mycotheologist
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Producing H2SO4 via electrolysis of ammonium sulphate
Alright so this is my current understanding (I don't really know how it works so don't take anything I say here as truth, I'm just learning) of the
electrolysis of aqueous ammonium sulphate:
NH4+ is reduced at the anode, yielding NH3. SO42- is oxidised at the cathode to produce
H2SO4 (I don't know if thats correct, I'm just going on what someone told me).
So the major issue here is that the NH3, being a strong base, will just react with the H2SO4, reforming ammonium
sulphate. So I'm starting this thread for ideas on how to get around this problem. This will be a good learning process because the solutions we come
up with won't just apply to this ammonium sulphate situation, they'll apply to other situations too.
One solution that comes to mind is using a membrane to separate the cathode product from the anode product, like they do when they wanna produce NaOH
(instead of NaClO) via the electrolysis of NaCl. I don't know how difficult it is to make a membraned cell but I reckon it would vastly expand the
home chemists electrosynthesis capabilities.
Another idea: maybe you could add something that would neutralise the NH3 before it can reach the H2SO4. For example, if you surrounded the anode with
some kind of mesh thats coated with insoluble strong acid, maybe the ammonia would get protonated straight after its produced at the anode. Alright
that would be way too impractical to build a device like that.
Another idea: maybe we could invent some kind of weird electrode that traps the product. That actually might work. Lets say you had a hollow metal
electrode that was coated on the outside with plastic. The only ammonium ions that can be oxidised by this electrodes are ions inside the tube. The
tube could be designed so that the NH3 gas can only move upwards where it travels to the top of the tube and out into the air. A problem with this is
that NH3 reacts with water to form NH4OH which will not rise up, out the tube and into the air. Water isn't the only solvent polar enough to conduct
electricity and solvate ions though. Maybe theres a suitable solvent that is unreactive towards NH3. I'm guessing polar solvents like DMSO and DMF
will conduct electrons and solvate ions in the same way water does, would NH3 react with these compounds? This actually seems pretty feasible. Another
idea that pops into my head is using an aquarium pump or something to create an upward current in the follow electrode. That way, the product, whether
it be a gas or a salt will go where you want it to go. It could be directed through some kind of filtration system. Thats getting way too elaborate
though, the hollow electrode with unreactive solvent idea is something I could test out myself.
Are there any major flaws in my thinking or is this actually a feasible idea?
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barley81
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You probably can't make sulfuric acid from ammonium sulfate at reasonable cost. Sulfuric acid is cheap, so there wouldn't be a point in doing so
anyway. However, you can make ammonium persulfate by electrolyzing a solution of ammonium sulfate and sulfuric acid according to Wikipedia. This is
used for etching printed circuit boards, and can be used to do some pretty cool experiments (oxidize silver to oxidation state +3 and nickel to +4).
Persulfate is also a strong oxidizer, so a platinum electrode is probably required.
Ammonium bisulfate is formed by decomposition of ammonium sulfate at 250˚C (wiki again). This can probably be used as the starting point for the
electrolysis, eliminating the requirement for sulfuric acid.
[Edited on 9-4-2012 by barley81]
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Adas
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Probably if the cathode would have very very small surface area and the current would be high, the ammonia gas would have no time to dissolve.
Rest In Pieces!
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barley81
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Ammonia dissolves extremely (!) quickly in water. A test tube of ammonia is "swallowed" very quickly by a tub of water (see ammonia fountain
experiment). Especially since the solution is acidic, ammonia would dissolve very quickly. There might not even be a visible evolution of gas. If you
ran the cell, you would just make hydrogen and oxygen, and perhaps persulfate. If you want something "close" to sulfuric acid, just decompose the
ammonium sulfate into ammonium bisulfate. Bisulfates are used as sulfuric acid substitute for some experiments.
Post #200!!!
[Edited on 9-4-2012 by barley81]
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Aperturescience27
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If you really need sulfuric acid from ammonium sulfate (or just want to prove that you can do it if you want to), you could mix ammonium sulfate and
some metal (I'm thinking zinc) and heat it to 250 C so that the ammonium sulfate decomposes to ammonia and ammonium bisulfate (which will be liquid at
that temperature), which reacts with the zinc to make zinc sulfate, ammonium sulfate, and hydrogen, the ammonium sulfate decomposes to bisulfate again
and reacts with more zinc. You could also use a metal oxide instead of a metal, and just not get hydrogen. Then you could electrolytically reduce the
zinc sulfate to zinc at a (presumably zinc) cathode, produce oxygen at a platinum anode, and get sulfuric acid left in solution.
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AJKOER
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I think it would be an interesting theoretical/educational exercise (meaning impractical and expensive) to attempt making very dilute H2SO4 (and side
products) by boiling (or warming) Al (or pre-treated aluminum by heating Al till it is red, forming Al2O3/weakened gamma Al) in aqueous or moist
(NH4)2SO4 (from say MgSO4 plus ammonia water) plus some sea salt (to serve both as an electrolyte, assist in removing protective Al2O3 and also to
salt out ammonia gas).
Logic:
NH4+ = NH3 (g) + H+
H20 = H+ + OH-
Al + 3 OH- --> Al(OH)3 + 3 e-
H+ + e- = .H
.H + .H = H2 (g)
2 Al(OH)3 + 3 (NH4)2SO4 + Heat = Al2(SO4)3 + 6 NH3 (g) + 6 H2O
Reference: search internet for "Atmospheric Corrosion of Aluminum in the Presence of Ammonium Sulfate Particles" by R. E, Lobnig, et al.
Note, the experiment above differs from the Lobnig reference with the combined effect of applying heat and some sea salt.
Last step to produce very dilute H2SO4, dilute to neutral pH (after separating out any formed basic aluminum sulfate and mixed salts with ammonia)
causing the aluminum hydroxide to precipitate out:
Al2(SO4)3 + 6 H2O = 2 Al(OH)3 (s) + 3 H2SO4
[Edited on 11-4-2018 by AJKOER]
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LearnedAmateur
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Can’t comment on the electrolysis itself but there is no reduction or oxidation going on - it’s a change in charge as opposed to oxidation number,
since all atoms retain the latter on either side of the equation. In order to get to the acid, two hydrogen atoms are pulled off the ammonium cations
and tacked onto the SO4(-2). An example of a relevant reduction would be SO4(-2) to SO2, where sulphur changes oxidation state from +6 to +4.
In chemistry, sometimes the solution is the problem.
It’s been a while, but I’m not dead! Updated 7/1/2020. Shout out to Aga, we got along well.
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RawWork
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In electrochemistry, there's one very practical parameter called "current density at specific electrode" that determines what will happen. At least
that's how I call it. That will enable us to make anything, even what many would consider impossible. For example hydrogen peroxide can be made from
water in at least two ways:
oxidation of oxide or hydroxide anion (however you imagine it)
reduction of oxygen
Even sodium can be made from aqueous NaCl or any other water soluble Na salt. Well for such case at least theoretically, because that's practically
very extreme and may not be possible, at least for Na. See yellow sodium light in water:
https://www.youtube.com/watch?v=obLwJyYprqg
Under strong enough current density at cathode, believe it or not, ammonium and ammonia will reduce to nitrogen, as wikipedia and crc handbook of
chemsitry and physics show us:
N2 + 2 H2O + 6 H+ + 6 e ⇌ 2 NH4OH (0.092 V)
There are many more ideas. We can even oxidize N to N oxides aka nitrates and nitric acid etc:
N2O4 + 2 H+ + 2 e ⇌ 2 NHO (1.065 V)
Hydrazine too maybe  .
[Edited on 11-4-2018 by RawWork]
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SWIM
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this seems to have worked.
Haven't tried it myself
Attachment: crystals-09-00667.pdf (2MB)
This file has been downloaded 386 times
Edit:
Hmmm, calls for irridium and ruthenium.
probably not a cheap setup to put together.
[Edited on 30-10-2021 by SWIM]
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Fantasma4500
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ive heard tantalum MMO is whats ideal for doing persulfate
just for fun i dumped a platinum anode in, quite dilute NaHSO4 as im hearing its very corrosive to anode
it has started turning a bit yellow, interestingly it smells like when you run NaCl and it has finished turning it all into perchlorate, this smell
was supposedly Cl2O or ClO2- turns out its probably ozone
which means if you run the cell continously you might end up with H2O2 too
i havent yet tested if theres persulfate in there but i assume i can concentrate it down and mix with NaCl- this normally generates chlorine gas
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