Polarimetry!
How a Polarimeter Works
The general workings of a polarimeter (specifically the one designed here) is as follows:- Monochromatic light is emitted (or filtered) from a source
- The light passes through a plane-polarizer
- The light then passes through the sample (and will be rotated by x-degrees)
- The light the passes through another linear polarizer (used to determine the x-degrees)
- The sample
- Path-Length of the sample
- Concentration of the sample
- The wavelength of the light
- The temperature of the system when measured
Building a Make-Shift Polarimeter
I often like to think of what I want to do, and then wonder around the hardware store until I have amassed the materials to do it. You will see that my parts lists usually reflect that.
This project has already been completed, so the images are already "pre-assembled". I hope to document-as-I-go for future projects.
Parts List:
- Electronics (On/Off switch, wires for connections)
- A box (to hide the electronics)
- A tube (to keep external light out)
- 2 linear polarizers (it IS called a polarimeter)
- 2 "lens-holders" (one for each polarizer)
- A light source
- A power source
- A cuvette (to hold the sample)
- A measuring device (for angles)
What I used:
- Wires, switches and such from RadioShack
- A home-made wooden box
- PVC piping and adapters
- 2 linear polarizing films from Edmund Optics
- 2 "lens-holders" made from leftover wood/PVC
- A green LED
- A pack-holder for 8 AA batteries (12V) also from RadioShack
- A shot-glass for a make-shift demonstration cuvette
- A protactor for measuring rotation angles
We will start out with the box. Any sturdy pre-assembled one will do, however I chose to make my own out of wood: cheap, easy to modify. In designing this you need to consider that:
- you will need to mount the light source at the top
- you might want to mount an on/off switch to it
- it will need easy-access to the electronics inside
From the image below, one can see the box I built:
I have my batteries in a pack (secured to the inside) as a power supply to deliver 12V; I have the wires run to connect to the light source (an LED at the top), and most importantly: it's all connected to a big red switch. It's not worth making if it doesn't have a big red switch :-) I bought a small circuit board to wire it together/practice my soldering, but this is largely unnecessary for this device. I also bought small alligator clips; this was just to make it easy to swap around my 12 Volts to other projects.
Now for a top view:
Here we can see that I have simply drilled a hole to fit the LED in. I bought a green LED with the resistor built in and ready to run at 12V. They typically report the peak wavelength emission for LEDs, WHICH IS VERY IMPORTANT, as there is a wavelength dependence on optical rotation! The "standard wavelength used is the 589.3 (Sodium D line), however LED's are cheap and you can swap them out to make your polarimeter run at any wavelength you can see (just make a note!). The greens are not at 589, but they're in that vicinity.
I have also made my own holder for the polarizer with wood left over from building the box. I have pegged it so that it will go onto the box with the same orientation each time (this could be important, depending on how you fix the scale) as well as leave enough space to not physically come into contact with the light source. Linear polarizers are not expensive, but not always that easy to find in non-specialty stores. Most photo places sell circularly polarized lenses which will not work here. I ordered some inexpensive polarizing film from Edmund Optics, I'd say it is certainly adequate for this purpose (cut-able, easy to modify).
For my cuvette is currently a shot glass. I would recommend something better for measuring (a small grated cylinder perhaps, or a small beaker as there is a concentration dependence on optical rotation as well). However, for demonstration purposes, this works fine: just make sure that whatever you choose doesn't rotate the plane of light on it's own!
For a picture of the polarizer fitted on top, I present:
That's right, the big-red-switch lights up too :-)
For the tube of the instrument, I chose some PVC pipe. It is cheap, comes in a wide variety of shapes and sizes and all sorts of adapters! Any large tube that will keep the light out will do. You can get an idea of what mike looks like finished as shown below:
I don't imagine it has to be as tall as mine is, I just simply cut the PVC pipe I bought in two and used it as it was. I have latched on the top/bottom parts so that I can easily remove the tube from the base-box to move the samples in and out. I know I said that you don't have to use PVC, but I will try and persuade you with the following picture:
Since we are measureing an angle change in the rotation of plane polarized light, we need to be able to rotate the top polarizer and threaded PVC adapters can be bought for only a couple of dollars and works magnificantly!
With all of these parts together, the polarimeter is complete! Now we just line up the empty polarimeter so the the polarizers line up and block the light from the LED (as viewed by your eye).
Then we place our sampe in, which will rotate the plane of light, and therfore make the LED visible again:
All that is left to do now, is rotate the top polarizer untill the LED is no longer visable. Measuring that amount of rotation ot the top will tell you how many degrees the sample rotated the plane of light! Dor this, all you need to do is attached a 360 Degree protractor on the top rotatable part of your polarimeter (with a zeroing mark for no sample). I don't have an image of this, as the protractor I bought was too large for the PVC piping I used. I will throw up a new picture when I get around to buying one.
Overall, I probably spent about $50 on this, but that price can be drastically reduced (just dont use oak boxes and fancy red switches ;-). One of these can simply and cheaply be made for a classroom, especially for demonstartion purposes!
Happy hunting for parts!
mark[at]markslaboratory.com