Overview
This lecture covers measuring the optical rotation of sugar solutions using a polarimeter, preparing solutions with various concentrations, and analyzing the results to identify an unknown sugar.
Optical Activity and Polarimetry
- Chiral compounds rotate the plane of polarized light; the rotation angle depends on several factors including concentration and path length.
- The polarimeter is used to measure the observed rotation (α) of polarized light passing through a sample.
Preparing Sugar Solutions
- Make a 0.1 g/mL sugar solution by dissolving 5 g sugar in 50 mL distilled water.
- Rinse all equipment with distilled water to avoid contamination.
- Prepare lower concentrations (e.g., 0.05 or 0.08 g/mL) by diluting the original solution with distilled water using the formula:
(initial concentration) × (volume used) = (final concentration) × (final volume).
- Example: To prepare 10 mL of 0.04 g/mL solution, combine 4 mL of 0.1 g/mL solution with 6 mL distilled water.
Using the Polarimeter
- Turn on the polarimeter and avoid touching the optical lenses.
- Calibrate the instrument with distilled water so that it reads zero.
- Fill the cell with the test solution, insert it into the polarimeter, and record the observed rotation.
- Clean the polarimeter cell and equipment with distilled water between measurements.
Data Analysis and Calculations
- Specific rotation ([α]) is a constant for each compound; observed rotation (α) is measured in the experiment.
- The relationship: α = [α] × c × l, where c is concentration (g/mL) and l is path length (dm).
- Plot observed rotation against concentration and fit a linear trendline in Excel; the slope represents [α] × l.
- Compare the slope with known values to identify the sugar type.
Key Terms & Definitions
- Chiral compound — A molecule that cannot be superimposed on its mirror image, leading to optical activity.
- Polarimeter — An instrument used to measure the angle of rotation of polarized light passing through an optically active substance.
- Observed rotation (α) — The measured angle by which polarized light is rotated by a sample.
- Specific rotation ([α]) — A characteristic value for each compound representing rotation per concentration per path length.
- Concentration (c) — Amount of solute (sugar) per unit volume, typically in g/mL.
- Path length (l) — The length light travels through the sample, measured in decimeters.
Action Items / Next Steps
- Prepare the sugar solutions as outlined.
- Use the polarimeter to measure rotations for each solution.
- Enter data and plot the graph in Excel; determine the specific rotation and identify the sugar.