Homemade Polarimeter for Optically Active Fluids

Hello everyone!

A polarimeter is used to determine the angle of rotation of the plane of polarization of optically active liquids. What is the plane of polarization and what are optically active substances?

After all, light is an electromagnetic, transversal wave. In the case of a transverse wave, the direction of oscillation is normal to the direction of propagation. These two directions span a plane, the so-called plane of oscillation. In the case of unpolarized light, this level is completely randomly distributed for all light particles (photons). However, there are polarization filters which, as it were, only allow one direction of oscillation. If unpolarized light falls on such a polarization filter, the light behind the filter is linearly polarized and only oscillates in the same plane of oscillation. If you now use a second polarization filter in connection with the first filter, you can use this to regulate the light intensity that passes through. If the two filters are perpendicular to one another, light can no longer penetrate the second polarization filter. If they are parallel, the intensity is maximum.

With an angle alpha between the two filters, the following intensity comes through: I = I0 * cos ^ 2 (alpha). This is the law of malus.

So what does an optically active substance do? If linearly polarized light penetrates an optically active substance, it rotates the direction of polarization. This rotation depends on the wavelength (color) of the light, on the length L of the liquid column and on the concentration c of the optically active substance.

This rotation by the angle alpha obeys the law: alpha = k * c * L.

The factor k is called the specific angle of rotation and this indicates by how many degrees the direction of polarization is rotated if the liquid column is 1 decimeter long and there is 1 gram of the optically active substance per milliliter of solvent (e.g. water).

Which parts are now required for the polarimeter? You need

• a polarizing filter sheet (f.e. ebay)
• a cuvette, i.e. a glass vessel for the water and the optically active substance
• two 1.25 "filter holders from the field of astronomy
• a laser mount
• a stable base plate
• Lasers with different wavelengths / colors
• a white screen
• Sugar as an optically active substance

The first polarization filter is permanently installed in its holder. The second polarization filter must be rotatable. These two filters must be arranged in such a way that no light can penetrate the second polarization filter at 0 ° on the scale or that it is perpendicular to the first filter. This is best checked by looking at the sun through both filters. The second, rotatable polarization filter is set to 0 ° and then the first is rotated until the intensity is minimal. Then you leave the first filter in its position and only turn the second filter for the experiment later.

The experiment is actually very simple. You put a certain, known amount of sugar (for example 3.6 grams per cube) into the cuvette and dissolve it with a known amount of hot water (for example 85 milliliters). Then you put the different colored lasers one after the other in the holder and rotate the second polarization filter until the intensity on the screen is minimal. This angle of rotation alpha is noted for the respective wavelength of the laser and the concentration of the dissolved sugar (in grams per milliliter).

In my case each sugar cube has a weight of 3.6 grams and the length of the cuvette is 0.798 decimeter. If you put for example 2 sugar cubes into 85 ml of water, the concentration will be 2*3.6 g / 85 ml = 0.0847 g/ml.

For 4 sugar cubes you can see the needed angle of rotation for the four different lasers in the pictures, 18° for the 405 nm laser, 15° for the 450 nm laser, 10° for the 520 nm laser and 8° for the red 640 nm laser.

At the end you have to calculate the specific angle of rotation k for each wavelength. To do this, simply divide the measured angle of rotation by the concentration and the length of the cuvette.

Example: The rotation angle for the 450 nm laser was 15° with a concentration of 4 * 3.6 grams = 14.4 grams of sugar in 85.4 ml water. Therefore the conzentration is 14.4 / 85.4 = 0.1686 g/ml.

To get the factor k you have to devide the rotation angle (= f.e. 15°) with the concentration and the length of the cuvette, so you get

15° / (0.1686 g/ml * 0.798 dm) = 111.5 °/(dm * g/ml)

In order to get a more precise value for the specific angle of rotation k, you can average your values at different concentrations.

At the end you plot the calculated specific angle of rotation k as a function of the wavelength and interpolate for the wavelength of the sodium D-line (589 nm). In my case I received 68 ° / (dm * g / ml). The table value is 66.4 ° / (dm * g / ml).
What use is a polarimeter to me now? Now you can use it to determine the unknown concentration of the optically active substance. You pour the sugar solution into the cuvette, determine the angle of rotation alpha for a certain wavelength and use the known values for alpha, k and the length L to calculate the unknown concentration c. This is how winemakers do it, for example, when they want to determine the sugar content of their grapes.

With this in mind, have fun recreating and Eureka ...

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