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woelen
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If you want to create elements from Hg-compounds or As-compounds, stick to aqueous chemistry. Do not use hot chemistry. Breathing volatile Hg or
As-oxides is something you must avoid at any cost.
Arsenic can be made in elemental form, using aqueous chemistry, but this is not straightforward. I myself have experimented with As2O5, which fairly
easily can be reduced to As2O3 or arsenious acid, but the step to elemental arsenic is not easy. One safe method (no risk of formation of AsH3) is
reduction with hypophosphite (which is a moderately strong reductor), but the reaction is slow and yield is low, resulting in a black powder of pure
As. Using stronger reductors, such as NaBH4 or powdered zinc is very dangerous. You get quite a lot of hydrogen gas, contaminated with AsH3 as side
product.
As2S3 can be converted to arsenic acid and sulfate by heating in conc. HNO3. The arsenic acid in turn can be reduced to arsenious acid and arsenic.
The oxidation of As2S3 by nitric acid must be done outside on a breezy day! You get fumes of NOx and tiny droplets of liquid, which may contain
dissolved arsenic acid. Loosely cover the beaker with a paper tissue. Gas can escape easily, but most tiny droplets are trapped by the paper.
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Upsilon
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Quote: Originally posted by woelen  |
Arsenic can be made in elemental form, using aqueous chemistry, but this is not straightforward. I myself have experimented with As2O5, which fairly
easily can be reduced to As2O3 or arsenious acid, but the step to elemental arsenic is not easy. One safe method (no risk of formation of AsH3) is
reduction with hypophosphite (which is a moderately strong reductor), but the reaction is slow and yield is low, resulting in a black powder of pure
As. Using stronger reductors, such as NaBH4 or powdered zinc is very dangerous. You get quite a lot of hydrogen gas, contaminated with AsH3 as side
product.
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Would it be possible to use tin(II) chloride as a reductant? SRPs make it look like it's feasible:
H3AsO3 + 3H+ + 3e- -> As + 3H2O (+0.24V)
Sn4+ + 2e- -> Sn2+ (+0.15V)
As + 3H+ + 3e- -> AsH3 (-0.23V)
A redox reaction theoretically occurs containing the first two half reactions, oxidizing tin(II) to tin(IV):
2H3AsO3 + 6H+ + 3Sn2+ -> 2As + 3H2O + 3Sn4+ (0.09V)
While the reduction of As to AsH3 shouldn't occur:
2As + 6H+ + 3Sn2+ -> 2AsH3 + 3Sn4+ (-0.38V)
So, overall:
2H3AsO3 + 6HCl + 3SnCl2 -> 3SnCl4+ 2As + 3H2O
Thoughts on this? It will probably be quite slow, but I would at least not have to buy a hypophosphite.
EDIT: Another possible reductant could be formaldehyde:
H2CO2 + 2H+ + 2e- -> H2CO + H2O (-0.03V)
2H3AsO3 + 6H+ + 2H2CO + 3H2O -> 2As + 6H2O + 3H2CO2 +
6H+ (+0.27V)
Simplified to:
2H3AsO3 + 3H2CO -> 2As + 3H2O + H2CO2
[Edited on 26-10-2015 by Upsilon]
[Edited on 26-10-2015 by Upsilon]
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blogfast25
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| Quote: Originally posted by Upsilon |
So, overall:
2H3AsO3 + 6HCl + 3SnCl2 -> 3SnCl4+ 2As + 3H2O
Thoughts on this? It will probably be quite slow, but I would at least not have to buy a hypophosphite.
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Yes, I believe that should work.
There's something much simpler though: deposition of As (+3) or (+5) from acid solution as As(0) on copper wire or strip. This is still used as a
forensic screening test for arsenic.
EC1 did that IIRW and there's a report on it.
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Upsilon
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| Quote: Originally posted by blogfast25 |
There's something much simpler though: deposition of As (+3) or (+5) from acid solution as As(0) on copper wire or strip. This is still used as a
forensic screening test for arsenic.
EC1 did that IIRW and there's a report on it. |
What exactly will allow these arsenic compounds to dissolve to form As3+ or As5+? As2O5 is apparently pretty soluble and can be made by the action of
HNO3 on As2O3.
The use of formaldehyde seems pretty appealing to me, however (see edit above). I will theoretically get arsenous acid directly from the reaction of
As2S3 and hypochlorite. The formaldehyde would then be added, leaving nothing with the arsenic but formic acid and water, which can easily be
evaporated off.
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blogfast25
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| Quote: Originally posted by Upsilon |
What exactly will allow these arsenic compounds to dissolve to form As3+ or As5+? As2O5 is apparently pretty soluble and can be made by the action of
HNO3 on As2O3.
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Arsenic is one of these elements that's neither a metal nor a non-metal. It's also amphoteric.
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woelen
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I actually tried using tin(II) for making elemental arsenic. It does work, but the arsenic you get is very impure and I could not purify it without
putting myself in high risk.
I dissolved SnCl2 in conc. HCl and added this to a solution of As2O5. When this is done you immediately get a dirty brown precipitate, which is a very
intimate mix (solid solution?) of As and basic tin(IV) chloride. I read about this process and the only practical way of purification of the As is by
sublimation. This is something which I do not want to do, I value my health too much.
The route with hypophosphite is much slower and the arsenic is formed as a compact fine crystalline powder. The arsenic looks black. With
hypophosphite, however, the reaction only works reasonably fast in the presence of a lot of chloride ion (as HCl) and this adds a large risk: I saw
tiny droplets of oily liquid on the surface, which must be AsCl3. When I saw the AsCl3 I immediately quit the experimenting (AsCl3 is volatile, is
quite stable in the presence of humidity and acid, much more so than PCl3, and EXTREMELY poisonous). So, know what you are doing, work in well closed
reaction vessels, a beaker is not OK.
You'll find that making elemental arsenic always is done through high temperature chemistry and sublimation of the element. Apparently there is no
satisfying aqueous method with acceptable yield. The best I found is the hypophosphite route with yields of atmost a few percent. It is suitable for
making a small sample of powdered arsenic when done carefully.
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Upsilon
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What about the possibility of formaldehyde as a reductant? Reposting from above:
H3AsO3 + 3H+ + 3e- -> As + 3H2O (+0.24V)
H2CO2 + 2H+ + 2e- -> H2CO + H2O (-0.03V)
Balancing for electrons, the protons conveniently cancel out:
2H3AsO3 + 6H+ + 2H2CO + 3H2O -> 2As + 6H2O + 3H2CO2 + 6H+ (+0.27V)
Simplified to:
2H3AsO3 + 3H2CO -> 2As + 3H2O + H2CO2
[Edited on 27-10-2015 by Upsilon]
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blogfast25
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| Quote: Originally posted by Upsilon | What about the possibility of formaldehyde as a reductant? Reposting from above:
H3AsO3 + 3H+ + 3e- -> As + 3H2O (+0.24V)
H2CO2 + 2H+ + 2e- -> H2CO + H2O (-0.03V)
Balancing for electrons, the protons conveniently cancel out:
2H3AsO3 + 6H+ + 2H2CO + 3H2O -> 2As + 6H2O + 3H2CO2 + 6H+ (+0.27V)
Simplified to:
2H3AsO3 + 3H2CO -> 2As + 3H2O + H2CO2
[Edited on 27-10-2015 by Upsilon] |
There's no shortage of reducers that will yield As(0) but all will yield powders, except Cu sheet/strips.
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Upsilon
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I suppose that is true; I wouldn't really mind having a powder too much instead of a lump, but might as well go for the lump if it's not any more
difficult. I'll get around to it when I'm ready.
I would like to discuss chromium next. I have chromium(III) oxide, but it is quite unreactive to both acids and bases. I did a small test thermite
with it, and it was quite slow, did not burn to completion, and did not form nice pieces of metal (probably not hot enough to melt the chromium
nicely). Will using a booster reaction provide better results? I figure that if I can at least get it to burn mostly to completion, then I could just
dissolve everything in HCl an electrolyze the solution - chromium will be deposited but aluminum will not.
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blogfast25
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| Quote: Originally posted by Upsilon |
I would like to discuss chromium next. I have chromium(III) oxide, but it is quite unreactive to both acids and bases. I did a small test thermite
with it, and it was quite slow, did not burn to completion, and did not form nice pieces of metal (probably not hot enough to melt the chromium
nicely). Will using a booster reaction provide better results? I figure that if I can at least get it to burn mostly to completion, then I could just
dissolve everything in HCl an electrolyze the solution - chromium will be deposited but aluminum will not. |
The chromium thermite works very well with small amounts of booster like NaNO3 or K2Cr2O7 and provided you use at least 50 g (or so) of mixture. I
have some nice 'large' round reguli from that. Always include a small amount of slag fluidizer (CaF2 or CaO) to improve yield.
Of course you can also electroplate Cr(0) from aqueous solutions, see the electrolytic chroming of objects.
Cr2O3 is usually totally unresponsive to HCl though...
[Edited on 27-10-2015 by blogfast25]
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Upsilon
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| Quote: Originally posted by blogfast25 |
The chromium thermite works very well with small amounts of booster like NaNO3 or K2Cr2O7 and provided you use at least 50 g (or so) of mixture. I
have some nice 'large' round reguli from that.
Of course you can also electroplate Cr(0) from aqueous solutions, see the electrolytic chroming of objects.
Cr2O3 is usually totally unresponsive to HCl though...
[Edited on 27-10-2015 by blogfast25] |
I wasn't intending to try and dissolve the Cr2O3 in HCl - I was referring to using it on the aftermath of the thermite if I could not get nice samples
of Cr. The small grains of Cr would be dissolved in HCl along with unreacted aluminum and aluminum oxide, then Cr could be electroplated while
aluminum would not be. However, since you say that the thermite alone can provide good results I may not do this; though if I am interested in further
purifying the metal from the slag then I may go through with the electroplating anyways.
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blogfast25
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| Quote: Originally posted by Upsilon | | However, since you say that the thermite alone can provide good results I may not do this; though if I am interested in further purifying the metal
from the slag then I may go through with the electroplating anyways. |
In theory thermites can provide very pure metals, if you start from very pure oxide and Al powder and get the formulation right.
For very purely plated metal, start from a very pure water soluble Cr compound,
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j_sum1
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I did a thermite with Cr2O3 and Al not long ago. I got a mixture of metallic Cr and Al2O3 for my efforts. Inseparable by mechanical means, and not
going to do anything favourable with any acids. The lumps look kind of cool though.
I may take up bloggers' suggestion and add a nitrate booster.
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Upsilon
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What do you mean by that? Treatment with HCl should dissolve everything but unreacted Cr2O3, then Cr3+ can be reduced to Cr by electrical means while
Al3+ won't be reduced.
[Edited on 28-10-2015 by Upsilon]
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blogfast25
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| Quote: Originally posted by j_sum1 | I did a thermite with Cr2O3 and Al not long ago. I got a mixture of metallic Cr and Al2O3 for my efforts.
I may take up bloggers' suggestion and add a nitrate booster. |
What can I say? Many feel called but few are chosen! 
Seriously though, getting aluminothermic reductions right is not as easy as it sounds. If I had a penny for each one that fails due to poor
execution...
1) use right stoichiometry, including boosters if needed.
2) always use a slag fluidizer.
3) larger thermites tend to give much better yields.
Just a few rules of thumb that can go far...
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elementcollector1
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Say, I've got a suddenly relevant question: I did a Vanadium thermite a few months back, and used the metal as my V element sample. However, I
remember some thread I started a few years back stating that V/Al thermites contaminated the produced V with up to 10% Al. I know I starved the mix of Al
when I lit it - did this help in any way?
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blogfast25
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Yes.
ALL chemical reactions have to be seen as equilibrium reactions, as is this one:
V2O5 + 10/3 Al < ==== > 2 V + 5/3 Al2O3
By increasing V2O5 in the mix you push the equilibrium to the right, thereby lowering the Al content in the metal obtained.
The process of adding extra target oxide to the mix is known in aluminothermic jargon as slagging. As I wrote above, aluminothermy has the
potential to produce highly pure target metals. Slagging is one measure that contributes to it.
Slagging does have one disadvantage: it lowers the end-temperature somewhat, as the extra oxide also needs to be heated up.
[Edited on 28-10-2015 by blogfast25]
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j_sum1
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| Quote: Originally posted by Upsilon |
What do you mean by that? Treatment with HCl should dissolve everything but unreacted Cr2O3, then Cr3+ can be reduced to Cr by electrical means while
Al3+ won't be reduced.
[Edited on 28-10-2015 by Upsilon] |
If I want Cr(0) and I don't want Al2O3, then I am not going to get there with my mix and an acid.
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Upsilon
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You said that you were getting Cr metal that was highly contaminated with Al2O3. Putting the whole lot of this in acid will leave you with an AlCl3
and CrCl3 mixed solution:
2Cr + 6HCl -> 3H2 + 2CrCl3
Al2O3 + 6HCl -> 2AlCl3 + 3H2O
The mixed solution could then be electrolyzed, since Cr3+ can be reduced in aqueous solution while Al3+ cannot.
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j_sum1
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I don't doubt I could do some electrolysis. However, at the moment I have some metallic Cr which I got from Cr(III). Acids are not going to purify
my mixture without going back to Cr(III).
I also don't think that HCl will have a lot of effect on Al2O3 in any reasonable time frame. This link suggests 110 days at elevated temperatures.
The goal is samples for the element collection.
I have a nice lump I purchased.
I have some electrolysis samples and will do more.
I don't have a good piece obtained via thermite. Current sample has the alumina contamination.
Acids will not get me from where I am to what I want.
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Upsilon
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Quick question - what are these "complexed selenium" pills that I am seeing everywhere? I cannot find any sort of formulation for them, but if they
contain selenium, then it would be interesting to have a go at extracting it.
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blogfast25
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| Quote: Originally posted by Upsilon | | Quick question - what are these "complexed selenium" pills that I am seeing everywhere? I cannot find any sort of formulation for them, but if they
contain selenium, then it would be interesting to have a go at extracting it. |
As with all these food supplements they contain very little of the advertised active ingredient.
For Se, that's a good thing, as otherwise pill poppers would suffer constant garlic breath! 
https://en.wikipedia.org/wiki/Selenium#Toxicity
And 'complexed' is likely to be a marketing buzzword, rather than an objective, scientific description.
[Edited on 29-10-2015 by blogfast25]
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Upsilon
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| Quote: Originally posted by blogfast25 |
As with all these food supplements they contain very little of the advertised active ingredient.
For Se, that's a good thing, as otherwise pill poppers would suffer constant garlic breath!
https://en.wikipedia.org/wiki/Selenium#Toxicity
And 'complexed' is likely to be a marketing buzzword, rather than an objective, scientific description.
[Edited on 29-10-2015 by blogfast25] |
Yeah I realized that not long after posting - most of these capsules contain only 200 ug of selenium. If I wanted a 1g selenium sample, I would need
5000 capsules. At around $4 USD for a bottle of 100 capsules, I would need 50 bottles which is $200 USD! I'm starting to grudgingly accept that
selenium may just be one of those elements I have to buy - selenium compounds seem to be ridiculously expensive while samples cost only a few bucks -
there's just no competition between the two.
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elementcollector1
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| Quote: Originally posted by Upsilon |
Yeah I realized that not long after posting - most of these capsules contain only 200 ug of selenium. If I wanted a 1g selenium sample, I would need
5000 capsules. At around $4 USD for a bottle of 100 capsules, I would need 50 bottles which is $200 USD! I'm starting to grudgingly accept that
selenium may just be one of those elements I have to buy - selenium compounds seem to be ridiculously expensive while samples cost only a few bucks -
there's just no competition between the two. |
Fear not, young element collector, for I hath found thy solution!
https://en.wikipedia.org/wiki/Selenium_rectifier
This has been on my to-do list for quite a while - dissolve away everything else, and you're left with a few thin plates of pure Se.
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Upsilon
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Interesting, I wonder just how much you could get out of one of those things? Too bad they're obsolete now and probably not as available as they used
to be.
EDIT: Ah! I hadn't even thought about selenates yet! 20g of sodium selenate will run about $25 USD which is more than reasonable. I'll keep this
option on the table as well.
[Edited on 29-10-2015 by Upsilon]
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