smaerd
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Electrolysis of bismuth and oxide layers
So I played all day with molten bismuth. Admiring its oxide layers and the wonderful colors that they have. Then I thought hey I wonder if I could
remove those oxide layers via electrolysis.
Experimental:
3g of NaCl was added to 200mL Distilled water. Aluminum cathode was used the oxidized bismuth metal as the anode. Ran 12v and about 20A through it and
within seconds all of the coloration of the oxide layer was completely gone.
Conclusion:
Pretty dramatic effect to witness. Hehehe
Next experiment: Putting the oxide layers back onto the bismuth.
Experimental:
The cathode for the anode were swapped from the previous experiment. All that was noticed was black splotches on the bismuth and degradation of the
aluminum. I also used a much lower potential 5V still a ridiculous amount of current about 20A. Still only black splotchy tarnishes.
Next a significantly lower current 5.5V and 1.5A power supply was used. There was little effect after twenty minutes on the bismuth. The mucousy
looking aluminum oxides clung slightly to the metal, the experiment was canceled.
Next an iron electrode took the place of the aluminum electrode. The beaker was cleaned out and fresh water as well as NaCl was added. 5V 1.5A PSU was
used again. The bismuth metal completely turned black(not splotchy). The electrolyte turned orange. The black layer was hard to remove and was
scrubbed and sanded off.
Tried using a paper salt-bridge and 12V 20A. No change was noted on the bismuth after 20 minutes.
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Wondering if anyone has any ideas about this or if it's even possible. Should I adjust the electrolyte's acidity?
[Edited on 17-1-2013 by smaerd]
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elementcollector1
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Use bismuth for both electrodes? Just a guess...
Elements Collected:52/87
Latest Acquired: Cl
Next in Line: Nd
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smaerd
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Tried two bismuth electrodes. I was able to get a slightly blue tint although it looks like it was painted on with a dull paint and not really shiny
as I had hoped. It's probably not possible to get that lustrous oxide shine by electrolysis.
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elementcollector1
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My bismuth is currently that dull gray you described, are you sure using it as an anode gets rid of that stuff?
Elements Collected:52/87
Latest Acquired: Cl
Next in Line: Nd
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Bezaleel
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I'm sorry if I missed someting very basic here, but isn't it peculiar that the oxide layer is removed if bismuth is used as the anode? Since the anode
is positively charged, it will attract either O2- or Cl- in this experiment. Then what reaction occurs that removes the oxide layer?
6 Cl- + Bi2O3 --> 2 Bi + 3 Cl2O + 6 e- ?
O2- + Bi2O3 --> 2 Bi + 2 O2 + 2 e- ?
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smaerd
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Good question I also found this slightly counter-intuitive. I re-ran the same initial experiment with a salt bridge separating the catholyte and
anolyte to be sure it wasn't an electroplating effect. Same effect only slightly slower(color is stripped).
I know bismuth oxychloride is formed with excess water and HCl. Though I haven't seen much evidence of this. I really don't understand much of what
is happening then again I haven't really studied bismuth chemistry to any appreciable means.
Element collector I mean the blue, magenta, golden, etc colors get stripped to what appears to be the elemental metal. Here are two awful pictures to
demonstrate. The first one is the blue pretty bismuth oxides, the second is the partially electro stripped material. (note black cable for
anode unless I wired my PSU wrong which I doubt...)
 
Maybe it has to do with the way the oxides are layered generating the different spectrum of light adsorption. Then when I apply the current it fully
oxidizes them removing the color. The metal doesn't look dull or gray neccesarily but it could be more dull then fresh metal.
Here's an upclose shot of bismuth metal(not mine - http://www.flickr.com/photos/paulslab/7087302199/)
So it would make sense that it generates a prism like effect on light with it's oxide layers. hmm...
Under this premiss maybe if I try a steel mesh type cathode and a bismuth anode I could get the colors so long as the distance isn't far. I'll give it
a shot and report back.
edit - Steel mesh didn't really work. Though doing a repeated slow dipping motion with the bismuth metal I was able to obtain the goldenish hue on
some parts, maybe if I lowered the current the gradient type effect would work better...
I'll also cast a nice bismuth electrode or two and play with more honest attempts at achieving this effect.
[Edited on 21-1-2013 by smaerd]
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Sublimatus
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Use 0.1 M NaOH solution as your electrolyte.
Varying the current and distance between the electrodes will dictate how quickly the oxide films form. The change in color should begin at the tip of
the crystal and then progress up toward your power source.
The colors will cycle through in a similar progression as in the image below (from left to right) until the oxide film becomes too thick to give you
interference colors.
I've never been able to achieve true reds, as seen in the large crystal featured on Theodore Gray's site. This has always perplexed me.
The oxide film will be unlike what you see with usual crystals, in that it will mostly be a flat color, rather than highlighted with different colors
in the recesses and along the raised edges. I never got around to trying it, but I suspect you might be able to control that by adding some sort of
resist wax onto the surface of the crystal.
I should apologize for not speaking up sooner about what little bit I know about this subject. There's still some small part of me that is sure I'd
one day become rich selling high quality bismuth crystals with the best definition and color.  The good methods of crystal growth and color are generally kept secret by those in the know, from what I can tell, but
this is counter to the principles of a scientific spirit.
Edit: Also, to get a colorless crystal to start with just take a colorful crystal and dip it in dilute hydrochloric acid. The color will disappear
instantly. While you can remove the color with electricity, doing it too many times can lead to ugly pitting on your crystal.
[Edited on 1/21/2013 by Sublimatus]
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smaerd
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Thanks for the advice sublimatus! I have seen crystals with such vivid pinks in them I almost wonder if they are doped with some kind of colorant.
I'll do this soon.
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Sublimatus
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Having played with bismuth and electrolysis for years, I am convinced there are methods I must be unaware of. That, or I don't have the best bismuth
one could get.
Take for example some of the better crystals on bismuthcrystal.com. Some are truly unbelievable, in terms of definition in the forms, and variance in
color. The man is a wizard.
That, or it's some funny photography. You can get some really deceiving photographs of bismuth, especially when flash is involved. The picture
below, taken by my friend, is demonstrative of this fact. The bismuth looks amazing, but the color is simply not distributed in that fashion in real
life. The crystal looks more a metallic blue, which shifts colors as you turn it in the light.
In any case, good luck. Bismuth is a chaotic creature. One moment you think you have it figured out, and then it starts misbehaving. Especially
when it comes to growing crystals, I think the system is highly sensitive to initial conditions. So much so that I'm of the opinion that it is
impossible to control in any meaningful way.
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AndersHoveland
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The brilliant colors of a bismuth crystal are due to the thin oxide layer, which has a dichroic optical effect. The intricate pattern of the crystal
is caused by the unique phenomena that bismuth is deposited more rapidly onto the corners of the metallic crystal than the crystal faces.
Bismuth oxide ceramic is an oxygen-ion conducting solid when it is heated (it does not have to actually be molten to conduct electricity).
I am not sure that bismuth can electrolytically separated in water. Most metals cannot.
[Edited on 22-1-2013 by AndersHoveland]
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Sublimatus
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Could you elaborate on what you mean by:
| Quote: |
I am not sure that bismuth can electrolytically separated in water. Most metals cannot. |
Bismuth can be recovered as pure metal by electrolysis. Electrolysis of bismuth trichloride in aqueous solution with graphite electrodes yields
chlorine gas and a spongy black material. Collecting this black material and heating it with a torch shows that it is fine bismuth metal.
The real trick is collecting it and melting it without loss. I've found that a pan of finely divided powder claimed by the method above left out will
suddenly begin to exotherm upon drying. Heating it with a torch yields yellow bismuth trioxide. This leads me to believe that the powder turns to
grey bismuth oxide hydroxide, which becomes bismuth trioxide up heating.
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MrHomeScientist
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hkparker had some curious experiences with making bismuth crystals some time ago - he's got a <a
href="http://www.youtube.com/watch?v=uRyPG6iJyjI">video</a> of it on his channel. His crystals always turned out metallic - no oxide layer at
all! He eventually uncovered the source was tin contamination, which somehow prevents the colors from forming but doesn't interfere with crystal
growth.
That's really interesting that you can remove the layer with electrolysis, that never occurred to me! Both that and reversing the current to put the
colors back on are very interesting concepts. I'm definitely trying this out on some of the crystals I've grown.
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