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Author: Subject: Arsenic from Skutterudite
elementcollector1
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[*] posted on 17-5-2014 at 11:50


I now have fairly conclusive evidence that copper is the noted impurity.
For starters, the green solution from earlier was filtered and mixed with very concentrated NaOH solution. This produced a yellow precipitate at first, but upon addition of more NaOH this darkened to a green and then brown precipitant, finally settling at dark brown. (Likely CuO.)
Then, upon filtering the solution, it was first yellow, then brown (these are likely colloidal colors), then green (same olive green as starting), and now it's dark blue, leading me to suspect sodium cuprate. Another possibility is chromium, yet although being more common in minerals like skutterudite it doesn't explain the brown colors.
I suppose I'll have to reacidify this with HCl until the mess of arsenic oxide and copper hydroxide precipitates, then... do something to separate those two?




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[*] posted on 18-5-2014 at 05:26


If there really is copper present, take a small amount of the original solution and add an excess of strong ammonia to it. The deep blue of copper (II) ammine complex is hard to mistake.

Copper would initially precipitate with NaOH to blue Cu(OH)2, which in very strong NaOH will dissolve to deep blue cuprate. CuO would only form on heating of the Cu(OH)2. The green could be explained by tetrachlorocuprate (but only if there's a lot of chloride in the solution).

You should also test for iron with K (or NH4) thiocyanate.




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[*] posted on 18-5-2014 at 09:24


The solution was extremely hot when precipitation was happening, likely due to NaOH neutralizing tons of excess HCl (of which there was assuredly plenty).
Don't have any thiocyanate, so that's a no-go.




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[*] posted on 18-5-2014 at 14:43


Results are in - this is not copper. Ammonia had no effect on the brown precipitate (which, if CuO, should have reacted immediately). The only other candidate I can think of is chromium, but given the prevalence of blue and violet colors at high pH, this seems so unlikely. In addition, to my eye, the green solution and the ammonia made a yellow solution, possibly indicative of chromate. Uugh, it only gets worse!
Testing on whether the pH~7, yellow liquid resulting from the green solution and sodium hydroxide - HCl combinations is pending. In the meantime, I thought some of the insoluble residue from the initial nitric acid dissolution that was mixed in with the white arsenic oxide might be elemental arsenic, so I plan to attempt to sublime some of this powder.
I think I might have to restart this with a new sample of skutterudite - I have so many solutions resulting from the initial dissolution that I frankly don't even know what happened to the arsenic anymore!

EDIT: The insoluble residue tests positive for arsenic and has a density of ~5g/mL, within acceptable bounds for As's 5.727 g/mL given my sloppy techniques. The only thing left to do for this is test whether the initial ore has the same density (which it well might!) According to MinDat, skutterudite has a measured density of ~6.5g/mL, so I might have to use some more refined techniques... Given that I only have 0.5g of the unknown substance, and significantly more of the initial ore (a few g remaining), I'd like to know if there are any more accurate volume measurements than water displacement.


[Edited on 5-18-2014 by elementcollector1]




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[*] posted on 19-5-2014 at 04:52


Quote: Originally posted by elementcollector1  
Results are in - this is not copper. Ammonia had no effect on the brown precipitate (which, if CuO, should have reacted immediately).


I'm not so sure about that: freshly precipitated CuO will be more sluggish towards NH3 than freshly prepared Cu(OH)2. Could cause a false negative. It would be better to test straight on the solution (neutralising the HCl with NH3 and then add excess).

Chromium: alkalise and add peroxide. A yellow/orange colour that causes precipitation with Pb(NO3)2/Pb(OAc)2 would mean Cr(III) (to Cr(VI)).




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[*] posted on 19-5-2014 at 05:20


Per the test for copper: I did that too. Solution turned clear, then yellow, no precipitate whatsoever.

Will get back to you on the other stuff.




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[*] posted on 19-5-2014 at 09:09


Quote: Originally posted by elementcollector1  
Per the test for copper: I did that too. Solution turned clear, then yellow, no precipitate whatsoever.

Will get back to you on the other stuff.


The yellow is bizarre. Seems to point to iron though. That would give a darkish precipitate when hot, as Fe(OH)3 also dehydrates.




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[*] posted on 19-5-2014 at 09:36


Quote: Originally posted by elementcollector1  
Results are in - this is not copper. Ammonia had no effect on the brown precipitate (which, if CuO, should have reacted immediately).

I just tested this: ~.1 g CuO was put in a vial, ~2 mL of 15% ammonia solution was dropped on it and it was shaken strongly. Nothing happened for about 5 minutes, then, after everything settled out, I could see the solution was faintly blue- green in color. Then I added a drop of con. Hydrochloric acid. Again nothing much happened for a while. About ten minutes after HCl (aq) addition, it is slightly more greenish - but still quite faint.
[EDIT] I'm not sure about the copper oxide purity, it was made by electrochemically oxidizing copper in aqueous solution. The ammonia is quite pure.

[Edited on 19-5-2014 by Zyklonb]




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[*] posted on 19-5-2014 at 11:48


Zb:

Nice work. But the solubility of CuO in strong ammonia is likely to depend on its history: a calcined grade isn't the same as a product freshly obtained by mild heating of Cu(OH)2 in watery suspension. Electrolytic copper oxide may yet again respond differently.




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[*] posted on 19-5-2014 at 13:34


I don't know if this has been posted already, but this is a page on how to produce arsenic from "Arsenious Acid", I apologize if it is inapplicable.

http://books.google.com/books?id=TFAQAAAAYAAJ&lpg=PA378&...




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[*] posted on 19-5-2014 at 14:46


Quote: Originally posted by blogfast25  
Zb:

[...] A calcined grade isn't the same as a product freshly obtained by mild heating of Cu(OH)2 in watery suspension. Electrolytic copper oxide may yet again respond differently.

Yes, I did heat the CuO strongly in air for several minutes, but I'm still not entirely sure what differences that would make.




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[*] posted on 20-5-2014 at 04:55


Quote: Originally posted by Zyklonb  
Yes, I did heat the CuO strongly in air for several minutes, but I'm still not entirely sure what differences that would make.


It's a fact that the 'reactivity' of oxides depends strongly on their thermal history. See e.g. fresh ferric hydroxide v. calcined Fe2O3 or hydrated alumina v. Bauxite etc etc.




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[*] posted on 20-5-2014 at 11:25


Mine should have been fresh, but I digress.

News with the gray leftovers: Almost definitely residual skutterudite. The mystery material was dried and calcined with a blowtorch, causing various brown, white, and black/silver mirrors (for lack of a better word) to appear on the test tube. I then took some of the old ore, back from when I was trying H2SO4 and NaOCl to dissolve the skutterudite (update: Still nothing.). This gave the same results upon sublimation, and upon cooling had developed the same coloration and density, which seems pretty conclusive to me. Ah well!

An interesting note on the sublimation: It's too small to tell, but what appears to be a pure black As crystal roughly 1 mm in length grew in the middle of one of the colored patches. It has very defined crystalline boundaries... whatever it might be. In addition, some of the mirrors were, in fact, bright shiny silver in color before cooling (they dulled to gray-black afterwards). This is the same result as attempting to sublime the arsenic-coated thick-gauge copper wire (it turned silvery and this flowed towards the bottom before the iridescent copper oxides took over the coloration). In fact, I tried cutting a wire in half while it was coated and subliming: The arsenic appears to have soldered the two wires together! :P

Another thought occurred to me: What if this brown precipitate is elemental arsenic? The amount could make sense given the concentration of the starting solution, woelen has mentioned impure arsenic as being brown before, and the supernatant solution (once filtered) tested only weakly for arsenic, taking days to form anything on the wire (and it still is only barely darkened), indicating most of the arsenic has now left the solution. Of course, this fails to explain the variety of colorations I'm getting, but it holds more water than the copper hypothesis (as where would significant amounts of copper enter the equation?)

I guess the only way to find out is to attempt to sublime it once it's dry.




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[*] posted on 22-5-2014 at 19:07


Have you been tired of the poor performance of your aluminium foil? Seeking something new and exciting? Well, try our brand new Arsenic Foil!



Anyway, this flaked off the test tube after I tried subliming some skutterudite ore. Sublimation works somewhat well - I get a mix of arsenic oxide (which fumes out of the test tube) and metallic arsenic, which forms a mirror on the tube walls. So, at least I have something...

Gonna try the following:
-Addition of carbon to reduce the As2O3 back to As
-Sublimation in a closed copper tube (just a length bought from the hardware store, hammered shut and possibly twisted at both ends)




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[*] posted on 23-5-2014 at 04:26


Nice! I tried scraping the stuff I had sublimed off the copper wire and there was so little that it became arsenic bits that were pathetically small. "See the dust in there? That's arsenic :)" screw that!
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[*] posted on 23-5-2014 at 17:03


Heh, yeah. Not dealing with that anymore.
Bought the copper tube, and decided to only hammer shut one end (which still needs some work before I decide that it's sealed). Unfortunately, the 1/2" cap is a rather loose fit - any ideas on tightening it, given that this is where the pure arsenic is supposed to collect? I really don't want to solder and unsolder it every time I wish to collect product, and can't exactly tape it shut because the pressure in there is going to be such that gas would escape easily...




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[*] posted on 26-5-2014 at 18:31


Well, that was idiotic and then brilliant. In that order!
First, I mixed in an equal volume of charcoal, in the hopes that there would be no arsenic oxides from the oxygen in the tube. This ended up being true, but having weird side effects.

Using a cap on one end didn't go so well, as nothing I did could keep it shut enough for the gases to escape. So, I took that apart, hammered both ends shut, and heated for a very long time, being sure not to touch one end (as that was where I wanted the arsenic to end up). The end result, after I hacksawed it open at the middle, was pleasing!



(Everything is a lot more metallic than it appears in the photo, because my inside lighting is frankly terrible.)
The larger lumps are actually coalesced arsenic, that appeared to have melted together! I assume this is due to the carbon dioxide (from the charcoal, the oxygen, and heat) causing enough pressure inside the tube to mess with the phase diagram and cause the arsenic to melt rather than sublime. Works for me - now I didn't have to worry about scraping off foil!

I am very glad to have a sample of arsenic in my collection, and also to never have to touch this stuff again.




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[*] posted on 27-5-2014 at 06:25


Congratulations! I always love seeing these elemental isolations. So you simply ground up the ore, mixed with charcoal, and heated in a sealed pipe to sublime the As? I wonder what the rest of the products were.
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[*] posted on 27-5-2014 at 06:31


Nice! I should buy some Skutterudite and try this, sounds like fun. And you're still alive, so that's a plus.



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[*] posted on 27-5-2014 at 10:53


Quote: Originally posted by Zyklonb  
Nice! I should buy some Skutterudite and try this, sounds like fun. And you're still alive, so that's a plus.


I think that was more luck than skill on my part - there were plenty of leaks from the tube, even after I triple-folded the flattened copper pipe!
In addition, make sure you do this outside, obviously.
I would honestly attempt the wet route for a sample of arsenic - for this, I got fed up and got lucky.




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[*] posted on 27-5-2014 at 13:08


Nice. Carbothermic reduction in action.



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[*] posted on 28-5-2014 at 09:58


Quote: Originally posted by blogfast25  
Nice. Carbothermic reduction in action.


To be fair, I should probably have used significantly less carbon. The stoichiometric amount for the amount of air in the tube would probably be milligrams, instead of the equal volume (to the skutterudite) I decided to add. This ended up being a drawback because the pressure of the CO2 ended up causing it to work its way out of the tube through tiny cracks in the flattened ends, offgassing and scaring the pants off of me every time.

I wonder what would have to be done for a perfectly sealed tube? Likely it would have to be welded shut.




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[*] posted on 28-5-2014 at 11:36


Quote: Originally posted by elementcollector1  
Quote: Originally posted by blogfast25  
Nice. Carbothermic reduction in action.


To be fair, I should probably have used significantly less carbon. The stoichiometric amount for the amount of air in the tube would probably be milligrams, instead of the equal volume (to the skutterudite) I decided to add. This ended up being a drawback because the pressure of the CO2 ended up causing it to work its way out of the tube through tiny cracks in the flattened ends, offgassing and scaring the pants off of me every time.

I wonder what would have to be done for a perfectly sealed tube? Likely it would have to be welded shut.


No, no, EC1: excess is good here, trust me. You may have slightly overdone it but that's partly why it worked.

Stoichiometric would have been as slow as grass growing.





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[*] posted on 15-7-2014 at 18:10


In recent news, it turns out the waste material had yet more arsenic to yield. This might need some purification and sublimation, but I was wondering if anyone wanted a sample of arsenic for their own purposes. Not sure on prices, as I am unsure of how much I actually have (some of the ore was dumped in that waste container too...)



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[*] posted on 26-10-2015 at 18:06


Hmm. It seems all the pics in this thread died. I'll just re-upload what I have, then:


tumblr_n67nvvEUSV1rlgzvyo1_400.jpg - 42kB

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tumblr_myj9ieQ9hD1rlgzvyo2_250.jpg - 25kB

tumblr_myj9ieQ9hD1rlgzvyo4_400.jpg - 75kB




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