The polarimeter with part of the Weissenberg camera (at left), 2018
Polarimeter
20 November 2023
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A curious property of many molecules is their ability to rotate plane-polarised light (light which vibrates in a single direction) after it is filtered through a special polarising lens. This is especially the case for enantiomers, which are pairs of molecules that are arranged such that they are mirror images of each other - similar to our hands. These compounds have almost identical physical properties, but behave differently in biological systems: while one version of a molecule might be beneficial, its mirror image version might be highly toxic. When creating medicines and other useful compounds, it is therefore necessary to analyse the amount of each enantiomer in a sample, along with its purity and concentration, to ensure that the compound is safe for use.
The polarimeter with part of the Weissenberg camera (at left), 2018
One method of doing this was by using a full-circle polarimeter such as the one displayed here. This type of instrument was developed in the mid-nineteenth century by Franz Schmidt & Haensch in Berlin, and one of the black and white photographs in this exhibition shows one in use around 1914 in the physical chemistry laboratory of the Old Chemistry building. As the photograph suggests, the polarimeter required a consistent light source in order to operate, which at this time was often a sodium lamp. Measurements were then made by inserting a tube containing a sample into the polarimeter, and manually rotating the polarising lens left or right until a peak dip was observed in the brightness of the light travelling through the instrument.
When this was seen, the positive or negative angle of the rotation of the lens could then be read off the scale at one end, with the small magnifying lenses allowing for more accurate measurements to be made. Depending upon the brightness of the beam and the direction of rotation, correlations could then be made to the type of enantiomers present in the sample, its concentration, and its purity. These days, however, polarimetry is done digitally and automatically, without the need for less accurate manual observations of changes in brightness.
