A Polarimeter (manual version)

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A polarimeter measures the angle of rotation caused by optically active substances like sugar solutions precisely. A yellow LED (585nm) at the base shines through a polarizing film, through the sample in the tube, and then through another polarizing film in the cap. To use it, first rotate the cap until the light you see through the cap is the darkest. Then match the 0 on the scale to the pointer on the cap. Next, place the sample in a 16mm diameter test tube and put it 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 as well as the temperature. Specific rotation is calculated by multiplying the height of the liquid in the test tube (in dm) by the concentration, then dividing the observed angle of rotation by that result. 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 degrees and Fructose at -92 degrees. A famous experiment involves making a sucrose solution, measuring/calculating the specific rotation, then adding a drop of Hydrochloric or Sulphuric acid, then observing the change in rotation over time caused by the reaction.