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Q & A: Measuring the concentration of sugar with polarized light

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Most recent answer: 05/22/2016
Hello,I am taking part of the semi finals of a competition, to then compete in the science fair. In my experiment there is a part where i need to check how much sugar is in the water, so i was wondering if there was any way you could measure how much sugar there is in an amount of water, using a method that is not too difficult. I have considered using without using a saccharometer or a laser kit, but for both i need to buy them at an expensive price. Swapping my experiment to testing the amount of salt water is fine as well. I also have considered using the method of evaporation, but i am not for sure it would bring out the perfect results i am wantingThank You
- Chai (age 13)
Auckland, New Zealand

One way of measuring the concentration of sugar in a solution is to observe how it affects the polarization of light. This is actually one of the ways sugar concentrations are measured in industry. I think you could do it very cheaply if you get creative.

Sugar molecules are chiral, and they "rotate" the polarization of light that passes through a sugar solution to a slightly different polarization. Polarization is related to the direction of the oscillating electric and magnetic fields that make up a beam of light. For example, the electric field could be oscillating back and forth parallel to the ground—we call this horizontal polarization. Or it could be oscillating perpendicular to the ground, which we call vertical polarization. It can also be polarized at any angle in between horizontal and vertical. A polarizer is a material that allows a certain polarization to pass through it, and blocks light of other polarizations.

If you shine polarized light through a sugar solution, the sugar will rotate its polarization to a different angle. How far it gets rotated depends on a) the type of sugar (you'll need to know something called the "specific rotation," which is different for different sugar molecules), b) what thickness of sugar solution it passes through, and c) the concentration of the sugar solution. You could measure the rotation with polarizers and deduce the concentration of the sugar. A simple procedure could be something like this:

Start by placing one polarizer at a fixed angle. This polarizer won't move at all during the experiment. Then place the second polarizer in front of the first one, and shine some light through them both. You could use a flashlight or the light from a sunny window as the light source. Rotate the second polarizer—you should see the intensity of the light change. Find the angle of the second polarizer that blocks as much as the light as possible. Mark this angle, maybe by drawing a vertical line on the polarizer. The two polarizers are now oriented perpendicular to each other, so light that makes it through the first polarizer won't make it through the second one at all.

Now put the sugar solution in between the two polarizers. The amount of light that makes it through the second polarizer should change, because the polarization was rotated by the sugar. Rotate the second polarizer until the amount of light that gets through is minimized again. Measure how far you had to rotate it from its original position (with a protractor, for example). This angle is the rotation of the light caused by the sugar solution.

To calculate the concentration of sugar, you can use this formula:

concentration in grams per milliliter = measured rotation angle / (specific rotation × thickness of the sugar solution in centimeters/10)

To do this measurement, you'll need some polarizers. You could buy two polarizing filters made for photography. You could try using two pairs of polarizing sunglasses. Or you could pop the lenses out of a pair of 3D glasses from a movie theater—they might even give them to you for free if you ask. You'll have to be careful to get glasses which use linear polarizers, not circular polarizers. Many 3D glasses use circular polarizers, but as far as I know the glasses designed specifically for IMAX movies only use linear polarizers. You can test whether you have circular or linear polarizer lenses by putting the two lenses on top of each other and seeing if the amount of light transmitted changes when you rotate one of them—for linear polarizers it will change a lot, and for circular polarizers it won't.

One thing to keep in mind is that the rotation of the polarization will depend on the wavelength (color) of the light. White light is a mixture of all colors, and the different colors will be rotated by different amounts. You could try putting some kind of color filter before the first polarizer to narrow the range of colors, or you could use an inexpensive laser pointer as your light source instead of white light. You should also try to find a specific rotation value that was measured near the wavelength you're using. It seems like a common measurement wavelength is 589 nm, which is emitted by the sodium-vapor bulbs that are commonly used in streetlights.

Finally, I'd suggest trying this technique first with a known concentration of sugar to see how accurate your result is.

Here's a description of a similar experiment, and somes values of specific rotation for different sugars (it's not clear what wavelength they used, however):

Good luck!

Rebecca H.

(published on 05/22/2016)

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