Vernier Software and Technology
Vernier Software & Technology

Understanding Polarimetry

Figure from experiment 6 from Organic Chemistry with Vernier


A polarimeter is a device that measures the rotation of linearly polarized light by an optically active sample. This is of interest to organic chemists because it enables differentiation between optically active stereoisomers, i.e., enantiomers. Enantiomers, chiral molecules, are molecules which lack an internal plane of symmetry and have a non-superimosable mirror image. One way to tell these molecules apart is to use polarimetry. Polarimetry is also helpful for biological applications because amino acids, nucleic acids, carbohydrates, and lipids are all optically active. Determination of the optical activity of a compound using polarimetry allows the user to determine various characteristics, including the identity, of the specific chemical compound being investigated.

Incident non-polarized light is transmitted through a fixed polarizer that only allows a certain orientation of light into the sample. The sample then rotates the light at a unique angle. As the analyzer is turned, the rotated light is maximally transmitted at that unique angle, allowing the user to determine properties of the sample. A (+) enantiomer rotates the plane of linearly polarized light clockwise, dextro, as seen by the detector. A (–) enantiomer rotates the plane counter-clockwise, levo.

A compound will consistently have the same specific rotation under identical experimental conditions. To determine the specific rotation of the sample, use Biot’s law:

{\alpha} = {\it{\text{[}}{\alpha}{\text{]}}{\ell}{\text{c}}}

where α is the observed optical rotation in units of degrees, [α] is the specific rotation in units of degrees (the formal unit for specific rotation is degrees dm-1 mL g-1, but scientific literature uses just degrees), ℓ is the length of the cell in units of dm, and c is the sample concentration in units of grams per milliliter.


In this experiment, you will

  • Become familiar with the use of the Polarimeter.
  • Experience how sample path length and concentration affect observed rotation.
  • Calculate the specific rotation for a known sugar sample using Biot’s law.

Sensors and Equipment

This experiment features the following Vernier sensors and equipment.

Additional Requirements

You may also need an interface and software for data collection. What do I need for data collection?

Organic Chemistry with Vernier

See other experiments from the lab book.

1Determining Melting Temperature
2Recrystallization of Benzoic Acid and Aspirin
3Determination of a Boiling Point: Simple and Fractional Distillation
4Identifying an Unknown Analgesic by Melting Temperature and Thin-Layer Chromatography
5Separation of Organic Compounds by Acid-Base Extraction Techniques
6Understanding Polarimetry
7Identification of Organic Unknowns Using Polarimetry
8Investigating Gas Chromatography
9Fractional Distillation of Esters
10Understanding Intermolecular Forces Using a Gas Chromatograph: Enthalpy of Vaporization
11Investigating Thermodynamic Relationships of Substituted Hydrocarbons
12Extraction of Spinach Pigments and Analysis by Electronic Absorption Spectroscopy
13SN1: Synthesis of t-butyl chloride
14SN2: Synthesis of 1-bromobutane
15Observing the Reaction Kinetics of Sucrose with Polarimetry
16The Synthesis and Analysis of Aspirin
17Isolation and Epoxidation of a Natural Product: Limonene
18Synthesizing Ethyl Acetate by Fisher Esterification
19Synthesis of Dibenzalacetone by Aldol Condensation
20The Diels-Alder Reaction of Anthracene with Maleic Anhydride
21Friedel-Crafts Acylation of Ferrocene
22Grignard Formation of Crystal Violet
23Synthesis of Fluorescein
24Synthesis of Methyl Orange and Its Application to Textiles
25Analysis of Natural Products
26Using a Gas Chromatograph: Identifying an Unknown Compound

Experiment 6 from Organic Chemistry with Vernier Lab Book

<i>Organic Chemistry with Vernier</i> book cover

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