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Author: Subject: How to view spectrum of aqueous solution ?
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[*] posted on 29-10-2015 at 12:42
How to view spectrum of aqueous solution ?

I found on this forum a photo of a spectrum of a salt:

I have this spectroscope:

How can I view a spectrum with this one ? I tried an Na2CO3 solution backlit by a cool white LED lamp (continuous spectrum) but no lines were visible.

What is the trick ?
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[*] posted on 29-10-2015 at 14:04

Would sodium carbonate even absorb any light in the visible region?

As below, so above.
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[*] posted on 5-11-2015 at 13:50

It's a colorless solution, so no, it doesn't.

It would be possible to measure absorbance at UV range.
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[*] posted on 6-11-2015 at 10:09

Were you referring to my holmium sulfamate spectrum (the link is broken for me)? You'd probably have an easier time seeing the spectrum of something colored - then you can easily identify absorption bands. You can also view the spectrum of a premixed flame which has clear emission bands.

Raney nickel can't hydrogenate dank memes.
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[*] posted on 6-11-2015 at 12:26

Firstly, a LED lamp is not a very good choice for spectroscopic purposes. It has strong emission bands and produces hardly any light at other wavelengths. You want something that produces more of a continuum, not bands or even sharp peaks (such as fluorescent lights).
Commercial spectrophotometers commonly use a tungsten incandescent light. A simple common halogen light is much better than a LED light for your purpose.

Secondly, even if there are absorption bands in the spectrum, you will find it difficult to see them unless you use a rather concentration solution of an intensely colored compound. You may be able to see sharp absorption bands that produce a really dark band, but it will be difficult to see a little 'dip' in intensity in an otherwise bright spectrum. I think you should start with a solution with an absorbence of at least 2 at a certain wavelength (ie. transmittance of 1%).

Thirdly, sodium carbonate solutions are colorless. So, there is no strong absorption in the visible part of the spectrum. Try something colored. Permanganate, dichromate, copper sulfate etc etc.

"If a rocket goes up, who cares where it comes down, that's not my concern said Wernher von Braun" - Tom Lehrer
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