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How does temperature affect the specific optical rotation of sugar solution at constant concentration and why? If, for example, the temperature is increased, will the optical rotation for a given path length and concentration increase or decrease, and why?

Mlinarm
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  • Clarification: the specific optical rotation of a compound is the optical rotation for the compound at a specific concentration and temperature. What you are asking about how the optical rotation of sugar is affected by temperature. See http://rudolphresearch.com/polarimeters-and-polarimetry/#.U_lBIPldVos – LDC3 Aug 24 '14 at 01:32

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The temperature changes it's optical activity. This depends on the wavelength of light used and the substance. From my own personal experience, a 633nm laser will result in little change in optical activity. A green (546.1nm) has a much more easily measurable change (if you are doing a lab).

We have the equation $$[\alpha]=\frac{\alpha}{l c}$$ where $[\alpha]$ is the specific rotation (at a specific temperature and wavelength), $\alpha$ is the observed rotation, $l$ is the path length (through the sugar water), and $c$ is the concentration (in this case the sugar).

I'm not exactly sure why temperature changes the specific rotation, but my best guess (from experiments that I have done) is that temperature changes the polarization of the substance. (Should make sense if you think of how water interacts with light at different temperatures) Our above equation is sometimes written as $$[\alpha]^T_\lambda = \frac{\alpha}{lc}$$ Where $T$ is temperature and $\lambda$ is the wavelength. So the only thing that can change in here is the original polarization of the substance.

MaybeALlama
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  • do you know value of $\frac{d\alpha}{dT}$ around RT? Other way, how large is $\alpha(RT+1K)-\alpha(RT)$ relative to $\alpha(RT)$? – aaaaa says reinstate Monica May 07 '15 at 23:55
  • Off the top of my head I can't think of $\frac{d\alpha}{dT}$ around RT. But if I understand the second part you are basically asking how much bigger x(a+b) - x(a) is than x(a). Right? If I understand the problem correctly it should be relatively trivial. – MaybeALlama May 08 '15 at 00:26
  • my question is how large alpha changes when temperature changes. is it like 0.001% difference between 24C and 36C or not? – aaaaa says reinstate Monica May 08 '15 at 00:29
  • You can rewrite $[\alpha]^T_\lambda = \frac{\alpha}{lc}$ as $[\alpha]^T_\lambda = \frac{\alpha}{l\rho}$ So I would, personally, solve the problem by figuring out the relationship of the substance has between temperature and density (since you should have a direct relationship between temperature and density). I hope this puts you on the right track. But when I used these equations I used tables for the specific rotation. – MaybeALlama May 08 '15 at 00:39
  • i am more interested in real-world data,i can figure out the math :) – aaaaa says reinstate Monica May 08 '15 at 00:44
  • Well the math will tell you the relationship. But I just dug out my notes from the experiment that I did (back in Jan 2014). I used a HeNe laser and a green (543nm) laser (which worked better) to test concentration of sugar in water. Eyeballing the data, it looks linear in the range 35C-70C. Small range, remember that $\rho \propto T$. That's all the data I have. I do know you'll find better data appending a google search with "filetype:pdf" to get papers. – MaybeALlama May 08 '15 at 00:52
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The temperature associated with a measurement of the specific or molar rotation of a given substance must be specified. Thermal volume changes or alterations in molecular structure (as induced by a temperature change) are capable of producing detectable changes in the observed rotations.

Santosh Chhajed
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