| (OT) More weird chemistry
August 4 2003 at 1:18 AM | Tom B. (no login)
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| I've been playing around with electrochemistry, looking for ways to make photosensitive stuff. One experiment of interest was electrolysis of a 2% Potassium Bromide solution with silver electrodes. When voltage was applied, a film of yellow stuff immediately began to collect at the anode, then later it became messier with all sorts of yucky grey stuff mixing in. But the thing was, I thought the initial yellow color seemed very bright, as though it was producing light. I Googled around and found this intriguing bit:
http://www.sas.org/E-Bulletin/2001-09-15/chem/column.html
When I repeated the experiment (with silver, not mercury) in a darkened room, I saw no light emission at all, but the current density (~200 mA) and KBr concentration was MUCH lower than described in the Klooster demo. I tried shining a UV penlight on the yellow stuff and I'll be darned if it didn't darken after a few seconds of intense exposure. So at least maybe I'm seeing some silver bromide formation. Might be fun to see if I can get electroluminescence from this setup. But anyway it's an interesting way to make silver bromide if you have no access to silver nitrate.
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| | Author | Reply | Martin
(no login)
| | Re: (OT) More weird chemistry | August 4 2003, 6:08 AM |
"But anyway it's an interesting way to make silver bromide if you have no access to silver nitrate."
Yes, definitely. We, Joe (Farina) and I, had been looking around for that possibility. It appears this method goes back at least to a certain German guy named Vollenbruch, who was able (~ 1904) to produce Lippmann emulsions on such grounds. Of course, the whole thing seems to be strongly inspired by Becquerel.
There are also a couple of patents to cover electro-chemical formation of silver halides. My search program being "down", I am experiencing problems in searching through the 3 000 patents I had downloaded. However, I remember three very interesting patents hold by Polaroid:
US 4060419
US 4102759
US 4102758
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| Tom B.
(no login)
| | Re: Re: (OT) More weird chemistry | August 4 2003, 7:51 PM |
Interesting - The difference in my setup is that it uses silver for both anode and cathode with all the halide supplied by the electrolyte. The only activity I see at the cathode is bubbles of gas (hydrogen perhaps). No apparent addition or removal of silver at the cathode. I suspect that as the bromide becomes exhausted, conventional water electrolysis takes over and oxygen is generated at the anode, leading to (gray-black) silver oxide/peroxide formation at the anode.
Free patent info:
http://patft.uspto.gov/netahtml/srchnum.htm
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| Dinesh
(no login)
| | Re: (OT) More weird chemistry | August 4 2003, 8:25 AM |
"Fascinating!" You say you got "..a film of yellow stuff..". What did you dissolve the KBr in? It sounds like maybe the yellow stuff was Sulphur from sulphides dissolved in the solvent. However, this yellow stuff does not seem to be the yellow glow reported in the Klooster demo. Why would the Mercurous Bromide give off light? Is it the interface between the HgBr and the Hg causing a band gap? The (exothermic) reaction energy given off as light? Triboluminescence? (Light caused by pressure - you sometimes see this by crushing a sugar lump in darkness). Could this work with other materials to choose the color of the electro-luminescence? What's the bandwidth? Sounds like a good way of getting a safe-light.
I used to press film onto glass plates by squegeeing really hard. When I peeled the film off the glass slowly, sometimes there would be a crackling glow between the film and glass. I assumed this was some form of triboluminescence.
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| Tom B.
(no login)
| | Re: Re: (OT) More weird chemistry | August 4 2003, 7:15 PM |
The solution was 1g KBr in 50 ml distilled water, so no sulfur was present. I think the peeling tape glow phenomenon is due to electrolumenescence.
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| Martin
(no login)
| | Re: Re: Re: (OT) More weird chemistry | August 4 2003, 9:11 PM |
I have had a look into Frieser, "Die Grundlagen der photographischen Prozess mit
Silberhalogeniden", Frankfurt 1968 (a 1500 p book about silver halides). It mentions low
levels of electroluminescence of silver halides. The effect seems to be strongly related to the
purity of the silver. With increased silver purity this effect is said to stop completely.
What also comes to mind in this context are photo-voltaic cells based on silver-silver
bromide (in the late 20s one of the old Kodak gurus, E. Sheppard, made extended research
on that issue).
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