Polarimeters: The Relationship Between Optical Activity and Path Length

Tuesday November 13 2018


Basic polarimeters are comprised of a light source, two polarizing filters, a sample tube containing a solvent of interest, and an eyepiece for direct observation of optical rotation. Also known as rotary polarization, this phenomenon is largely dependent upon the chemical composition and molecular structure of the sample material. It is a unique form of optical transmissivity that is only observed in fluids, solids, and heterogeneous mixtures with distinct biomolecular or mineralogical structures.

Quartz, with its adjacent crystal planes, was the first material used to observe optical rotation. This led to a rapid increase in the understanding of optical transmissivity in crystalline structures and multiple discoveries in the field stereochemistry. Polarimeters are the leading instrument used for qualitatively demonstrating rotary polarization in classroom environments. This differs from the quantitative measurements that distinguish optical activity as more than a chemical curiosity.

Harold M. Gladstone, Ph.D. developed a basic experiment for observing variations in rotary polarization and the path length of incident light. It offers a simple way to exhibit quantitative measurement of optical activity using basic polarimeters.

Relationship of Optical Activity and Path Length

The relationship between path length and optical activity is particularly interesting and can be easily demonstrated in a classroom environment. First, the average of multiple light readings is taken by rotating the eyepiece fully in both clockwise and counterclockwise directions. This provides a reference for zero path length.

A solution comprising 17.1g of sucrose can then be loaded into a sample cell of known dimensions. Place the cell vertically and carefully measure the height of the liquid up to 2mm, then record the angle of rotation of the plane of polarized light once the sample cell is loaded into the polarimeter. Repeat this step while increasing the sample height in increments of 2mm.

This data can be used to plot a graph and determine the linearity between the optical rotation and liquid height. The specific rotation of sucrose at a path length of 10cm, according to the student’s graph, can then be compared against existing literature to measure percent error. Using Glas-Col’s robust, vertical polarimeter this experiment can be carried out reliably in under 2 hours.

Polarimeters from Glas-Col

Glas-Col was founded as a developer and supplier of novel heating mantles for laboratory experiments. In the decades since, we have introduced an expansive product range designed to meet the rigorous standards of modern scientific testing applications.

Alongside our industrial and commercial products, we have introduced the Glas-Col Polarimeter for academic institutions. It was designed to demonstrate the rotating planarization of light in a solution and has been widely adopted in high schools and colleges across the US. If you would like any more information about our polarimeter, read our previous blog post: The Applications of Polarimeters. Or, contact us directly with any questions.