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Author: Subject: Arsenic from Skutterudite
blogfast25
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[*] posted on 1-1-2014 at 05:30


Like plante I'm convinced no arsine will evolve. But as a precaution it is easily destroyed (look at the Marsh test, for instance), if it did evolve.



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[*] posted on 1-1-2014 at 08:23


No, I meant redox potentials or other data instead of this useless should–shouldn't back and forth.



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[*] posted on 1-1-2014 at 12:41


Redox potentials for reactions in concentrated acids?

On Arsine Wiki mentions that it "will react violently in presence of strong oxidizing agents, such as potassium permanganate, sodium hypochlorite or nitric acid". No chance of free arsine with strong acids.

Interestingly, the Wiki entry on BiH<sub>3</sub> mentions:

"The methodology used for detection of arsenic ("Marsh test") can also be used to detect BiH3. This test relies on the thermal decomposition of these trihydrides to the metallic mirrors of metallic As, Sb, and Bi. These deposits can be further distinguished by their distinctive solubility characteristics: As dissolves in NaOCl [my emph.], [...]"

So this CoAs3 may well dissolve in hypochlorite, presumably to arsenite.

[Edited on 1-1-2014 by blogfast25]




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[*] posted on 1-1-2014 at 12:54


And what would it leave behind? CoO? Co(OH)2? I would presume the cobalt would not go into solution.



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[*] posted on 1-1-2014 at 13:03


Quote: Originally posted by elementcollector1  
And what would it leave behind? CoO? Co(OH)2? I would presume the cobalt would not go into solution.


Co(II) hydroxide, possibly (depending on oxidation potentials) a III or IV hydroxide. It would not dissolve.

[Edited on 1-1-2014 by blogfast25]




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[*] posted on 1-1-2014 at 15:19


Put one gram of ore in a test tube with about 15 mL of bleach. So far, no results. I'm guessing this will take a while, but if I attempt to heat the mixture, the NaOCl will immediately disproportionate - which might ruin the dissolution entirely.



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[*] posted on 1-1-2014 at 15:56


I would go for the strong inorganic acid.



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[*] posted on 1-1-2014 at 16:29


Update, 1 hour later: Slow bubbling is observed. There is some sort of darker layer on top of the ore - this is likely CoO. So, I think this reaction is occuring - just very slowly.



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[*] posted on 1-1-2014 at 16:46


Any chance you could take some photos of the process?



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[*] posted on 1-1-2014 at 17:10


Let it be known that I tried. The bubbling is almost invisible, and the bottom layer is only visible with flash on.







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[*] posted on 1-1-2014 at 19:44


Update, 3 hours later: The black layer mentioned previously has grown larger, and more noticeable. This substance is noticeably different from the skutterudite. As you can see from the photo above, the ore sample is bright and silvery. This stuff is a black, flocculent powder that very easily forms a suspension, leading me to suspect Co(OH)2. The same slow rate of bubbling is observed.



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[*] posted on 1-1-2014 at 19:56


I suspect cobalt arsenate would also be insoluble.



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[*] posted on 1-1-2014 at 20:01


Yeah, but apparently that's sort of a pinkish-purple (see here: http://en.wikipedia.org/wiki/Erythrite), so I feel pretty safe about discounting that.



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[*] posted on 2-1-2014 at 06:04


That cobalt arsenate [Erythrite] is an octahydrate so it must be fairly soluble. It's described also as 'cobalt bloom', which also suggests solubility. Pinkish/purplish/red is of course the colour of the Co<sup>2+</sup> hydrated ion, see CoCl<sub>2</sub>, for instance.

But we're not sure whether As(III) or As(IV) is being formed, must check the potentials for that...

And we're in alkaline conditions, which would precipitate the cobalt as a hydroxide.

[Edited on 2-1-2014 by blogfast25]




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[*] posted on 2-1-2014 at 10:53


A few relevant reduction potentials (CRC Ed. 86 values):

Hypochlorite to chloride = + 0.81 V
HAsO2 to As(0) = + 0.248 V
H3AsO4 to HAsO2 = + 0.56 V
Co<sup>2+</sup> to Co(0) = - 0.28 V
Co<sup>3+</sup> to Co<sup>2+</sup> = + 1.92 V

This would indicate that hypochlorite can oxidise As all the way up to H3AsO4 and Co to Co<sup>2+</sup> (but not Co<sup>3+</sup>;).

If so, with an excess of alkaline sodium hypochlorite, sodium orthoarsenate (Na3AsO4) would be formed and Co(OH)2 would precipitate. Filtering would then allow separation of the Co and the arsenic.

According to my Holleman, solid H3AsO4.H2O can be obtained by concentrating an acidic solution of this acid and cooling to below 15 C. Here I would suggest acidifying the sodium arsenate solution with H2SO4 (sodium sulphate being very soluble), boiling to further destroy any excess hypochlorite and to concentrate the solution. Chilling should see ortho arsenic acid hydrate crystallise, more or less free from Na, sulphate and chloride.

[Edited on 2-1-2014 by blogfast25]




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[*] posted on 2-1-2014 at 12:46


Quote: Originally posted by blogfast25  
A few relevant reduction potentials (CRC Ed. 86 values):

Hypochlorite to chloride = + 0.81 V
HAsO2 to As(0) = + 0.248 V
H3AsO4 to HAsO2 = + 0.56 V
Co<sup>2+</sup> to Co(0) = - 0.28 V
Co<sup>3+</sup> to Co<sup>2+</sup> = + 1.92 V

This would indicate that hypochlorite can oxidise As all the way up to H3AsO4 and Co to Co<sup>2+</sup> (but not Co<sup>3+</sup>;).


I don't have my CRC handy, but what's the reduction potential for Co2O3 to Co(OH)2? I suspect the oxidation of cobalt will be more favourable in basic solution.




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[*] posted on 2-1-2014 at 12:52


Judging from this info, would this be a suitable reaction?
3 Co + 2 As + 8 NaOCl + 6 NaOH -> 8 NaCl + 2 Na3AsO4 + 3 Co(OH)2

From this and a general formula of CoAs3 for skutterudite, this suggests that 7 As get 'ignored' by this reaction for every 3 Co. If I try with the formula for skutterudite, the reaction is not balanceable (presumably because it's assumed to be a compound).

Update on the test tube: The ore is now all darkened and intermixed with the brown-black powder. I'm going to add some more bleach, fill the test tube up (should be about 30 mL, or 1.94g NaOCl).




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[*] posted on 2-1-2014 at 13:24


DA: I'll have a look tomorrow.

EC1: for an overall reaction equation with the ore, just write one for 1 Co (eq. 1), then one for 1 As (eq. 2). Then combine for CoAs3, as 1 x (eq. 1) + 3 x (eq.2) = overall reaction equation (theor. stoichiometry). What you've got there seems correct, so just add one of eq. 2 and you're done.

It's rather a lot of NaCl, which may make obtaining the pure H3AsO4.H2O harder.


[Edited on 2-1-2014 by blogfast25]




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[*] posted on 2-1-2014 at 13:33


I get:

2 CoAs3 + 12 OH- + 11 OCl- --> 2 Co(OH)2 + 6 HAsO32- + H2O + 11 Cl-.




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[*] posted on 2-1-2014 at 14:05


DA:

Balances in everything, including charge, so it must be fine. Just add the Na<sup>+</sup> to get rid of the charges.

With commercial bleach being so weak you'll have a lot of water and a lot of sodium chloride!

I'll see if CRC make any mention of the solubility values of ortho arsenic acid.

:cool: Another approach could be fusion of the ore with KOH + KClO3 or KNO3. These oxidisers may also get As to ortho arsenate (or maybe arsenite (III)?) but with the advantage of temperature and much higher concentrations of leachates.

[Edited on 2-1-2014 by blogfast25]




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[*] posted on 2-1-2014 at 15:27


For that matter, I could add NaOH and heat in a water bath: The bleach would disproportionate into chlorate, and the reaction would be faster.
Update on the test tube, 24 hours in: In addition to the previous update, about 1mm of dark brown powder has settled. Bubbling rate seems to have slightly increased? It's so small I can't properly tell, but the bubbles do look bigger.

EDIT: I can't believe this didn't occur to me earlier, but... what is the bubbling? None of the reactions outlined mention any sort of gas, and it obviously can't be steam, 24 hours in and room temperature. Is it air? I did shake this thing a few times to ensure mixing, but I usually leave it for a few hours and it still bubbles when I get back...

[Edited on 1-2-2014 by elementcollector1]




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[*] posted on 2-1-2014 at 15:38


Cobalt catalyse hypochlorite decomposition to oxygen and chloride.


For bfesser:

Reference from my own work.




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[*] posted on 2-1-2014 at 15:48


Oh. Hmm. So, this reaction will actually lose potency over time.
That is quite the problem - I might have to try this again with some different reagents. I have some sodium nitrate I can experiment with.




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[*] posted on 3-1-2014 at 06:02


Try fusing with NaNO3 (MP = 308 C).

For overall reaction equation, assume 4 NaNO3 === > 2 Na2O + 4 NO + 3 O2

and use the O2 for Co + 1/2 O2 === > CoO

and 6 As + 9 Na2O + 5 O2 === > 6 Na3AsO4

For balancing, you may be a bit short of Na: supplement with NaOH.

[Edited on 3-1-2014 by blogfast25]

As a starting point I would use 1 mole ore (= 3 moles As) + 5 moles NaNO3 + 4 moles NaOH, perhaps adding 10 - 20 % excess NaNO3.



[Edited on 3-1-2014 by blogfast25]




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[*] posted on 6-1-2014 at 00:01


Ive been following this thread quite closely, and actually just purchased a rather large and beautiful Skutterudite specimen. I was a bit concerned however about the handling of the Skutterudite itself, how dangerous is a large 'rock' of this stuff? I have been handling it only with gloves just in case, but I haven't been able to find much of anything regarding its safety online, just a few vague references such as this: http://csmsgeologypost.blogspot.com/2013/04/skutterudite-cobalt-nickel-arsenide.html, stating "It is interesting to note that the local population (Berbers) knew about the toxicity of the outcropping arsenates long before the onset of commercial mining—they used it for insect control and rat poison.".


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