, the more concentrated the substance the more the light will rotate and the greater the observed optical rotation.  Similarily, 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 (Fig.16).

Fig. 16  Diagram of a Polarimeterhttps://en.wikipedia.org/wiki/Specific_rotation

Polarimetry is an important technique which can be used to identify substances (since the specific rotation is unique to the substance just 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 usefullness of many drugs is dependent on using the correct enantiomer.

Using the polarimeter you built above, we will now conduct many experiments in polarimetry.

*Items are in red can go in student lab notebooks*Items in blue are sample results for the teacher

Experiment #1  Optical Rotation and Light Intensity

ProcedureRecord 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.

Fig. 17 Light Intensity vs. Rotation Angle

*Another light sensor could be substituted

Experiment #2 Optical Rotation of Enantiomers

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.

Procedure1. 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.

      Fig. 18  Shift from red to blue at the angles of minimum light intensity

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.

Fig. 19  Angle of Rotation vs. Concentration for Tartaric Acid (notice how the D(-) and L(+) enantiomers have slopes with the opposite signs)

Experiment #3 Path Length and Biot’s Law

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.

Procedure

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.

Fig. 20  Angle of Rotation vs. Path Length

Experiment #4  Determining the Concentration of an Unknown Sugar Solution

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.

Procedure

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.  

Fig. 21  Angle of Rotation vs. Concentration for Sucrose

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

Experiment #5 Identifying Sugars Using Polarimetry

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.

Procedure

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.

Fig. 22  Biot’s Law

Fig. 23 Biot’s Law with Experimental Data

*Sample graphs for the data I collected are observed below

Fig. 24  Dextrose

Fig. 25  Fructose

Fig. 26 Galactose

Fig. 27  Maltose

Fig. 28  Equation for Percent Error

Experiment #6      Which soda do you prefer?

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.

Experimental Procedure:1. Prepare 100 ml of the following solutions.  

11.3% sucrose solution (use cane sugar as the source of sucrose) 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.

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?

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?

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?

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?

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?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?

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?

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.

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.

Glucose    Fructose

Both glucose and fructose have stereogenic centers.

17. Use Biot’s Law to calculate the specific rotation for sucrose.  How does this compare to the literature value?

29% error!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?

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.

-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

Experiment #7    Problems with Mexican Soda

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.

Fig. 29  Decomposition of Sucrose into glucose and fructose

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.

Procedure

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.

Fig. 30 Preparation of HFCS standard solutions

*Solutions were diluted to 200 ml

%Fructose Fructose (g) Glucose (g)

0% 0 22.6

10 2.0 18.0

20 4.0 16.0

30 6.0 14.0

42 8.4 11.6

55 11.0 9.0

75 15.0 5.0

100 0 22.3

Fig. 31  Calibration curve for HFCS standards

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

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 rougly two months prior.

Assessment of Student Performance/Learning:

Student success at building a working polarimeter. Students successfully prepare necessary solutions Students successfully prepare a report in their laboratory notebook following the

rubric below:AP Chemistry Laboratory Notebook

Citations / Links to more information:

High Fructose Corn Syrup What it is and What it ain’t http://www.bodyrecomposition.com/research-review/straight-talk-about-high-fructose-corn-syrup-what-it-is-and-what-it-aint-research-review.html/

Modified version of An Inexpensive Homemade Polarimeterhttp://article.sapub.org/10.5923.j.jlce.20150301.02.html

Build a Homemade Polarimeterhttp://pubs.acs.org/doi/pdf/10.1021/ed078p644

Sugar Identification Using Polarimetryhttp://www.xula.edu/chemistry/documents/orgleclab/StereoPolar.pdf

Understanding Polarimetry with Vernierhttp://www.vernier.com/experiments/chem-o/6/understanding_polarimetry/

Experiments with Polarized Lighthttp://www.lhup.edu/~dsimanek/14/polaroid.htm

The Story of Mexican Coke is a lot More Complex than Hipsters Would Like to Admithttp://www.smithsonianmag.com/smithsonian-institution/story-mexican-coke-more-complex-than-hipsters-would-admit-180956032/

Why Coke’s Cane Sugar May Seem Just Like the High Fructose Kindhttp://www.acsh.org/news/2016/04/08/why-cokes-cane-sugar-soda-may-seem-just-like-the-high-fructose-kind

Experiments with Polarized Lighthttp://www.lhup.edu/~dsimanek/14/polaroid.htm

CreditsEducator Designer: Kevin AmundsonMRSEC Lab Advisor: Nicholas Kearns, graduate student Zanni group UW-MadisonMRSEC Lab PI: Dr. Martin ZanniOthers: David Gagnon, Program Director Field Day Lab UW-Madison              AnneLynn Gillian-Daniel, Education Director MRSEC UW-Madison            Dr. Nelson Cardona Martinez, SusWEF Director U of Puerto-Rico-Mayaguez            Christopher Gillette, RET fellow            David Vajko, RET fellow            Alycia Riehl, RET fellow            Kathia Rodriguez, RET fellow Puerto Rico

           Samirah Mercado Feliciano, RET fellow Puerto Rico            Dr. Tom McDounough, Zanni group