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experiments on salt emission spectra


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Hello all,


Picking up on Hermann Trude's idea (in a recent post that I was unable to find though) of watching atomic emission spectra of salt by burning them in ethanol/water solutions, I got the following results:


- the pure ethanol/water flame was blue

- CaCl2 gave characteristic red flashes

- NaCl yielded an orange flame, not much different from the orange color of a candle's or matche's flame

- neither KCl, nore BaF2 (which is insoluble in water by the way) yielded any perceptible changes from ethanol's blue flame.


Do these results seem right to you ? Would you suggest any other salts ? I'd love to see some green, or pink, for example ...


Thanks !



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Hmm, I would think the barium salt would give a green flame (like barium salts typically do). Weird that yours didn't. KCl should give hints of purple with orange.


Now, the way I would do it would be to soak strips of cardstock in solutions of different ions (Na, K, Ca, Ba, etc.) and let them dry out, then burn them to get the flame colors. Copper will also give green; copper with chloride gives blue or blue-green. Not sure about the pink, though ;)


EDIT: One thing I forgot is boric acid. Boric acid with methanol gives green flames, and boric acid with ethanol (I think) gives light blue flames.

Edited by Melvin
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BaF2 is close to insoluble in everything. How do you expect it to color the flame if none of it will dissolve?


Red from calcium? Orange from sodium? Huh? Calcium gives a nice orange in my experience and sodium is the ubiquitous yellow..


You'd be better off spraying the solution through a flame, that way ensuring that the coloring agent is around when the alcohol burns. In a still dish, you're burning the fumes and the only way the coloring agent gets into it is by spattering.


Cardstock burns yellow and is going to wipe out any other spectral lines you might see. A fused bead of salt on the end of an iron wire held in an alcohol flame should give you better emission for the difficult ones.


Boric acid in methanol is great, because it forms trimethyl borate in equilibrium, which is volatile and can really color the flame a nice minty green. Ethanol is significantly less impressive and tends to give a mixed color flame of orange, blue, and green with boric acid, according to woelen.


A good way to show off the purple line of potassium is to mix a bit of potassium chlorate and sugar and light it. This isn't really an indoors demonstration though.


You won't see pink from anthing. Try some lithium for a nice red though.


Copper acetate in methanol or ethanol is moderate for giving blue greens, but it's somewhat hard to tell the diffference from the alcohol flame.


Indium gives a gorgeous, oddly saturated blue color, but it's not an everyday compound and teasing the color out is fairly hard. I think it is visible when I was burning a small sample of the metal in a propane-air flame.

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I'm planning to make this into a demo... currently my idea is to first start with the usual platinum loop and bunsen burner and then introduce the ethanolic solutions so people can see it better, talk a little bit about the reasons for the emissions and then hopefully demonstrate it using a spectrophotometer made from a CD, a cardboard tube and a webcam.

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