woelen
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Arsenic pentoxide
I purchased some material, which was sold to me as arsenic pentoxide, formula As4O10. The material even has an analysis sheet, telling the material
has at most 1% As4O6, at most 2% of water and between 0.1% and 0.2% of iron as iron(III). Guaranteed As4O10-content is 97.5%.
I looked up Internet sites, and also my books, and they all describe this solid as deliquescent and VERY soluble in water, giving arsenic acid,
H3AsO4.
The material I have looks like white dry chalk-like chunks which are fairly easily crushed with some force and do not look deliquescent and humid at
all. I expected some humid glassy mass, which is more what I expect when I read 'deliquescent'.
The solid only dissolves in water with great difficulty. When I add the solid to water, then nothing seems to happen. The liquid becomes milky. Even
after one hour, the liquid still is milky. Only when I boil the water for many minutes, the liquid becomes clear and colorless.
The material dissolves more easily in dilute solution of NaOH. The solution then also become colorless, but with a small amount of brown flocculent
solid, which most likely is iron(III)hydroxide. After a while, the liquid is totally clear and there is a very small amount of orange/brown material
at the bottom.
The material also seems to dissolve slightly more easily in acids, but for that, one also needs heating for a long time.
Finally, I tested the material by dissolving it in dilute HCl, and adding a strong reductor (dimethylamine borane). This scared the shit out of me,
the liquid started bubbling  (I did this inside, which at second thought does not seem very smart). Of course, when borane-chemicals are added to
acid, normally some hydrogen is formed, but now I was afraid these bubbles were AsH3 or at least contain some AsH3. Only after several minutes, I also
obtained a brown, almost black flocculent precipitate, which must be elemental arsenic. This precipitate quickly dissolves again, when hydrogen
peroxide is added.
Fortunately, nothing bad happened and I think the bubbles indeed just were hydrogen. I also did not perceive any 'funny' smells.
What do you think I have? It must be some arsenic compound, but is it really As4O10? My experimental results seem to differ from what I read. Has
anyone else on this forum experience with arsenic pentoxide?
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Polverone
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I have some so-called arsenic pentoxide as well, described on the bottle (rather vaguely) as a hydrate. Mine is a chalky powder and does not seem very
soluble in water either. I have not tried to reduce it because I am wary of arsenic volatilization.
PGP Key and corresponding e-mail address
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Sauron
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And a good thing it is to be wary of, indeed.
I have not found any real upside to As chemistry. It all seems to be rather nasty and best avoided.
Gone are the days of Dr.Erhlich as we have better syphilitics, organoarsenicals not needed thank you.
So while this element plays a benign role in semiconductors, which industry consumes some arsine gas, otherwise this is a nasty sapot on the periodic
table.
This is altogether not true of P, or Se.
Or am I being ignorant?
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The_Davster
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Ugh, arsenic oxide, that brings back some memories.
First course of analytical chemistry, I had to do a titration using a sodium arsenate solution that we prepared from ultrapure As2O3(fine powder).
The lab manual just said it was toxic, making no distinction between it and stuff like KI. So it was everywhere, the morons used it like any other
chemical, and the balances were well dusted after
But I digress...
As I have only worked with As2O3, it is all I can compare it too, but the solubilities do seem similar. However, As2O3 is a good standard for
titrations, due to its non-hygroscopic nature. It would appear that As4O10 is deliquescent from what you mentioned, so perhaps leave a few grains out
in an uncapped vial and see if it dissolves in its water of hydration?
I agree with Sauron, arsenic chemistry is risky, with no particuarly redeeming charastics. Although I do own a little of the metal just as a
collectors piece.
[Edited on 24-7-2007 by The_Davster]
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not_important
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A decent way to reduce As(V) is to use SO2 or sulfites in acidic solution. As4O10 will oxidise strong hydrochloric acid to chlorine, forming AsCl3 as
well. Stannous chloride will reduce either the (V) or (III) oxides to elemental As and AsH3, any reducing agent that can form hydrogen under the
reaction conditions will generate some AsH3.
As(V) oxide on aging tends to form a 'glaze' of the (V) oxide, the corresponding acid formed from absorbed water, and a bit of the mixed III/V oxide.
This can slow down solution, it's almost like dealing with a polymer that slowly unzips in water. Beyond that there's references to the hydrated
forms going colloidal, suggesting that the true solubilities may be lower - the colloidal form appearing to be dissolved without careful examination.
There are interesting aspects of arsenic's chemistry, but its toxic nature does require some care.
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12AX7
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Does it have any interesting coordination chemistry, besides, say, hexafluoroarsenate? (One of the stronger acid conjugates, I might add.  )
Tim
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woelen
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Thanks for all the responses. I recognize Polverone's description very much. I also did a few experiments with the material now and found some
interesting results:
1) The stannous chloride reduction in HCl works like a charm. It producec arsenic as a dark precipitate (grey/brown, almost black). Sad that stannous
chloride is so expensive, otherwise I would make a few grams of arsenic in this way as a fine dry powder.
2) Adding the solid to 2 ml or so of appr. 40% HBr (made in place from KBr and H3PO4) results in immediate formation of bromine. Heating produces more
bromine, but also a colorless oily liquid, which forms nice mirror-like drops on the surface. I have been VERY careful with this oily liquid, which
most likely is AsBr3. On strong dilution of the acid, the oily drops disappear and I get a milky white liquid (most likely As2O3, the result of
hydrolysis).
3) A pea-sized chunk of As4O10, added to water does not dissolve completely in 24 hours, but it does in 48 hours. Quite a lot of solid can be
dissolved in water, but it is a very slow process. Finally, the obtained liquid is colorless and clear.
4) With silver nitrate, a chocolate-brown precipitate is formed of Ag3AsO4. This was mentioned in one of my books as a sensitive detection for
arsenic(V). Arsenic(III) gives a light yellow precipitate with Ag(+).
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not_important
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I would say that you are correct regarding AsBr3. I know that with HCl there is a range of concentrations that will give you a clear solution of
AsCl3 in hydrochloric acid, further dilution results in As2O3 as you saw.
Be careful about heating mixtures of arsenic or its compounds with halogen acids. In the classic Noyes and Bray, and similar methods, analysis for
rarer elements, distillation from aqueous HCl or HBr is used to separate arsenic as the volatile trihalide.
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