woelen
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Reduction of arsenic pentoxide? What is the best method?
I have a small quantity of arsenic pentoxide and I decided to make some elemental arsenic from this in a finely divided form. I know of the dangers of
this compound, so no warning is needed on that.
I took appr. 1 gram of As4O10 and added a big excess amount of sodium hypophosphite. If you do this, then NO reduction occurs at all. I acidified the
liquid with sulphuric acid. Still no reduction at all. Next, I added some hydrochloric acid. When this is done, then there is reduction of the
material, but only slowly and for a small amount. After an hour of keeping the liquid very hot, I ended up with approximately 50 mg of pure arsenic.
Very nice product, but the yield is pathetic (definitely less than 7% of theory).
So, hypophosphite/hypophosphorous acid is not useful. It gives a clean very fine crystalline product, but the yield really is low. I do not understand
why.
Next, I tried on a small scale with sodium dithionite. When this reductor is used in alkaline solution, then no reaction occurs at all. When the
liquid is acidified with excess acetic acid, then still no reaction occurs. When a neutral solution of Na2S2O4 is added to an acidified solution of
As4O10 (acidified with dilute HCl), then quickly a lot of precipitate is formed, but the precipitate is very impure. It contains a lot of sulphur and
arsenic sulphide as well. Its color ranges from orange/brown to chocolate brown, depending on the concentration of the solutions and the temperature.
I tried all kinds of variations, but either there is no reaction, or a very impure and inhomogeneous product is obtained.
I also tried using sulfite as reductor, but this does not work at all.
What reductor could be used to reduce all arsenate/arsenic acid to elemental arsenic in a clean way. Clean finely crystalline arsenic is very dark,
almost black and somewhat glittering and that is what I want, not the dirty contaminated stuff which I obtain with dithionite in acidic solution.
[Edited on 9-4-09 by woelen]
[Edited on 9-4-09 by woelen]
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Jor
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SnCl2 works. But gives SnO2 byproduct, wich is impossible to remove, without redissolving the As.
I will give a few options, wich i would try (I do not know if they work.)
How about borohydride? Or will that give nasty extremely toxic AsH3?
Maybe an exotic one like Cr(II) in HCl, or is that too strong of a reducer and thus forming AsH3?
Formaldehyde? Iodide?
Maybe heating Zn with As2O5 (Excess), without any water/acid? And then redissolving the remaining arsenic oxides in slightly alkaline water (otherwise
you get AsH3), followed by dissolving the remaining zinc in HCl? Or just dissolve both the arsenic oxides and zinc in conc. NaOH, although I'm not
sure if arsenic disproportionates in alkaline medium just like white P (and to some extent red P does). However, reacting zinc with As2O5 could create
(carcinogenic) smoke if the reaction is too violent (I haven't tried it before), as well as maybe some Zn3As2, wich will give AsH3 with water.
Maybe any of these work?
Why didn't you try this first on a very small scale, if you didn't know it would give good yields? 
Hope you still have the remains of the gram of As2O5!
Maybe i could also give it a try 
After all you donated me a few grams of As2O5
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Magpie
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This paper may be useful:
http://www.springerlink.com/content/p6665813n4481124/
Their method uses SO2 to reduce As(V) to As(III). It states that As(III) can then be reduced to As metal with cadmium.
[Edited on 9-4-2009 by Magpie]
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Eclectic
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Maybe thiourea dioxide? Didn't I read here a while back it was stronger than dithionite?
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Jor
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O yes, one I forgot is hydrazine sulfate or hydroxylammonium chloride. Those might work.
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PHILOU Zrealone
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Maybe with air exclusion and heating red phosphorus
2 P + As2O5 --> P2O5 + 2 As
Or with aluminium powder with air exclusion and heating... beware of exothermic reaction and gaseous As generation, so start with little quantities...
10 Al +3 As2O5 -->5 Al2O3 + 6 As (g)
With sulfur and gentle warming?
5 S + 2 As2O5 --> 5 SO2(g) + 4 As
5 S + 3 As2O5 --> 5 SO3 + 6 As
[Edited on 9-4-2009 by PHILOU Zrealone]
PH Z (PHILOU Zrealone)
"Physic is all what never works; Chemistry is all what stinks and explodes!"-"Life that deadly disease, sexually transmitted."(W.Allen)
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not_important
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I do not recall ever seeing a reference to a wet reduction or an arsenic compound to the element. The methods I have seen were all dry thermal ones:
1. reduction of the oxide with carbon
2. reduction of the oxide with hydrogen
those run above 600 C so the arsenic sublimes.
3. decomposition of AsH3
4. reduction of AsCl3 or AsBr3 with H2
these can be done at a low enough temperature that the As forms a mirror or layer of black powder.
To back this up I would expect As compounds to lie between the corresponding P and Sb compounds in terms of difficulty of reduction. Again, I don't
recall seeing any non-electrolytic wet reductions of Sb compounds, as As would seem to be more unlikely.
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S.C. Wack
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There do seem to be some wet methods in the old literature, at least as sensitive tests, don't know about quantitative reductions. SnCl2 or
hypophosphite, both in boiling conc. HCl, are supposed to work slowly. The SnCl2 method is said to produce a product containing a few % of Sn in some
form.
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woelen
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I have done the hypophosphite method in HCl, but this does not work very well. Even after a long time of heating, only a small amount of As is reduced
to the element. This is a sensitive test for As though. The liquid turns dark brown, almost black, even when only a tiny amount of As is formed. This
most likely is due to the formation of very fine particles.
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chief
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Antimony is said to be even more poisonous ..., and can be found in many hardened lead-alloys, eg. those pieces which balance a cars tires ...
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not_important
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I'm not sure those old wet tests are producing pure As, or a compound or mixture. Copper is supposed to become covered with elemental arsenic, and if
rinsed and dried and then heated the arsenic sublimes, but I'd seen other references stating that the dark deposit was a compound that broke down when
heated. Only in the oldest books were these reaction unequivocally stated to be elemental arsenic, and I've never seen them used as a preparative
method, only the thermal ones.
As for the thermal ones, the carbon reduction may be done with the addition of Na2CO3, and a mixture of Na2CO3 and NaCN on fusion will also reduce the
oxides or arsenic to the elemental form.
Active metals do reduce solutions or arsenic compounds to the element plus AsH3, which reduces their usefulness as preparative routes.
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woelen
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I certainly will not do any experiments or preparations in which arsenic compounds are heated to several hundreds C. The risk of smoke, dust or vapor
with arsenic in it is too large, I do not have the equipment to handle such conditions in a proper way. So, if there is no satisfactory wet way of
isolating As from its compounds, then I'll have to refrain from isolating this element in decent quantities.
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S.C. Wack
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Other people seem to be happy with reduction with As+3. Here they use warm very conc. HCl with their SnCl2, using tartaric acid to produce a
precipitate free of tin. The actual analysis using iodine titration seems typical.
http://books.google.com/books?id=40QPAAAAIAAJ&pg=PA316
But a couple different authors say that As+5 is not so fast to be reduced.
Really there are very many articles over the years using either hypophosphite or SnCl2 in some way for quantitative analysis of arsenic, but this does
not necessarily apply here.
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not_important
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Ah, good find SCW - that may well fit woelen's needs by keeping the tin contamination out.
Most of the analytical methods really only work well with small amounts. Even into the 1930s people preparing elemental arsenic in gram amounts all
seemed to use the traditional reduction with sublimation into a tube lined with rolled iron sheet, or some slightly updated form of that.
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JohnWW
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Reduction of As2O5, As2O3, or arsenates with an electropositive metal like Al is likely to result in formation of the arsenide, AlAs.
BTW Antimony, mentioned above, is used to harden Pb and Pb/Sn alloys, especially in pewter.
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Nick F
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I was thinking about making some arsenic recently too. I have a very nice sample from elementsales.com, but it is rather small and I would like to get
hold of a bit more.
I have a 20g single crystal of realgar (which I would never dream of destroying if it wasn't very cracked internally and therefore not a very nice
crystal specimen), apparently heating of this substance in a vacuum produces sulphur and arsenic, and the arsenic can then be further purified by
subsequent vacuum sublimation.
The best idea that I could think of would be to seal the crushed realgar in an evacuated quartz tube and apply a bunsen burner flame to the outside of
the tube where the realgar is, thus hopefully decomposing it and allowing the sulphur and arsenic to condense at different distances away from the
heated zone. The arsenic could then be removed by cracking open the tube. This process would then be repeated to purify the arsenic.
What I like about this idea is that everything is contained during the heating, thus avoiding the danger of arsenic fumes, and since the arsenic is
being deposited from the gas phase it would allow crystalline arsenic to grow, which looks much nicer than precipitated powder and is safer from an
inhalation point of view. Also, since the realgar is initially a large single crystal it should be quite pure.
Unfortunately I don't have anything that gets hot enough to melt a quartz tube closed, and at the sublimation temperature of arsenic (~ 617*C) glass
would soften and get sucked in by the vacuum.
If you do have the ability to melt quartz and a vacuum pump then I would suggest that you reduce your pentoxide with carbon, as not_important
suggested, in a sealed evacuated quartz tube, to hopefully grow nice shiny arsenic crystals.
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woelen
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Nick, I can imagine that the trick with realgar can do the job, but I do not think it will work very well. Do you really think the sulphur and arsenic
are well-separated? I'm afraid that most of the two will be deposited in largely overlapping areas. You might be able though to remove the sulphur
with boiling toluene or CS2 afterwards.
The trick with As2O5 and carbon is EXTREMELY dangerous. One of the products will be either CO or CO2 and a lot of pressure will build up in the sealed
quartz tube while the reaction proceeds. Most likely it will explode when sufficient pressure has built up. Such a sealed tube only can be used when
there are no gaseous reaction products.
For the time being I have put the project to rest. I hope that I once will obtain some arsenites or As2O3. These are reduced more easily. The arsenic
in oxidation state +5 is reluctant to reduction, just as selenate and perbromate (and also phosphate, sulfate and perchlorate). The elements in the
top right corner of the period table are easier to reduce when they are in an oxidation state somewhat lower than their maximum (e.g. chlorate vs.
perchlorate, selenite vs. selenate and arsenite vs. arsenate).
[Edited on 28-4-09 by woelen]
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Hydragyrum
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In the past, I have successfully reduced As(V) to As(III) using SO2 and catalytic iodide (very important: the iodide does all the work, the SO2 merely
reduces the I2 back to iodide) and it is quite easy - however, that was with an organic group attached, and not purely inorganic arsenic, but I guess
it will be similar in your case.
However, of more interest to you perhaps is that an aqueous alkaline solution of As(III) can be induced to plate out As electrolytically - never done
it myself, but apparently it would be worth a go - and no fumes or dust expected either: probably relatively safe to do, compared with other methods.
PS. Apparently, Sb can also be plated out, but films deposited in presence of chloride can be explosive (don't know why, and no similar warning is
given for As, but just to be safe: avoid all chloride when electroplating As!).
[Edited on 29-4-2009 by Hydragyrum]
Chemistry is life (and a whole lot more)
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Jor
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Today I recieved 100g of sodium hypophosphite, and I also have a few grams of As4O10, so I think I'm gonna give the first experiment woelen did as
well.
I have also been looking for a As(III) compound. After looking in As chemistry it's quite interesting sometimes so I would like to have a few grams of
As2O3. I have yet to find a source for As2O3. I order via a local supplier from the big companies, but I doubt I can buy As2O3 without getting really,
really suspicious.
I can buy As(V) sulfide or realgar, but these are very expensive, and the supplier list high shipping costs as well.
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Magpie
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Jor, do you have rock shows in your vicinity? If so, you should be able to find a small piece of orpiment (As2S3) for not too much money. I bought
my piece for $3.
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Jor
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How do I convert the arsenic sulfide to arsenix trioxide? Would heating in air be sufficient? And wouldn't you get volatised (subliming) As2O3?
Or is it maybe easier to start from realgar, As4S4 where As is 2+ ?
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Magpie
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Jor, the only thing I have done with my As2S3 (orpiment) is to convert it to the arsenate ion:
As2S3 + 12OH- + 14H2O2 ---> 2[AsO4]3- + 3SO4-- + 20H2O
This was done for qualitative analysis purposes.
I don't have an element collection so have no need for pure arsenic. It intimidates me, especially in any gaseous form. But if I had to get it in
elemental form I would start by looking at the Marsh test.
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