Sciencemadness Discussion Board - Gas discharges

Gas discharges

indigofuzzy - 10-2-2007 at 01:12

I've been developing a growing curiosity about the colors and spectra created by electrical discharges through gasses. I've seen many pictures of the noble gasses and halogens glowing, but I''m curious about compounds - I've seen water vapor and CO2 discharges,, but I also wonder about things like ammonia, alkanes, alcohols, etc.

So, earlier this evening I was doing an experiment to find out what color a gas discharge through butane gas would glow. (The only gas I have access to until i finish moving) The set up was simple, I had two sewing needles for electrodes, supplied with power form a cold-cathode fluorescent tube power supply, creating an arc approximately 5mm across in free air. To this, i introduced a small amount (less than 1/2 cc) of butane gas (fully expecting it to ignite, i was quite prepared for that)

The good news: I got a lovely cyan arc through the butane. Curiosity satisfied.

The weird part: An odd smell arose when butane was added to the arc. It was a somewhat sour, somewhat "damp basement", unpleasant smell. The smell is much more intense if a weak enough arc is used that the butane doesn't actually ignite.

It may be helpful to note that my source of butane was a cigarette lighter, and I have no clue as to what kind of additives/impurities might be lurking inside. (The same smell does not occur if simply releasing butane into the air, nor does it occur when the lighter is lit. It only occurs with a high-voltage arc passing through the gas.)

So, does anyone have ideas on what may be causing the odd odor?
And does anyone know of some gasses worth running arcs through that create nice colors or spectra?

YT2095 - 10-2-2007 at 02:34

the green color is quite possibly coming from your metal electrodes, possibly copper used in the dip prior to plating .
the smell is hard one as you`d be getting all sorts of random products and even fixing free nitrogen from the air to these products.

indigofuzzy - 12-2-2007 at 02:21

I doubt the green is from copper. I've used pure copper as an electrode before and not seen any green at all unless I heat the copper in a flame to a blazing orange color. The cyan color of the flame was much more "sky blue" than green.

I figured out this afternoon a better way to describe the smell: It's strikingly similar to car exhaust!

I'll post some data about the actual spectrum of emission as soon as I get ahold of a diffraction grating, although the color does look similar to that of the Hydrogen Beta line. If that's the case, (I'll find out when I get my grating) then I'd have to wonder what happened to the alpha line...

YT2095 - 12-2-2007 at 02:34

well you have 656, 486, and 434 and some diminuative ones at roughly 600 region.
the 486 line is just above the blue green and closer to the blue, perhaps that`s the one partly resonsible?
Nitrogen has lines in the 525 area down to 600 also.

unionised - 12-2-2007 at 11:42

Is the arc the same colour (roughly) as the blue flame of a gas cooker? IIRC that's a CO emision. There are also blue bands from N2 an O2+ I think but it's been a while since I looked a the spectrum of air. The smell could be any of a whole collection of partially oxidied components. Butyric acid and butyraldehyde stink.

12AX7 - 12-2-2007 at 14:56

I recall the colors are CHn radicals. Look up fire on Wikipedia.

Tim

indigofuzzy - 12-2-2007 at 23:01

Quote:

Is the arc the same colour (roughly) as the blue flame of a gas cooker?

Yes, very similar color, actually. Like the lighter bluish-cyan part at the edge of the flame.

indigofuzzy - 12-2-2007 at 23:15

After reading the suggested wikipedia article, it makes me wonder if other alkanes would produce the same cyan colored discharge as butane. It really does look like the emission form the CH radical is the dominant color. I'll double check by viewing the spectrum on the surface of a CD.

Once my roommate and I get moved into our new place, I'll have to try propane and (if i can get ahold of it) methane. I should also try acetylene, to see in the triple bonds have any effect on color. I'll post again when I have new results.

My first Spectrum Tube

indigofuzzy - 4-4-2007 at 23:05

My first spectrum finally arrived today

It's filled with elemental Hydrogen, and is beautiful when powered up. I figured I'd share some pictures. I'll be trying over the coming year (as fast money allows) to collect all 18 tubes that I've found online: Hydrogen (check), Deuterium, Helium, Nitrogen, Oxygen, Neon, Sulfur, Chlorine, Argon, Bromine, Krypton, Mercury, Iodine, Xenon, Air, Water Vapor, Carbon Dioxide, and Carbon Monoxide.

Anyway, here are the pictures:

Me holding up my new spectrum tube:

The tube glowing: (I love the two-tone effect)

A photo through my cheap diffraction grating:

I'm thinking Krypton might be my next tube, the pictures I've seen seem to show a purplish-white glow at the electrodes, and a yellowish-white glow in the capillary. (Pretty colors first, round out the collection later)

Twospoons - 5-4-2007 at 00:07

Helium is very pretty - sort of pink/orange. IIRC there are half a dozen emission lines across the visible spectrum. Balloon helium should be good enough for a trial, and easy to get.

Maya - 7-4-2007 at 03:33

Xenon, Krypton followed by Argon are the prettiest in that order...

indigofuzzy - 8-4-2007 at 02:36

I love Xenon! I'm saving that tube for my birthday in august. I have a Xenon strobe tube that can sustain a low-energy blue arc (if used in a manner entirely contrary to its intended purpose

) so I can admire its color and spectrum in the mean time.

So my roommate and I cam up with a crazy idea: After I finish collecting all 18 spectrum tubes. (the 18 I've found online so far), I'm going to assemble them, and some 1/2" copper pipe into a double helix sculpture with the tubes as rungs. It will stand approximately 7 feet (213 cm) tall.

This is (roughly - I'm using a graphing calculator program because I don't have a 3d modeling package!) what the sculpture will look like:

(the colors are based on photos of discharge tubes, and are probably pretty far off, and I'm sure I have too many blue tubes in the model.....)

My roommate and I would be willing to build more sculptures like these upon request, after I have completed this one.

[Edited on 4.8.2007 by indigofuzzy]

12AX7 - 8-4-2007 at 16:31

That would rock.

Remember to put a big high voltage resistor in series with each tube -- they will not share (and light up) together!

Tim

indigofuzzy - 8-4-2007 at 22:14

I was actually going to use a separate neon tube inverter for each tube, and connect them all to a microcontroller, thereby allowing the tubes to chase, fade, and do crazy things like that. (Including, flashing the appropriate tube at the appropriate time to the song "The Elements" by Tom Lehrer....)

indigofuzzy - 5-5-2007 at 10:53

I just got my Krypton tube.

Photos of this tube are coming out looking nothing like reality....

Here's a somewhat color corrected version:

roamingnome - 5-5-2007 at 11:07

In regards to your hypothesis of butane type radicals being formed...

this congers up many proposed reactions. like having two gases passing through your arc and thus reacting/condensing

woelen - 10-6-2007 at 13:43

I have pictures of discharge tubes on my website for all noble gases, hydrogen, nitrogen and oxygen. Really remarkable to see the difference in colors.

Just click on the links for these elements: http://woelen.scheikunde.net/science/chem/compounds/index2.h...

I also tried experimenting with discharges through different gases at normal pressure, using a 8 kV AC power supply. I did experiments with air, O2, Cl2 and CO2. But I always obtained bright blue colors, or bright orange colors, if traces of Na(+) ions are present. The last I had with my CO2 generator from NaHCO3 and HCl. It is amazing to see how difficult it is to get really rid of the last traces of Na(+) ions. Even bubbling through water is not sufficient.

So, indigofuzzy, I wonder how you do your experiments. Are you using reduced pressure? At normal pressure I only obtain sparking and then the result is always the same, regardless of the gas used.

Jdurg - 10-6-2007 at 17:03

I love all my element samples that are discharge tubes. I have every "clear" gas as a discharge tube; Hydrogen, Deuterium, Helium, Nitrogen, Oxygen, Neon, Argon, Krypton and Xenon. The Noble Gases are in tubing bent into the shape of the element symbol. A VERY cool looking display when lit up. I'm buying a house in a few months and will look forward to building cabinets in my future den in order to display these tubes.

I would REALLY love to see the Hydrogen, Deuterium, Nitrogen and Oxygen samples be in discharge tubes shaped into their elemental symbols. Sadly, nobody does that at this point in time.

The oxygen tube is probably the most disappointing as it is incredibly faint and difficult to see. I think a lot of that is because the high voltage discharge makes some of the O2 form O3 instead of giving off light. I wonder if long term usage of this tube will eventually cause it to just completely fail as the O2 forms O3 and the O3 attacks virtually everything.

indigofuzzy - 16-6-2007 at 22:19

I was doing the experiments at normal atmospheric pressure, usually by drawing an arc between a flat metal plate and a sewing needle, or between two sewing needles, with a 3.5kV(or thereabouts) ccft power supply.

An arc through good ol' air is a distinct pinkish-violet color. (Though my diffraction grating doesn't show any distinct lines.) With Butane, the discharge is a pale cyan, with my diffraction grating showing strong green, aqua and blue emissions.

As for the glass tubes shaped like their elemental symbols: This is much easier to do with the noble gasses, because they are monatomic and easy to ionize. A mercury vapor lamp shaped into "Hg" should be quite doable though. As for N2, H2, O2, etc, I've heard that these can degrade the electrodes inside the tubes more so than the noble gasses. It may be more an issue that a big N tube filled with nitrogen might not last very long. But, alas, i may be wrong. Maybe there just hasn't been enough demand for them...

BTW, as a side question, if one were to want to make a fluorine spectrum tube, what could the tube itself be made of? IIRC, F2 would eat holes in ordinary glass. Is any known substance 1) resistant to fluorine, 2) transparent, 3) hard enough to withstand 1atm outside it, and about 1/700 atm inside? The only thing that comes to mind is CaF2.... Hmmm. Well, it would complete the halogens (which all seem to glow blue....)

[Edited on 6.17.2007 by indigofuzzy]

[Edited on 6.17.2007 by indigofuzzy]

not_important - 16-6-2007 at 23:02

sapphire - fused Al2O3 - will form a thin layer of AlF3 on exposure to F2, and resist further reaction, and the Al2O3 is certainly strong enough and temperature resistant enough to function for this application.

Perhaps electrodeless discharges are needed, get rid of the electrodes to reduce if not eliminate the corrosion problem.

[Edited on 17-6-2007 by not_important]

Nerro - 17-6-2007 at 01:19

God, I would love to see molten alumina at least once during my life. It's just like thermite, you jsut have to have seen it once

Does Deuterium give a slightly "redder" emission than 1H? My reasoning is as follows, the extra neutron shields a portion of the nucleus' attractive force which allows the electron to be excited just a little bit easier, which would mean the emission would be slightly less energetic and hence "redder".

And don't any of these gasses emit dangerous amounts of UV radiation? (just curious for safety concerns here)

[Edited on 17-6-2007 by Nerro]

12AX7 - 17-6-2007 at 07:46

Deuterium's higher mass causes a slightly higher reduced mass of the nucleus-electron system (assume the electron is orbiting as in the Bohr model). A proton is already 2000 times more massive than an electron, while the deuteron is about 4000 times. Therefore, the difference in anything observable is 1/2000 - 1/4000 or about 0.05% or so. The difference is as narrow as fine structure splitting and needs a good spectrometer to measure.

The 1st series (Balmer??) spectral lines of hydrogen are high energy / UV. The highest, up to 13.6eV, will be blocked by glass, but quartz will pass radiation down to 200nm, some of which should be present.

Tim

[Edited on 6-17-2007 by 12AX7]

Jdurg - 18-6-2007 at 05:40

I believe the Krypton tubes also emit UV light when they are on. Still, I don't think it will be a HUGE concern unless you plan on leaving these tubes on for any extended period of time. At that point, I'd be more concerned with the heat given off by the discharge tube causing damage to the tube itself than I would be about the UV light.

Indigo - with regards to H2, D2, N2 and O2, you really don't have to worry too much about degredation of the electrodes. The amount of gas inside the tubes is so fantastically small that the electrodes would have to be frighteningly tiny in order for them to degrade to any appreciable extent. What is a problem is that these gases emit far greater amounts of heat when the discharge is placed through them so you get more of a mechanical failure of the tube rather than a corrosion based failure. This is why for all discharge tubes of these gases you should never leave them on any longer than you absolutely need to. You don't want the tube to fail due to mechanical stresses caused by the heat given off from the gas inside. O2 is the most troublesome because high energy electrical discharge will turn 3 O2 molecules into 2 O3 molecules which might react pretty rapidly with the glass and/or the electrodes. Not to mention that O2 gives off a very weak light when excited. Again though, the corrosion on the inside should be of little consequence as halogen discharge tubes have some fairly corrosive materials inside and they are pretty sturdy.

indigofuzzy - 19-6-2007 at 04:57

Quote:

Originally posted by JdurgIndigo - with regards to H2, D2, N2 and O2, you really don't have to worry too much about degredation of the electrodes. The amount of gas inside the tubes is so fantastically small that the electrodes would have to be frighteningly tiny in order for them to degrade to any appreciable extent. What is a problem is that these gases emit far greater amounts of heat when the discharge is placed through them so you get more of a mechanical failure of the tube rather than a corrosion based failure. This is why for all discharge tubes of these gases you should never leave them on any longer than you absolutely need to. You don't want the tube to fail due to mechanical stresses caused by the heat given off from the gas inside.

Aha! So that's (part of) why the Noble gasses are so special inside tubes. More light, less heat.

Anyway, I ran across a few sites selling spectrum tubes, and have collected pictures of some of the discharges. (H2, D2, He, Ne, Ar, Kr, Xe, Cl2, I2, O2, S8, N2, Hg, CO, CO2, and Air) I was wondering if anyone had some pictures to fill in the gaps for me, or could describe what color some of the discharges are for: Br2, H2O, and NH3.

Sidenote: the range of prices is startling. some places want as little as $12.99(USD) per tube, while others want as much as €55.00 ($73.72 USD)

I'll be getting my next tube in early July -- Mercury Vapor. (And my best friend is getting me Xenon for my birthday

)

12AX7 - 19-6-2007 at 16:04

I've seen what is presumably water vapor, though much may also be hydrogen and air. Namely, the discharge at the electron gun end of my college's proton "accelerator" (500keV can hardly be called a college-level accelerator :rolleyes: ), which had a few microns of impurities (adsorbed water, leaking hydrogen, residual air, etc.). Likely, H2O readily dissociates to H, OH and other bits, as well as H2 and O2, but all perhaps happening in the near infrared band so that all you see in the visible spectrum is the lines of a mixture of H and O atoms.

I'd like a mercury vapor quartz tube, just because UV-B is energetic enough to kill stuff (i.e., cause chemical reactions).

Tim

YT2095 - 20-6-2007 at 00:25

has anyone ever seen a Copper Bromide discharge tube?
if ever you do, I`de recommend getting one

12AX7 - 20-6-2007 at 14:29

Heh! Don't forget the tuned mirrors on the end...

Tim