Understanding Polrimeters and the Relationship Between Optical Activity and Path Length

Basic polarimeters are comprised of a light source, two polarizing filters, an eyepiece for direct observation of optical rotation and a sample tube containing a solvent of interest. This phenomenon is also known as rotary polarization, and is largely dependent on the sample material’s chemical composition and molecular structure.

In fact, it is a unique form of optical transmissivity only observed in fluids, solids, and heterogeneous mixtures with distinct biomolecular or mineralogical structures.

Due to its adjacent crystal planes, quartz was the first material used to observe optical rotation. The use of quartz led to a quick increase in the understanding of optical transmissivity in crystalline structures, followed by multiple discoveries in the field of stereochemistry.

The leading instruments used for qualitatively demonstrating rotary polarization in classroom environments are polarimeters. The use of polarimeters differs significantly from the quantitative measurements distinguishing optical activity as more than a chemical curiosity.

Harold M. Gladstone, Ph.D. developed a basic experiment to observe the rotary polarization variations and the incident light’s path length. The experiment, using basic polarimeters, offers a simple way to display the quantitative measurement of optical activity.

Relationship of Optical Activity and Path Length

The inter-relationship between path length and optical activity is of particular interest, and can be easily demonstrated in a classroom environment. The first step involves taking the average of multiple light readings by rotating the eyepiece fully – both clockwise and counter-clockwise. This provides a reference for zero path length.

Then, a solution comprising of 17.1 g of sucrose is loaded into a sample cell of known dimensions. The next step involves placing the cell vertically and carefully measure the height of the liquid up to 2 mm. Once the sample cell is loaded into the polarimeter, this is followed by the record of the angle of rotation of the plane of polarized light. This step must be then repeated while increasing the sample height – in increments of 2 mm.

Data from this procedure can be used to plot a graph and determine the linearity between the optical rotation and liquid height. The student’s graph, illustrating the specific rotation of sucrose at a path length of 10 cm, can then be compared against existing literature to measure percent error. The use of Glas-Col’s robust, vertical polarimeter ensures that this experiment can be conducted reliably in under 2 hours.

Polarimeters from Glas-Col

Originally founded as a developer and supplier of novel heating mantles for laboratory experiments, Glas-Col has since then introduced an expansive product range designed to meet a wide variety of modern scientific testing applications, under the most rigorous standards.

Glas-Col has also introduced the Glas-Col Polarimeter for academic institutions, alongside its industrial and commercial products. This polarimeter was designed to demonstrate light’s  rotating planarization in a solution. Today, it has been widely adopted in high schools and colleges across the United States.

This information has been sourced, reviewed and adapted from materials provided by Glas-Col.

For more information on this source, please visit Glas-Col.

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