bolbol
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Arsenic in Nitric Acid.
So i tried dissolving arsenic in nitric acid. I used 4 grams and placed it in 30 mls of conc. Nitric acid. I observed nothing so I thought I'd leave
it alone and let it react.
When I came back the next day I saw the solution turned into a very nice dark green color(actual green not cyan like copper) and upon stirring I saw
that 90% of the arsenic sample is still present and I also observed that with stirring NO2 gas started coming out of the solution and solution turned
into yellow(typical color of NO2 dissolved in water)
So what's going on? This reaction in theory is supposed to produce HAsO3. And as far as I know that is colorless. What could be the cause of the green
color? Impurities? If so how come it disappears after stirring.
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woelen
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The As and HNO3 gives arsenic acid, NO, and NO2. NO and NO2, when put together, give N2O3, which is blue. Mixed with the yellow/brown of NO2, this
gives a green color. On shaking, the N2O3 easily escapes and breaks down, giving bubbles of NO and NO2.
I had similar observations, when making bismuth nitrate. The Bi dissolves, giving a green solution, which on heating lost its color and finally only
colorless Bi(3+) remained. I also observed this effect when dissolving Hg-metal in conc. HNO3.
Also a word of caution for you. If the solution bubbles, then you MUST cover the open end of the beaker with a piece of tissue or wadding. The
bubbling causes fine droplets to escape from the liquid and these droplets contain compounds of arsenic! These linger and may cause long term low
level exposure!
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bolbol
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Thanks. I didnt know about N2O3. And is there an explanation as to why this doesn't happen when making silver nitrate?
The only difference between the relations is that this one's taking days and it's still not finished consuming the Arsenic. And no its not bubbling or
anything.
Thank you again
Edit: I did some research and on Wikipedia I found that arsenic acid only exists in solutions but at the Same page it says the crystals are white and
hygroscopic... So now I'm confused, what happens when I evaporate the water and excess nitric acid from the solution?
[Edited on 4-1-2015 by bolbol]
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HgBrO3
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If high enough temperatures are reached the arsenic acid will dehydrate to AsO3 otherwise known is arsenic trioxide.
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bolbol
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That doesn't seem right
As in arsenic acid is at 5+ oxidation while arsenic trioxide which is As2O3 has As in 3+ oxidation
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woelen
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This is not right, arsenic acid will not decompose to AsO3 (actually, AsO3 does not exist at all).
In conc. nitric acid, H3AsO4 is formed from As. On heating, this decomposes and As2O5 (better: As4O10) is left behind. You have to heat quite strongly
before all water is removed and at those temperatures, some of the As4O10 volatilizes and some of it may decompose to As2O3 and O2. On standing in
contact with air, As4O10 absorbs water.
Arsenic acid itself cannot be completely dehydrated, the best you can get is H3AsO4·½H2O. On further heating it decomposes, giving the above
mentioned As4O10. Based on the formula, arsenic acid most likely is not H3AsO4, but some condensed species, like H4As2O7·2H2O or something even more
complicated.
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AJKOER
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With respect to the action of Nitric acid on Arsenic per Atomistry (link: http://arsenic.atomistry.com/chemical_properties_arsenic.htm... )
"Aqueous nitric acid up to 50 per cent, concentration has little action on arsenic, but the concentrated acid or aqua regia causes rapid oxidation to
arsenious and arsenic acids. When the acid is more dilute some ammonia may be formed."
An interesting preparation of As2O3, starting with household derived chemicals, is via FeCl3, which can be made from FeCl2 that is in turn prepared
from Iron in dilute HOCl (see prep for the later outline at http://www.sciencemadness.org/talk/viewthread.php?tid=62034 ) with a touch of NaCl to serve as an electrolyte for the self sustaining
electrochemical cell reaction:
"No reaction occurs between arsenic and solutions of antimony and bismuth chlorides. Ferric chloride is reduced thus -
6FeCl3 + 2As + 3H2O = 6FeCl2 + As2O3 + 6HCl "
On the topic of oxides, per Atomistry.com:
"Two well-defined oxides are known arsenious oxide, As2O3, and arsenic pentoxide, As2O5. The former is the most important compound of arsenic, being
the form in which the element is most used. A suboxide, As2O, and a tetroxide, As2O4, have been described, but the existence of neither as a pure
compound has been established."
Also, on preparation of As2O3 from N2O:
"Arsenic is oxidised, mainly to arsenious oxide, when heated in nitrous oxide; the reaction becomes appreciable at 250° to 270° C. and ignition
occurs at 400° to 450° C. This reaction takes place specifically between arsenic and the nitrous oxide and is not due to reaction with oxygen after
thermal decomposition of the nitrous oxide, as such decomposition does not occur below 400° C. and is very slight at 460° C. Nor does the reaction
resemble that which occurs in oxygen, except that, like the reaction in the dark with the latter gas, it is a surface reaction. No chemi-luminescence
is observed, however, and there is no upper critical oxidation pressure. At 360° C. the product contains at least 99 per cent, of pure arsenious
oxide, and at 420° C. it contains about 5.8 per cent, of arsenic pentoxide."
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Per Watts (https://play.google.com/books/reader?id=ijnPAAAAMAAJ&pri... ), Arsenic is readily oxidized by HOCl with the evolution of light. Also, the
lower oxide, As2O3 is converted into As2O5. To quote the relevant section from Watts':
"Reactions.--1. HClOAq acts generally as an oxidiser; it easily parts with 0 while HClAq remains. Thus, As is rapidly oxidised with evolution of
light; P, S, Se, Br, I are converted to H3P04Aq, H2S04Aq, &c., even by dilute HClOAq; lower oxides or salts are converted into higher, e.g. SO2Aq
to H2SO4Aq, FeO to Fe203, As203Aq to As2O5Aq, FeS04Aq to Fe2(S04)3Aq, Fe2Cl6Aq, and Fe2O3, MnSO4Aq to MnO2; sulphides yield sulphates, c.g. H2SAq
gives" H2SO4,Aq and S; "
Note, the strength of the Hypochlorous acid is not specified, so you may have to distill off half of dilute HOCl to form a concentrated acid as to
quote from Watts again, same page:
"Gay-Lussac found that, on distilling a dilute solution to one half, the distillate contained five-sixths of the total HClO (C. R. 14, 927)"
----------------------------
[Edit] Now, one may wonder why cheap, safe and fairly readily available Hypochlorous acid (as per the preparation I outlined it can be produced in
around two hours as I cool the HOCl/CaCO3 mix to allow the separation of the white Calcium carbonate), is not cited as the avenue of choice to prepare
As2O5? My answer, other than ignorance, is that the acid has to be prepared fresh given its limited shelf life. In other words, it isn't one of the
acids likely to be sitting on your shelf ready to use.
[Edited on 8-4-2015 by AJKOER]
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woelen
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I found this old thread and tried myself.
I added a piece of arsenic (as big as a pea) to 2 ml of estimated 60% HNO3 (exact concentration I do not know, I made this by means of distillation
from H2SO4 and KNO3 and some water, the acid, however very vigorously dissolves copper, so it must be quite strong). Nothing happens with the arsenic.
I heated the acid. At a certain point there is a lot of bubbling on the piece of arsenic, but the piece does not get smaller. So, I think it simply is
boiling of the acid and decomposition of the acid. There was quite some brown gas above the acid on heating.
Next, I went outside with the test tube and there I added 2 ml of conc. HCl to the still quite hot liquid with the lump of arsenic in it. When this is
done, there is a fairly vigorous reaction and the arsenic dissolves. The piece becomes smaller, but it does not dissolve completely. In the liquid, a
lot of white solid is formed, which sticks to the glass, but which on heating the test tube can be "peeled off" on the inside of the test tube. The
white solid, however, did not dissolve. I allowed the test tube to cool down, diluted the liquid 5 times with water and stoppered it. Now I still have
it standing, white solid at the bottom, piece of arsenic still in it and a colorless liquid above it.
I went outside, because I feared formation of volatile AsCl3 which could escape from the test tube. I, however, doubt that any AsCl3 was formed, but
better safe than sorry.
The white stuff I have now probably will be As2O3. Is there a typical reaction for detection of As2O3?
I am surprised by the low reactivity of arsenic, I expected its reaction to be much more vigorous.
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I did another experiment, also with conc. HNO3. This time I took a small spatula of As2O3 (I have a beautiful vintage bottle of this, even with a
price on it of Deutsche Mark 3,20, and wonder oh wonder, the bottle still was full of a perfectly white powder. I myself later put the orange
skull-and-bones sticker on the bottle.).
Picture of bottle.
The As2O3 does not seem to react with the nitric acid. I added a few drops of water. Still there is no reaction with nitric acid. Heating also does
not dissolve the white solid. Next, I added a small pinch of copper metal. The metal quickly dissolves, I get a blue solution and some brown color
above the solution. The white solid does not dissolve. The production of some free NOx does not seem to dissolve it more quickly.
Next, I added a little conc. HCl. When this is added, then with some difficulty the white solid dissolves, but this reaction certainly is not
immediate. You need quite some shaking.
In very old literature a good method of making NO2 or NO/NO2 mixes was adding As2O3 to nitric acid, but my experiment does not confirm this. The
material is not very reactive. Can anyone else confirm this?
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