497
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Electrolysis of aqueous SO2 yeilding H2SO4
I don't believe there's a thread on this so I figured I should start a new one. The real issue is the cell construction so it seemed to fit
technochemistry best. Low temperature synthesis of sulfuric acid requiring sulfur, oxygen and water:
SO2 + 2H2O > H2SO4 + H2
Has anyone done it? Does anyone know anything about it? What do you think?
It can produce H2SO4 in at least 80% concentrations, I haven't read an actual tested maximum but it looks to be pretty damn high, the conductivity
starts to get really low around 98%.
My plan so far consists of: a coated stainless steel vessel pressurized to about 60 or 70 psi, charged with a quantity of liquid SO2 below some kind
of separator and an amount of dilute aqueous H2SO4. I'm thinking carbon electrodes, one immersed in (or just above?) the liquid SO2, then some kind of
separator, and another carbon electrode at the top. As the cell runs the lower compartment will accumulate more H2SO4 which may sink below the layer
of SO2, or maybe just dissolve it. Keeping it hotter will reduce the amount of SO2 dissolved in the electrolyte. I'm not sure whether that would be
good or not.
What do you think? Could this be workable? It may not be necessary to have it under pressure, it could just be run cold with dissolve SO2. I'll try to
work up a diagram, I may not have explained it clearly.
The attached file is a detailed study of this type of cell with hydrogen production as the main goal. http://www.savefile.com/files/1387739
[Edited on 18-2-2008 by 497]
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Pulverulescent
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I like to avoid the coloured drain-cleaner too, but your process seems a tad too fiddly for my liking.
I've made H2S04 by plating-out Cu from CuS04, years ago, using a Pt/Ir anode and graphite cathode. It worked well, but I didn't pursue it to see what
concentration I could get. The blue colour vanished completely as the copper plated up on the cathode(in lumps)leaving dilute acid.
A Pb02 anode might work as well, but graphite might be subject to oxidation.
Platinum is expensive and lead dioxide anodes are difficult, but once you've got the anode you can easily produce H2S04 from something as inoccuous as
blue vitriol.
You'll still have to boil off(cough!)some water to get 98%(cough!).
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Pulverulescent
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On second thoughts, Pb02 should be converted to PbS04 by the acid present; this happens in storage batteries.
I'm wondering if lead itself, would make a suitable anode.
Formation of a coating of insoluble PbS04 might make the lead anode inert to the solution, allowing electrolysis to continue.
Concentration could be upped by adding CuS04 as the electrolysis proceeds and 98% might just be achievable. At that concentration undissolved sulfate
would be anhydrous and white. The strong acid could then be decanted.
P
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Mr. Wizard
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PbO2 anodes are available at low cost. Charge up an old automotive lead battery to as high a charge as possible. This only works if the circuit is
complete, and one of the cells isn't 'open'. A shorted cell won't stop the process, but an open one will. Once the battery is charged and gassing, or
the electrolyte is at a high specific gravity, dump all the acid into a plastic pan. The cells that were working now have PbO2 anodes and Pb cathodes.
The working cells usually have a clear electrolyte, whereas the bad or shorted ones will boil , become warm and be turbid.
Now you have a fairly strong acid solution that can be used for other things, and you have plates in the battery that consist of PbO2 anodes on a lead
core, and you have a mostly lead cathode, although it can also contain Sb, Ca, and Sn.
Rinsing the plates with distilled water many times will extract even more acid, and make the job a little easier.
Use precautions against Pb poisoning and acid burns to skin and eyes. If you don't know these precautions, don't do this.
Melting the lead from the plates can be a nasty job, with sulfuric fumes being emitted. Another hazard is the Calcium hardener added to the plates
can lead to Stibine gas being generated by the dross scraped off when you melt the plates. Water reacting with the Calcium generates hydrogen which
then can generate Stibine. This is analogous to the Marsh Test for Arsenic.
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Pulverulescent
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Thanks for the timely advice, Mr Wizard, I didn't know Sb was that reactive, or that Calcium was included in the alloy.
I'd thought of making grinding media from battery-plates---Sb-hardened balls for milling BP. It's nice to know the possible hazards before starting.
And it's easy to forget that auto-batteries are electrolysis cells, generating and decomposing H2S04 by charging and discharging.
P
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Pulverulescent
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On further musing, it occurred to me that if Pb02 anodes can withstand the conditions in perchlorate cells, they won't be affected by H2S04 at all.
In a battery, it's the porous cathode that's converted to sulfate on discharge; charging the battery regenerates the acid.
P
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jimmyboy
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Why not try oxidizing SO2 with chlorine gas? NaCl is so much easier to electrolyze... or heck make some from oxidizer/HCL - just an idea
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Pulverulescent
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'Sorry, jimmyboy, your idea just won't work. . .thionyl chloride is produced by reaction of chlorine with S03.
P
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Sauron
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Well, no. SOCl2 is produced by oxidizing SCl2 and/or S2Cl2 with SO3, neat or in high % (>60%) oleum. In the latter case, Cl2 is pumped in (the
reaction is under autogenous pressure) in order to reconvert pyrosulfuryl chloride, which would otherwise represent a product loss.
The lab procedure is described in Brauser.
An alternative is to react SO2 with phthaloyl chloride in a pressure reactor, which is described in a JACS article by Thomas Kyrides which I have
posted on this forum previously. UTFSE. Or use the ACS Pubs search engine.
Chlorine reacts with SO2 and a catalyst, such as camphor or activayed carbon, to form SO2CL2 not SOCl2. Maybe that is what you had in mind?
Anyway you were quite correct re jimmyboy.
Sic gorgeamus a los subjectatus nunc.
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Pulverulescent
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Yes, Sauron, you're entirely right, that *is* what I had in mind!
Seriously though, I was half right, and in an area I have little interest or experience in.
Sulfur/chlorine compounds, though, seem surprisingly complex, as does the nomenclature.
P
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Sauron
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The nomenclature (old style) is ass-backwards, but the molecular formulas are straightforward.
The sulfur chlorides are relatively easy, if messy, to prepare, and unpleasant. You make them by direct union of the elements. Or, you get them as a
byproduct of the chlorination of CS2 to CCl4.
Thionyl chloride is IMO an overrated reagent, at least for chlorination of carboxylic acids. There are better ways.
Sulfuryl chloride is a lot easier to make than thionyl chloride and has its uses, but is a highly irritating and corrosive compound and no great fun
to be around.
All of these things are way too reactive with water to ever make the way the fellow was musing about. And anyway SO2 is not oxidized by Cl2. They
shake hands and form sulfuryl chloride. SO2Cl2 happily disassociates to SO2 and Cl2 although usually a lot slower than it formed.
Sic gorgeamus a los subjectatus nunc.
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microcosmicus
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| Quote: |
I've made H2S04 by plating-out Cu from CuS04, years ago,
using a Pt/Ir anode and graphite cathode. It worked well, but
I didn't pursue it to see what concentration I could get.
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To answer this question, I made an experiment. I dissolved
CuSO4 in 93% H2SO4 to get a dark brown saturated
solution, then electrolyzed it with graphite electrodes,
plating out the copper. To avoid competing process
problems, I had to keep the current low --- bubbles would
arise at the cathode if I cranked the potential up --- so it went
sloooowly but, after letting it run overnight, I got the most of
the brown out of the liquid.
Unfortunately, the result looked more like milk because the
graphite electrodes corroded significantly, but it is should serve
as a proof of the concept of producing concentrated H2SO4
electrolytically or, at least, as encouragement to explore this
idea further. When I get some platinum electrode wire, I will
repeat this experiment.
[Edited on 25-3-2008 by microcosmicus]
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Pulverulescent
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Micro, was your acid clear before you added CuS04, or was it the oxymoronic dirty-drain-cleaner we all love to hate?
I can only guess the reason for the milkiness; pure graphite should disintegrate somewhat to a black sludge which settles out slowly.
Compressed carbon rods can contain a binder/glue which could possibly cause a white suspension as it oxidises, releasing the binder.
The anode I used was Pt/Ir wire 0.5mm thick, but I started with a water solution of CuS04.
IMO, graphite is pretty useless as anode material (though it will just about work for chlorates), as its oxidation reduces H2S04. Suspended carbon
colours H2S04 brown, but at 98% it's fully oxidised to C02, leaving a clear acid.
When you get your platinum, you'll get good results you can post, so lazy assholes like me don't have to get off our butts.
But, seriously, I'll have to find my old anode, for a bit of wire-pulling.
'Might try electrolysing the brown stuff to see if it can be cleaned-up that way.
P
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microcosmicus
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| Quote: |
Micro, was your acid clear before you added CuS04, or was
it the oxymoronic dirty-drain-cleaner we all love to hate?
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My acid was the good clear stuff from a chemical store.
I deliberately dirtied it up by adding the Cu2SO4.
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Compressed carbon rods can contain a binder/glue which could possibly
cause a white suspension as it oxidises, releasing the binder.
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As for the carbon rods, they were intended to be pencil
leads, but i heated them to 1500C before using them, so
the organics were gone and the only binders left were
clay. Of course, that would also make a cloudy suspension.
This was certainly not the best choice of electrode
for a harsh environment like H2SO4 --- in particular.
I imagine that oxygen liberated at the anode could
combine to make H2SO5 (a.k.a. pirahna acid)
which would eat right through them. As a matter
of fact, when I tried a large overpotential and so
had vigorous gas production at both electrodes,
the electrodes would be corroded in a matter of
minutes, if not seconds.
Also an update: When I look at the product now, a
day later, it is quite clear with maybe only a trace
of brown left, in contrast with what I started with,
which is brown as a piece of the glass used to make
the dark bottles for storing beer, photo developer,
and such light-sensitive chemicals. Since the fluid
is viscous and graphite is light, it took a long time
for the gook to settle. This experience inspires me
to build myself a centrifuge.
[Edited on 26-3-2008 by microcosmicus]
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Pulverulescent
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Micro, If you can get clean 93% H2S04 easily you're just 5% short of concentrated.
Boil off a few per cent H20 and you're (cough!) there.
Hard pencil graphite contains a fine clay to give the required hardness; soft pencils use less clay.
Compressed carbon, though, will mess up all electrolyses, including chlorate-making.
'Just an idea--- prolonged electrolysis of H2S04 should produce some H2S208, which, if used for concentrating dilute HN03 should oxidise the issuing
N02 to N205 by producing some H202 as vapour.
That way, strong, water-white HN03 might be obtainable without resorting to vacuum distillation.
RFNA looks great, though! And I've nitrated small amounts of hexamine with amber HN03.
White acid is safer, I'm told!
P
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