woelen
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Elemental arsenic from arsenate?
I recently purchased a small quantity of Na2HAsO4.7H2O, which was a spare chemical from an old lab. I tried to see whether I could get elemental
arsenic from this, suspended in water.
I really did not succeed. I had the impression that with suitable reductors, one could get a precipitate of elemental arsenic as a flocculent
brown/black precipitate, but whatever I try, I simply do not succeed. Not with borane-based reductors, not with milder reductors like sulfite and not
with hypophosphite.
Finally, I have thrown zinc metal to acidified aqueous solutions of the stuff, yes, I even dumped some solid NaH (80%) in a solution of the stuff (I
did both of these outside, with wind from behind, just in case some AsH3 is formed besides the H2). But even then, not a single sign of elemental
arsenic.
It is possible to reduce aqueous solutions of arsenate to elemental arsenic, and if so, what procedure should be used?
[Edited on 14-7-08 by woelen]
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ShadowWarrior4444
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http://www.esemag.com/0101/ferric.html
H2S seems to be indicated as a suitable reducer, perhaps in the presence of a ferric salt.
Though, I wonder if the NaH2 in the original compound is somehow interfering with your other reducing agents. Perhaps oxidizing the compound first,
then attempting to reduce the arsenate.
[Edited on 7-14-2008 by ShadowWarrior4444]
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unionised
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Would reduction to arsine (by, for example, acid and zinc) followed by thermolysis to arsenic count?
You ought to be able to get a visible mirror. Getting enough to do anything with might be more difficult.
I think reduction by borohydride to arsine is practically quantitative- it's used in "hydride generation" AA spectroscopy.
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DJF90
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It appears that what you have is an "acid salt", and so maybe you should try neutralising it with NaOH, producing Na3AsO4. This may reduce easier?
Perhaps try using H2S as suggested?
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not_important
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| Quote: | Originally posted by ShadowWarrior4444
...
H2S seems to be indicated as a suitable reducer, perhaps in the presence of a ferric salt.
... |
Forms arsenate or sulfides, not the metal.
Traditional method would be to mix (as dry materials) your Na2HAsO4 with sufficient bisulfate or NaH2PO4 to combine with all the sodium to form the
free arsenic oxides, add excess carbon over that needed to reduce the oxides, heat to redness in a test tube with a long tube of rolled up thin iron
sheet inserted in it and loosely stoppered with cork or metal foil. The arsenic distills off and condenses on the interior of the rolled up iron
sheet, when cold the sheet is removed and unrolled and the arsenic brushed off of it. You can use a glass tube, but it's harder to remove the
arsenic.
The sulfide or oxides can be reduced using ammonia at a temperature that cracks the ammonia into nitrogen and hydroge, again the arsenic will be in
the gas phases.
Copper will reduce acidic solutions of arsenates, forming a thin grey film on the copper. Iron tends to form arsenides or mixed sulphides if sulfate
is present. http://www.engr.uconn.edu/~nikos/asrt-brochure.html
Electrochemical methods can be used http://cat.inist.fr/?aModele=afficheN&cpsidt=14504635
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woelen
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I tried adding Na2S to the solution. No reaction is visible. This is in strong contrast with antimony, which gives a yellow precipitate of antimony
sulfide or thioantimonates.
Making arsine and then decomposing that is not something I want to do, I value my health too much. Apparently there is no simpe aqueous method for
precipitating arsenic?
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ShadowWarrior4444
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| Quote: | Originally posted by woelen
Making arsine and then decomposing that is not something I want to do, I value my health too much. Apparently there is no simpe aqueous method for
precipitating arsenic? |
Perhaps a bit of electrochemistry is in order. I wonder if there are any patents for plating arsenic, or at the very least refining it into a spongy
deposit.
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S.C. Wack
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Dithionite is supposed to work. But then so is hypophosphite, but in hot conc. HCl. The same conditions for SnCl2, which definitely works.
That one goes back a long ways:
http://www.nature.com/nature/about/first/chemistry.html
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Jor
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Aha, but how would you purify the arsenic containing some stannic oxide, without having to dissolve it again? Stannic oxide does not dissolve in
anything AFAIK, but molten akalies.
[Edited on 15-7-2008 by Jor]
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woelen
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S.C. Wack, your tip of using hypophosphite in hot conc. HCl is the magic key to arsenic 
I did the following:
- dissolve some As4O10 in conc. HCl. This takes a fairly long time of careful heating and occasionally swirling the test tube.
- add some solid NaH2PO2 to another test tube with conc. HCl. A white suspension is formed, most likely this is due to formation of insoluble NaCl in
the concentrated acid.
- dump all of the second test tube (excess hypophosphite is used) in the first one, and carefully heat. After some time a brown/black precipitate is
formed.
- add a lot of water. Any of the white solid now quickly dissolves, the brown/black As remains in the liquid as a solid.
- let the solid settle at the bottom, and decant almost all of the water. Pour new water on the solid and let settle again. Repeat two more times.
In this way, I obtained a suspension of rather pure arsenic in water. Whether I let it dry, or use this for further experiments, I have to decide on
yet. I only have 100 mg or so, I do not want much of this toxic chemical.
The method with hypophosphite is much better than the tin(II) based method. With tin(II) one always gets tin(IV) in the mix, which is not easily
separated, as correctly mentioned by Jor. With the hypophosphite all other products are water-soluble and hence can be removed more easily.
[Edited on 15-7-08 by woelen]
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