Altreon
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Electrolytic oleum
Hello. This is my first post on the sciencemadness forums. I sincerely apologize for any errors or previously documented information mentioned
hereafter.
In 2024 I tried electrolyzing a test tube for 2 weeks using two 0.3mm-thick and 10cm-long platinum wires. The electrolysis bubbled on both the cathode
and anode and smelled like things burnt by an ozone lamp(?). In the first day, all the brown contaminants that were in my somewhat dirty sulfuric acid
(stored in HDPE!) clumped up at the top of the liquid layer and eventually disappeared, turning the liquid from clear/brown to cloudy/white + brown
particles and finally to a clear liquid. I stopped once the electrolysis started producing a faint white smoke that didn't go away after the current
was stopped.
Unfortunately I did not recall any tests I performed on the liquid, aside from it creating a mysterious azure blue when I accidentally forgot to wash
the Pt cathode coated in NiSO4 soln. The Pt cathode had a very shiny black coating that would only go away when used as an anode in the next H2SO4
elec. run, which I suspect is the vitreous carbon that I also observed in elec. of glacial acetic acid with a few drops of H2SO4 (I could make another
post if there is interest in this).
I initially did this in an attempt to synthesize anhydrous H2S2O8 as it seemed I successfully produced some peroxide containing compound via the elec.
of 70% sulfuric acid, that didn't get destroyed by the platinum, that bubbled on cessation of elec., and bubbled harder with permanganate. It seems I
failed to account for it decomposing into H2SO4, SO3, and O2 in pure sulfuric acid.
After these promising results, I electrolyzed concentrated sulfuric acid for over a month using the same wires plus a third wire on the anode at 5A
using a 100mL ground glass bottle as a container, filled with 90mL of H2SO4. I tried 2A but the rate was abysmal and I had to heat it up because it
seemed(?) that conductivity decreased over time. After a month, the white fumes were able to escape from the ground glass joint, condensing some very
nice sulfuric acid vapors onto the immediate area around the flask. The spraying stopped after electrolysis was stopped, but smoke still emanated when
the bottle was opened.
Unfortunately again I was only able to test on this seemingly pure solution of dilute oleum once, but I remember pulling some through a pipette and
watching it fume and smoke through the air as I dropped it on a moist kitchen sink, along with adding iodine and observing the beautiful emerald green
of iodine cations only formed in such harsh dehydrating and oxidizing conditions as in oleum. By the next day the iodine solution became a regular
brown color, and I was unable to store any of the dilute oleum for testing due to personal issues.
I am posting this because I would like to hear the community's opinions on this synthesis of dilute oleum. It does not require heat and does not
require you to attend to the reaction at all. There are the obvious problems involved with obtaining Pt electrodes, and I have failed to find a
suitable metal that could survive as the cathode, and the wires cost the equivalent of $16 each, but if you can get past that, this method seems
viable for producing some (currently) unknown concentration of oleum. Even if the oleum turns out to be too dilute in the end, I am sure you could
purify and concentrate drain-cleaner sulfuric acid with this method, since it seems that the electrolysis ozonates all the organic impurities until
all you have left is clean sulfuric acid.
(For the cathodes: Ni/Cu produce a mix of H2S and SO2 along with green/voluminous black surface corrosion respectively, Mo slowly decomposes into Mo
blue above the H2SO4 surface and white milkyness below without any oleum content, while W very slowly turns brownish and turns the soln. cloudy,
though it might still produce oleum as I am currently conducting this tungsten experiment as I speak. I believe that a C cathode just crumbles while
Ag very quickly turns into white milkyness, notably unlike clear AgHSO4 soln. Ti dissolves into a hot purple soln relatively quickly. The most
resistant metal so far aside from Pt is W.)
[Edited on 13-12-2025 by Altreon]
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Altreon
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Registered: 11-12-2025
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Tungsten is not (or does not appear to be) a suitable electrode material. Similar to molybdenum, it looks very promising with a steady flow of H2, but
the cloudy acid produced is seemingly about as concentrated as the original acid because of its reaction with wood.
It seems Pt for both the cathode and anode is the only way this can work. Maybe Au could work for the cathode (anodic Au dissolves to form a purple
soln. in dilute sulfuric acid) and PbO2 could work for the anode though I've never seen those electrodes.
Edit: the sulfuric acid formed from W electrolysis reacts especially violently with water. The sulfuric acid had already cooled down to room temp.
Nothing precipitated, so I'm not sure if a compound without tungsten (i.e. oleum) actually did form and I did not electrolyze for long enough.
[Edited on 16-12-2025 by Altreon]
[Edited on 17-12-2025 by Altreon]
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Mister Double U
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Hello Altreon,
Regarding a Tungsten cathode: Do you smell any SO2 coming from the cathode? If not, you are decomposing water and thus concentrating the acid.
After running your cell for some time you could dilute your acid down and titrate to determine the concentration. If you compare that to the original
acid you will see if you concentrated the acid (you would need to draw a same weight sample from both groups).
If you have a big enough sample you could do it with a known molarity Sodium Bicarbonate solution in lieu of a ph indicator. You are done when
bubbling ends.
That might be more reliable than the wood test.
Best greetings!
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Altreon
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Registered: 11-12-2025
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Mood: oxidizing
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There was no detectable amount of SO2 coming from the W cathode, but I can't consider this as reliable because Mo works the same way while a nearly
invisible reaction occurs with the sulfuric acid, which may or may not dilute it.
I spilled all the electrolyzed sulfuric acid, which is why I compared it with its rxn. with wood. I will repeat this electrolysis with more sulfuric
acid and try to titrate it this time.
On a different note, do you know of any ways to qualitatively determine if W is present in soln that doesn't involve thiocyanates?
[Edited on 17-12-2025 by Altreon]
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Altreon
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Registered: 11-12-2025
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Actually practically doing this
TL;DR I turned 93% sulfuric acid into >5% oleum in 4 days.
I have electrolyzed approximately 200mL of 93% sulfuric acid (conc. measured via density) with a 12V5A DC power supply along with two intertwined
10cm*0.2⌀ Pt wires (practically only 3cm touched the soln) as cathodes, a gauze-like 1x2cm2 Pt electrode with a PTFE handle as an anode,
a 250mL amber glass bottle, a crumpled up ball of PTFE plumbing tape, and about half a week.
The following is a list of notable observations during the relatively simple synthesis:
- Starting from the second day, a small amount of smoke started being produced. This gets progressively worse over time, and the ball of PTFE was my
solution to stop the smoke from escaping. It is made of vaporized H2SO4 and it will condense on any nearby surface (i.e. bottle
walls, electrodes, alligator clips) given the chance. I decided to stop the experiment when the smoke started passing through the tape in large
quantities.
- The solution initially gets quite hot, far too warm to touch. The temperature decreases to mildly warm after a day and decreases only slightly over
the following days.
- The final concentration of oleum is >5%, determined using a graduated cylinder and online density vs. conc. charts (93.2g/50mL ≈ 1.864 g/mL ≈
>5% oleum). It smokes profusely even when cooled, though the vapors don't condense.
- The main problem with the synthesis is the fuming. I did not measure the starting mass of H2SO4 so I cannot say whether loss
of yield to evaporation of SO3 is a significant problem.
- This synthesis took over a month to make a comparable conc. of oleum out of 100mL 98% H2SO4 when I just used 3 Pt wires as
electrode. Surface area is important as with any electrolytic experiment, which is a problem considering the only valid anode is Pt and I have yet to
find a cheap and suitable cathode other than Pt.
- It is important that the electrodes reach near the bottom of the soln. to allow for mixing. Otherwise, the temperature spikes in the upper part and
the product smokes away.
- The electrodes and the tape seal will all be covered in oleum. Let the system cool before opening it if you don't want to be greeted by a rapidly
heating smoke bomb.
Miscellaneous images:
   
Edit: Another thing I forgot to mention is that a black insoluble precipitate initially appears on the Pt cathode. It flakes off after a few days into
large yellow fragments that bubble and eventually disappear. I believe that these are organic impurities from my contaminated
H2SO4 that are being sulfonated and oxidized to gases by trace Marshall’s acid.
- There is also a phase where the soln. bubbles on its own even after you disable the current. This happens before the smoking starts and I believe it
is due to unstable Marshall's acid (H2S2O8) produced by the reduction of HSO4-. This acid does
not appear to be produced in isolable quantities, although electrolysis of less concentrated and very cold H2SO4 is one of its
main synthesis routes.
- I will be testing if solutions of NH4HSO4 in conc. H2SO4 produce NH3SO3 when
electrolyzed. Sulfamic acid is surprisingly insoluble in most solvents and this may be a viable synthesis route assuming the bisulfate is soluble.
I've tried analogously making Na2S2O7 but NaHSO4 seems quite insoluble in acid so this might not work.
Another interesting idea would be the synthesis of (K or NH4)2S2O8 as it does not seem anyone on this
forum has ever tried.
Does anyone know which among (K and Na)2S2O7 is more suitable for making oleum the standard way? Of course the
potassium salt is normally harder to obtain but electrolysis of conc. H2SO4 over the corresponding bisulfate would make the
synthesis of both easy.
Does BaS2O7 exist? BaSO4 is significantly soluble in conc. H2SO4 and assuming the pyrosulfate
exists and is insoluble, it would probably be much easier to thermally decompose than any alkali metal pyrosulfate.
[Edited on 15-4-2026 by Altreon]
[Edited on 16-4-2026 by Altreon]
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