Sciencemadness Discussion Board

Why do Mg salts not color the flame ?

metalresearcher - 22-1-2021 at 12:58

Weird question.
Salts of Ca, Sr, Ba color the flame resp. orange, red, yellowish green. Same applies to the alkali metals.

https://upload.wikimedia.org/wikipedia/commons/4/45/Magnesiu...

And the spectrum of Mg has a clear green line.
So why do Mg salts held in a flame not show a greenish color ? And Mg does burn with bright white rather than green ?

[Edited on 2021-1-22 by metalresearcher]

[Edited on 2021-1-22 by metalresearcher]

Junk_Enginerd - 24-1-2021 at 10:26

I'm no expert on this but my reasoning is: Yes, there's a clear green line. Also lots of blue ones and also red ones. Red+green+blue=white.

Our sun is also mainly green, but looks pretty white.

The white when magnesium burns is probably more related to the extreme temperature at which it burns. Copper metal also burns white. It's so hot and bright that we can't really see any color.

DraconicAcid - 24-1-2021 at 11:40

When magnesium burns, it's mostly blackbody radiation (although I've long thought there was a hint of green in the light).

As for not colouring the flame, I'm not sure. But there are a lot of metals that don't colour flames noticeably, even though they should give interesting spectra. Maybe the flame just isn't hot enough?

ETA: This is a spectrum I took of burning magnesium with a cheap plastic spectrometer (one step up from the paper/CD pocket version). Lots of blackbody radiation, some sodium and lithium impurities, and a significant green line. I honestly don't know what that cyan line is- it's not a magnesium emission, afaicfo.

[Edited on 24-1-2021 by DraconicAcid]



[Edited on 24-1-2021 by DraconicAcid]

3147Mg.png - 354kB

phlogiston - 24-1-2021 at 14:14

Quote:
Salts of Ca, Sr, Ba color the flame resp. orange, red, yellowish green.


Only in the presence of a chlorine donor. The colors come from respectively CaCl+, SrCl+and BaCl+. In fact, barium and calcium metal burn with a bright white light, very similar to magnesium. (Strontium probably also does, I've never seen burning strontium though). There are many pyrotechnics compositions containing barium nitrate or barium sulphate as the main oxidiser that emit bright white light because they do not contain a chlorine donor.

You might ask what color MgCl+ emits, and I don't know. If it exists, and forms in fireworks flame conditions, it can't have a very brightly coloured emissions spectrum because magnesium is the preferred metal fuel in coloured fireworks because its reaction products are more volatile than those of aluminium, and therefore produce less black-body radiation that tends to wash out the color.

[Edited on 24-1-2021 by phlogiston]

unionised - 24-1-2021 at 14:21

Quote: Originally posted by DraconicAcid  


I honestly don't know what that cyan line is- it's not a magnesium emission, afaicfo.


[Edited on 24-1-2021 by DraconicAcid]

Probably a MgO band
https://www.sciencedirect.com/science/article/abs/pii/001021...

Bedlasky - 24-1-2021 at 20:40

Phlogiston: I use strontium nitrate for making red flame. You don't need chloride for this.

Fulmen - 25-1-2021 at 03:16

Some elements work in virtually any conditions, others need very specific conditions. The boiling point of the compound is one factor, as solids produce back body radiation (continuous spectrum) while gaseous species produce narrow spectrum emissions. But since these narrow bands are produced by electron excitation they are affected by chemical bonds.

phlogiston - 25-1-2021 at 03:56

Bedlasky, that is interesting. Can you share the composition?

clearly_not_atara - 25-1-2021 at 07:25

https://physics.nist.gov/PhysRefData/Handbook/Tables/magnesi...

tl;dr: the strong lines of Mg are actually in the UV at 280 and 285 nm, and these are more than ten times as strong as anything in the visible spectrum. this is also part of the reason why burning Mg is so dangerous to your eyes.

oddly i've never heard of a UV lamp that uses this effect

Bedlasky - 25-1-2021 at 16:37

Quote: Originally posted by phlogiston  
Bedlasky, that is interesting. Can you share the composition?


Composition of what? I use just strontium nitrate.

xxxhibition - 26-1-2021 at 10:20

I found out a Mixture of 55% - 60% Strontiumnitrat and 45% - 40% Hexamin
gives you a deep red saturated Flame.
:cool:

metalresearcher - 26-1-2021 at 11:01

Quote: Originally posted by DraconicAcid  
When magnesium burns, it's mostly blackbody radiation (although I've long thought there was a hint of green in the light).

As for not colouring the flame, I'm not sure. But there are a lot of metals that don't colour flames noticeably, even though they should give interesting spectra. Maybe the flame just isn't hot enough?

ETA: This is a spectrum I took of burning magnesium with a cheap plastic spectrometer (one step up from the paper/CD pocket version). Lots of blackbody radiation, some sodium and lithium impurities, and a significant green line. I honestly don't know what that cyan line is- it's not a magnesium emission, afaicfo.

[Edited on 24-1-2021 by DraconicAcid]



I checked burning Mg ribbon again with my spectroscope and saw like you, beside the yellow Na-D line, indeed the green and turquoise lines as well, and when the brightness faded, these two lines appeared better as the white blackbody radiation fainted.


[Edited on 2021-1-26 by metalresearcher]

Metallophile - 26-1-2021 at 11:46

The green and turquoise lines around 520 and 500nm look kind of like Titanium to me. It has a whole bunch of bright(ish) lines from 498-506nm.

https://physics.nist.gov/PhysRefData/Handbook/Tables/titaniu...

DraconicAcid - 26-1-2021 at 12:12

The brightest visible line from magnesium is at 518 nm, which is the green one.

https://physics.nist.gov/PhysRefData/Handbook/Tables/magnesi...