A Polarimeter (manual version) by the-real-aga

Last crawled date: 3 years, 1 month ago

A Polarimeter is used to measure the angle of rotation caused by optically active substances, such as a sugar solution.
A yellow LED (585nm) in the base shines through a polarising film, through the sample in the tube, then through another polarising film in the cap.
First the user rotates the cap until the light they see through the cap is the darkest. Then the scale is rotated to match the 0 with the pointer on the cap. Next the sample is placed in a 16mm diameter test tube and put in the device. If the substance is optically active, the darkest point will have changed, which can be measured by rotating the cap and reading off the scale ring - this may be either to the left or right of zero.
The concentration of the sample in g/ml must be known, and the temperature.
Specific rotation is calculated by multiplying the height of the liquid in the test tube (in dm) by the concentration, then divide the observed angle of rotation by the result of that.
The specific rotation is written as the calculated angle followed by the temperature and wavelength, 585nm in this case.
Common examples are sucrose (table sugar) at +66 and Fructose at -92 degrees.
A famous experiment is to make a sucrose solution, measure/calculate the specific rotation, then add a drop of Hydrochloric or Sulphuric acid, then observe the change in rotation over time caused by the reaction.