Sciencemadness Discussion Board

Copper flame colour

Bedlasky - 3-11-2020 at 20:26


A long time ago I did some flame tests on copper wire. And I found very strange behavior. When I dipped oxidized copper wire in to HCl and than put it in to flame, it firstly emits green colour, than blue and finally orange! When orange colour faded wire stop emit light. Final product of decomposition was CuO because copper wire was black. I read that CuCl2 decompose at 1000°C in to CuCl and Cl2 and that Cu+ emits blue colour in flame. CuCl decompose at 1490°C. So maybe orange colour was emit during decomposition of CuCl? Or it was caused by catalytic oxidation of Cl- in to Cl2 (CuCl2 catalyse oxidation of HCl in to Cl2). But this sounds unlikely because orange flame appeared in the end of emission - if this is caused by oxidation of Cl-, orange colour should start at beginning.

I did today few other tests. I made [CuCl2]- solution from CuSO4, HCl and ascorbic acid. I put some anodized aluminium wire in to this solution and than put it in to the flame. Flame had green + blue colour, exactly as with copper wire. But without orange flame.

Than I cleaned wire in HCl and put it in to CuSO4 solution. Than in to flame. Flame had green colour.

What cause orange colour of flame I don't know. Any ideas? Look at video, blue colour merged with green, but orange is good apparent.

Edit: Maybe there is other explanation for orange flame - CuCl and CuCl2 vaporized from copper wire in to the air and than decomposed in to metallic copper and chlorine. Finely divided copper quickly reacted with oxygen to form CuO.

[Edited on 4-11-2020 by Bedlasky]

metalresearcher - 3-11-2020 at 23:26

Was it orange or bright yellow ?

In the latter case, the wire or plastic was probably contaminated with sodium. Sodium ions are very 'aggressive' in disrupting frame color.

EthidiumBromide - 4-11-2020 at 00:04

I can confirm this happens with CuCl2 on copper wire. 99% pure CuCl2 was used, the copper wire was bright and shiny, so any contamination from anything other than Cu and Cl- can be ruled out.
Besides, this orange isn't simply the flame changing color. It's more of these thin, wispy orange ribbons or streaks, besides the blue/green flames (sodium contamination would just change the color of the entire flame without these "ribbon" effects). Judging by the video, you got this very same result, Bedlasky.
I'd love to have a piece of platinum wire to test the reagent grade CuCl2 for the flame color it would give. Unfortunately I hesitate to buy anything platinum, I just can't justify buying something that expensive with my current budget, even if it can last forever.

Since this happens with CuCl2 on copper wire, maybe it has something to do with Cu(I)? Time to prepare some CuCl powder and test what it does in the flame. Of course, I invite you to do the same, Bedlasky

woelen - 4-11-2020 at 00:27

Another reason for orange flame color may be formation of very small solid particles at the rim (cooler parts) of the flame. Solid particles which are hot emit (at least approximately) so-called black-body radiation and this has a color ranging from dull red to white hot, with orange colors in the lower mid-range. This is the reason why nearly all fires emit orange light. Smoke particles give the orange light. The greens and blues from many fires are coming from emission of light from gaseous (possibly ionic) compounds.

CuCl and CuCl2 are somewhat volatile at red hot temperatures and in the cooler parts of the flames it may condense to solid particles again, which then give an orange glow.

[Edited on 4-11-20 by woelen]

zed - 4-11-2020 at 03:51

Flame? And, what pray tell, is the source of that flame? Some kind of natural gas? Propane?

That would probably make your orange color, glowing carbon particles, due to incomplete combustion.

Bedlasky - 4-11-2020 at 05:14

Metalreseacher: Orange. But this isn't certainly sodium, because flame would be orange from beginning. This orange flame was more like a narrow high singular peak at the end.

EthidiumBromide: [CuCl2]- is technically solution of CuCl in HCl. So I tried it. I observed blue and green flame, but orange not. I think that elementar copper affects this phenomena. I don't have Pt wire, so I can't try it on unreactive surface.

Woelen: This sounds reasonable. But why this happened only on copper wire and not on anodized aluminium wire?

Zed: Natural gas, so mostly methane. Flame alone have clear blue colour without any orange flashes.

zed - 4-11-2020 at 05:54

Ummm. The question we used to ask is: Why is the flame blue?

And I'm still thinking, the orange color, is glowing carbon particles.

Perhaps, if you place a white ceramic dish plate above your experiment, you will detect traces of soot.

TheMrbunGee - 4-11-2020 at 06:06

I do not know why but many (if not all) metals tint the flame orange at some temperature (when they get hot enough). I did not understood if orange flame disappeared at one point while still heating? That would be weird and worth investigating.

EthidiumBromide - 4-11-2020 at 06:34

But this is not a tint of the flame. These orange wisps briefly form when heating the CuCl2 on copper, but die down when the flame returns to normal. So it can't be the metal alone that's causing this. I wish I could provide a good photo of this but I can't. This single photo I found online vaguely shows what it looks like:

main-qimg-080314455228de552de0f438c540026d.jpg - 26kB

Notice that aside from the blue/green tinted flame there is an orange streak in the middle. These streaks can be much taller than the flame itself, even 10-15 cm above it if a large quantity of CuCl2 is used.

The only question is what are these streaks? I have never observed something like this happening in the case of other flame tests (Li, Na, Sr, B, CuSO4 and so on), both chloride and copper need to be present in some form (either CuCl2, CuCl or Cu dipped in concentrated HCl).
Maybe Woelen is correct in suggesting this might be the CuCl2/CuCl vaporising off the surface of the copper in the flame and this vapor/smoke causes the orange glow.

[Edited on 4-11-2020 by EthidiumBromide]

Bedlasky - 4-11-2020 at 08:19

Zed: I don't think that this is carbon particles. When I put wire in to flame, it firstly glows green. After a while there was also bright blue flame (along with green). After some time there was really tall and narrow orange flame, taller than rest of the green/blue flames. Than all colours dies and there was just burning gas and red glowing copper wire. If orange colour was due to carbon particles, it glow orange all the time and don't form this narrow tall flame.

TheMrbunGee: Orange colour disappeared (along with other colours) at once. Orange persisted little bit longer, but not too much.

EthidiumBromide: I think that Woelen's explanation sounds reasonable. But why didn't do this my [CuCl2]- solution on anodized aluminium wire? It have both - copper and chloride, but it didn't emit orange light. I'll try it again today, maybe I found rod from synthetic sapphire, this is good and inert material for this.

TheMrbunGee - 4-11-2020 at 11:38

If those are just streaks - I suggest those are tiny particles of CuO flaking off and heating up to top temperatures quickly.

Bedlasky - 4-11-2020 at 16:37

So I did few other tests. I dissolved some CuSO4 in conc. HCl. I used 3 different materials for flame test. I dipped wire (or rod) in to [CuCl4]2- solution and than put it in to the gas flame.

1. Anodized aluminium wire

2. Copper wire

3. Sapphire rod

1. Green and blue flame without orange tall flame.

2. Green and blue flame, after some time tall orange flame appeared. At the end it turned more in to red.

3. Green and blue flame, after some time at higher parts of gas flame I saw red glowing, which became little bit more intense with time.

Anodized aluminium probably reacted with Cu2+ even through oxide layer. This probably cause absence of orange/red glowing.

So Woelen and TheMrbunGee are probably right. Orange/red glowing is caused by some glowing particles of copper or its compounds (CuCl2, CuCl, CuO, Cu - this is hard to tell, it can be mix all of these compounds).

Bedlasky - 9-11-2020 at 17:49

I did some writeup in czech and in english