Vernier Software & Technology

# Understanding Polarimetry

## Introduction

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.

## Objectives

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.

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.

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

### Experiment 06 from Organic Chemistry with Vernier Lab Book

#### Included in the Lab Book

Vernier lab books include word-processing files of the student instructions, essential teacher information, suggested answers, sample data and graphs, and more.