amazingchemistry
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Using UV-VIS to determine purity?
I have found a couple of nice Spectronic 20 spectrophotometers on ebay recently, and I was wondering if besides their use in inorganic quantitative
analysis, they could be used to qualitatively determine the purity of organic compounds. My thoughts were that since these instruments work by
comparison with a blank, one could take a reagent whose purity is known, zero out the instrument, and then look at the sample for any discrepancies in
absorbance/transmittance. Am I correct in thinking this?
Always remember to check your pride at the door and ask that "stupid" question. Learning comes from having your "stupid" questions answered.
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phlogiston
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If you are researching pigments, I guess so but the majority of organic compounds hardly absorb light in the visible part of the spectrum (they appear
white/colorless), and as such cannot be detected in that way.
If you have conjugated systems in your molecule, there will be absorbance in the UV, but that is not very specific. I.e. if the impurities also have
conjugated systems, they will have broad absorbance peaks in the UV as well.
Often, because the tail of these broad peaks extends a bit into the blue part of the visible light spectrum, many such compounds are yellow or
yellowish.
So, in summary, for organic compounds I don't think it work for this purpose unless you are interested specifically in compounds with characteristic
absorbances in the UV-VIS range.
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bfesser
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If you're simply looking for a rough qualitative test for impurities, try melting point.
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Dr.Bob
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Yes, unless you are RB Woodward, the UV-Vis will be useful only for specific tasks. Some organics absorb as certain frequencies, like DNA, which can
be characteristic, but most organics absorb similarly, so impurities might absorb similarly to the desired compound. if you have a solution of a
pure compound, they can be useful for determining concentration, but not so much purity. Look up Beer's law in Google for more info. (The chemist
Beer, not the law about people looking more attractive after frat parties...)
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Rich_Insane
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In the lab where I used to work, we used UV-VIS to calculate concentrations of pigments. Typically, what happens is that you take a chemically pure
sample (from Sigma or something) and perform a serial dilution. The known concentrations are analyzed at a certain wavelength (it might be useful to
do a full-spectrum scan and look for prominent peaks). Linear regression is used to generate an equation into which the absorbance can directly be
substituted to give the concentration.
Unfortunately, when you have a very impure mashup of compounds, you will end up with a messy spectrum, and there is no way of knowing if the given
absorabance is due to possible augmentation or detraction to the absorbance (at a particular wavelength) by impurities.
UV-VIS does work great for reasonably pure, highly colored pigments as well as for measuring the density of cells in culture.
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amazingchemistry
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Beer's Law was what I meant by "inorganic quantitative analysis applications"  I
know you can even determine pH colorimetrically. I was looking for an "extra bonus" to do with organic compounds and UV VIS
Always remember to check your pride at the door and ask that "stupid" question. Learning comes from having your "stupid" questions answered.
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Dr.Bob
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It also works well for "organic quantitative analysis applications", but mostly when the material is mostly pure, but diluted somehow. I am just
saying that it is not much use for qualitative purity assessments, except in very specific applications where you know a lot about the possible
impurities.
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