SUCROSE CONCENTRATION THROUGH
POLARIMETRY
A polarimeter passes polarized light through an optically active solution and measures the degree to which light is rotated by the solute molecules. Figure 1 shows how non-polarized light is transmitted through a fixed disc with very thin slits in it. The light passing through the disc is now polarized because only light of one orientation can travel through the slits. As polarized light passes through the solution, the plane in which the light oscillates is rotated a unique amount depending on the identity of the solute. A second polarizer is attached to the analyzer wheel. Light passed through the second polarizer reaches a detector and is reported as light intensity. When the user rotates the wheel, the light intensity changes at each polarizer position. Light intensity reaches a maximum when the optical rotation of the outgoing plane-polarized light matches the orientation of the slits on the analyzer disc.
Figure 1. How polarized light travels through a solution in a polarimeter
The specific rotation ([α]) is the angle that an optically active solution rotates polarized light as it travels through the sample. Sucrose, glucose, and fructose are optically active molecules. These enantiomers lack symmetry and have non-superimposable mirror images of themselves. At 20 °C, [α] is 66.47° for sucrose, 52.7° for glucose, and -92.0° for fructose. Dextrorotatory enantiomers rotate the plane of polarized light clockwise at a positive angle as seen by the detector. Levorotatory enantiomers rotate the plane of polarized light counterclockwise at a negative angle. Polarimetry is used routinely in quality and process control in the pharmaceutical industry, the flavor, fragrance/ essential oil industry, and the food industry. Many organic chemical manufacturing industries also rely on polarimetry to determine product purity. The optical purity of a product can be determined by comparing known specific rotation values with the measured specific rotation of compounds like amino acids, antibiotics, steroids, vitamins, lemon oil, sugars, and polymers.
Biot's Law:[α]λT= αobservedℓ × c
Biot's law allows the determination of solution concentration. The accepted specific rotation ([α]) for a given solution depends on the light source wavelength (λ), solution temperature, concentration, observed angle of rotation, and path length of light traveling through the sample. If temperature and wavelength for [α] are not given, assume λ = 589 nm and temperature is 20 °C. The units for specific rotation are degrees·mL·dm-1·g-1, but this is usually
abbreviated to degrees. Observed rotation (αobserved) is based on the measured optical rotation in units of degrees.
Path length (ℓ) is the length of the sample cell in units of dm. The cell that you will be using is 1.00 dm in length and the light source wavelength equals 589 nm. In this investigation you will determine (c), the sample
Objectives
• Use the observed rotation of light to determine the unknown concentration of a sucrose solution.
Materials and Equipment
• Data collection system• Polarimeter with sample cell
• Beakers (4), 50-mL
• Beaker, 250-mL
• 3% Sucrose solution, ~15 mL
• 6% Sucrose solution, ~15 mL
• 10% Sucrose solution, ~15 mL
• Unknown Sucrose solution, ~15 mL
• Lint-free non-abrasive lens wipes
• Rinse bottle filled with distilled water
Safety
Follow regular laboratory safety precautions.
Procedure
Make sure the polarimeter is turned off and the USB cable is unplugged. Align the notch on the analyzer wheel with the orientation line printed on the polarimeter.
Turn on the polarimeter. Open SPARKvue and connect the polarimeter to your device. Open the Quick Start file called Intensity versus Angle.
Note: If the file is not available, create a graph display with Intensity on the y-axis and Angle(°) on the x‑axis.
Label the 50-mL beakers as follows: 3%; 6%; 10%; Unknown. Collect about 15 mL of each sucrose solution in the appropriate beaker.
Convert the known sucrose solution concentrations from percent to g/ml by dividing each percent value by 100 and enter the results in Table 1 (Example: 20% × 100) = 0.20 g/mL).
Remove the cap from one end of a clean, dry sample cell and set it aside. The cap includes an outer ring, rubber washer, and end glass as shown in Figure 2. Handle the end glass by its sides.
Remove the other cap. Make sure the end glass, rubber washer, and outer ring are assembled in the same order as shown in Figure 2, then hand-tighten the cap to secure it.
Figure 2. Sample cell diagram 1. 2. 3. 4. 5. 6. 7.
Figure 3. Symmetrical peak selection
Fill the sample cell with distilled water and slide the end glass over the threaded opening as shown in Figure 2. Place the rubber washer and outer ring over the end glass and hand-tighten the cap.
Dry the outside of the sample cell and the visible portion of the end glass on either end of the cell.
Tilt the cell to allow all bubbles to collect in the air trap. If the cell has bubbles that will not move, gently tap the cell on top of a paper towel pad until the bubbles move.
Set the sample cell inside the polarimeter. Slide the cell all the way to the right. If the cell has a logo or the air trap is marked, take note of its location to keep cell position consistent across multiple runs.
Start collecting data. Move the analyzer by rotating the wheel towards the back of the polarimeter from zero degrees to 360°. The wheel notch will align with the orientation line when a full rotation is complete.
Note: If a subsequent run begins around 360° instead of 0°, move the wheel just past 360°, stop collecting data, and start a new run at the new wheel position.
Stop collecting data.
Locate the peak closest to zero degrees and complete the following to determine the angle where maximum intensity is reached:
Open the Graph Tools and toggle the Move Tool to the Select Tool.
Add a selection box to capture the top third of the peak with symmetry as shown in Figure 3.
Open the Curve Fits menu and apply a Gaussian fit to the selected data. The function parameters will appear. The value for 'c' indicates the angle of maximum intensity. Record this value in Table 1.
Pour the contents of the cell into the 250-mL waste beaker. Shake out as much liquid from the cell as possible.
Rinse the cell with a small amount of the 3% solution and repeat steps 8-15 using the 3% solution. Repeat with the 6% solution and with the 10% solution.
The observed angle of rotation for each sucrose solution, αobserved, is determined by subtracting the angle at
maximum intensity of the reference substance (distilled water) from the solution angle at maximum intensity: αobserved= solution angle – reference angle
Calculate αobservedfor the three known sucrose solutions. Enter your answers in Table 1.
Navigate to page 2 in the SPARKvue file. Enter the Table 1 values for solution concentration (g/mL) and observed angle of rotation into the SPARKvue table.
Use the Graph Tools menu below the graph display to scale the graph and add a linear fit to the graph. Sketch the results in Graph 1. Include the best fit line and linear fit values for slope (m), y-intercept (b), and r (correlation coefficient). Add a title to your graph along with labels and units on the axes.
Note: Use an alternative graphing program to generate a graph, best fit line, and calculations for slope, y-intercept, and correlation coefficient if the SPARKvue file is not available.
Rinse the cell with a small amount of the unknown solution and repeat steps 8-15 using the unknown solution. Remove both caps. Rinse the sample cell and cap components with distilled water. Shake excess liquid from the 8. 9. 10. 11. 12. 13. 14. a. b. c. d. 15. 16. 17. 18. 19. 20. 21.
Data Collection
Table 1. Experimental and calculated data for sucrose solutions
Substance Concentration (g/mL) Angle at max intensity (°) rotation (αObserved angle of
observed)
Distilled water (reference) 0.0 0
3% Sucrose 6% Sucrose 10% Sucrose
Unknown sucrose concentration
Questions and Analysis
Biot's law states there is a direct relationship between angle of optical rotation and solution concentration, therefore the law can be rearranged as a linear equation (y = mx + b) where y equals αobservedand x equals
concentration in g/mL. Use the linear equation values from your data to determine the concentration of the unknown sucrose solution. Show your work and report the concentration both in g/mL and as a percent concentration. Is your answer reasonable? Explain.
Your instructor will reveal the actual value for the unknown sucrose percent concentration. How does your calculated value compare to the actual concentration value?
Identify sources of error that could have contributed to differences between the calculated and actual solution concentrations.
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