Hexabromobenzene
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Obtaining bisulfate, alkali, sulfuric acid with high efficiency by electrolysis in several step
As you know, I have developed a very effective diaphragm for electrolysis from bag vacuum cleaner bag, which works even with 3 volts of voltage
And now I have a good idea of what to do with it. You can get sulfuric acid, alkali, bisulfate by electrolysis. This process is known, however,
its efficiency is small because of the exhaustion of ions.
I propose a method in several stages. First, we electrolyze sodium or potassium sulfate with a diaphragm with an excess of sulfate at the bottom of
the anode container (potassium sulfate is better). At first, a bisulfate will be obtained, which can continue to be converted into sulfuric acid by
electrolysis or other methods
But pay attention. With electrolysis of potassium / sodium sulfate to bisulfate current efficiency does not fall since potassium/ sodium ions are not
exhausted and little hydrogen ions. For every 26.8 Amperhours you will convert the mol of sulfate into mol bisulfate and mol alkali. Also, if cathode
container contains sulfate you will have more current efficiency , but it will reduce the depletion of ion sulfate.
And so electrolysis with very high efficiency, you can get a bisulfate that can be turned into sulfuric acid and alkali. Bisulfate is best converted
into sulfuric acid in separate electrolysis or other methods.
Potentially, if you use a small anode from lead dioxide or even lead with a small addition of fluorine ions or even chloride, you have a chance to get
persulfate. Especially good potassium salts due to the low solubility of perisulfate
[Edited on 21-8-2025 by Hexabromobenzene]
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Chemister
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That's funny I was just thinking about how to make sulfuric acid electrolysis cells more efficient and work at larger scale, and making bisulfate
instead of sulfuric acid directly was one of the things I was thinking about to reduce the issue of hydrogen ions going back into the cathode chamber.
Why is it that potassium sulfate is better than sodium for this? Is it just because potassium sulfate is more soluble, or is there some other reason?
Something that could be useful to make this process more economical is that it should be possible to convert the alkali back into sulfate using
calcium sulfate as a sulfate source:
Carbonate the hydroxide using CO2 or air
2KOH + CO2 → K2CO3 + H2O
Let the carbonate solution sit over calcium sulfate. Calcium sulfate is much more soluble than calcium carbonate meaning that the carbonate with
eventually react with the calcium sulfate to form calcium carbonate leaving the sulfate in solution.
K2CO3 + CaSO4 → K2SO4 + CaCO3
Net reaction:
2KOH + CaSO4 + CO2 → K2SO4 + CaCO3
Calcium sulfate is only slightly soluble in water (~2.6g/L), so even if there is a huge excess, only a small amount will end up in solution, which
means that you probably don't really have to worry about it contaminating your electrolyte unlike other salts like magnesium sulfate. Calcium sulfate
is also extremely cheap and abundant, and should be available pretty much everywhere.
This is likely more effort than it is worth if you are using sodium sulfate, since sodium sulfate is already quite cheap, but might be worth doing if
you are using potassium sulfate since (in my experience at least) potassium salts can sometimes be kind of pricey. This could also be used to make
sodium sulfate from sodium carbonate and calcium sulfate which might be cheaper than just buying the sodium sulfate directly.
Also, if you had a way of carbonating the hydroxide continuously, you could constantly extract electrolyte from the cathode chamber, carbonate it, and
allow it to pass through bed of calcium sulfate before draining back into the cathode chamber in order to continuously replenish the sulfate and
prevent alkalinity from building up in the cathode chamber. Finding a source of pure CO2 to carbonate the hydroxide continuously is difficult, but
with sufficient airflow it may be feasible to use the hydroxide to pull CO2 directly out of the air. I have been working on a wet scrubber project
that om paper can absorb CO2 at a rate sufficient to keep up with an electrolysis cell running at a moderate current, but I won't know for sure until
i actually finish building it and can test it. Again, this is probably more trouble than its worth if you are trying to make bisulfate or sulfuric
acid at a small scale, but might be worth it if you are trying to make huge amounts.
[Edited on 21-8-2025 by Chemister]
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Hexabromobenzene
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Potassium sulfate dissolves in water less than sodium sulfate. But potassium bisulfate dissolves much better than sodium bisulfate. Even if you pour
potassium sulfate on the bottom of the anode chamber, it will dissolve and become a solution of bisulfate
Potassium hydroxide also absorbs carbon dioxide from the air much better. After a couple of weeks in the open air, potassium hydroxide becomes potash.
Sodium hydroxide does not completely become carbonate even after a month. Perhaps a mixture of hydroxides will work
I have potassium sulfate from ash. Since the wood was burned with coal, it was contaminated with coal ash and some of the potash became sulfate.
Potassium sulfate is not soluble in a strong solution of potash above 1.3 and crystallizes into large crystals.
Potassium sulfate is also present in pure wood ash, but it is much less. About 20% of potash. There are also chloride impurities, which is a problem.
[Edited on 21-8-2025 by Hexabromobenzene]
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clearly_not_atara
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It makes no sense to convert KOH into K2SO4. It's like converting gold into lead. KOH is far more useful.
If you can only get sodium sulfate you can just use that. If you want to convert Na2SO4 to K2SO4 the obvious question is "what is your potassium
starting material?" because anyone who cannot get K2SO4 obviously cannot get most things and we cannot easily guess what they can get. This is not an
issue that can be solved in general.
If you cannot get any sulfates then the obvious question is "what forms of sulfur CAN you get?" and the solution is basically going to be to oxidize a
neutral-to-alkaline solution of sulfur compounds with H2O2 which will form sulfate ions in good yield. FWIW it is usually possible to obtain potassium
bisulfite KHSO3 because of its use in winemaking and food preserving.
And in the case where you can obtain gypsum (CaSO4) but not any other sulfates, then you can of course convert this to the sodium or potassium salts
by metathesis with Na2CO3 or K2CO3. You can probably obtain gypsum because it is aka plaster.
[Edited on 21-8-2025 by clearly_not_atara]
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Chemister
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True, but the KOH produced by this process may not be easily recoverable since it will be mixed with a ton of sulfate and dissolve in a bunch of
water. By carbonating the used electrolyte and treating it with calcium sulfate you could simply resuse the electrolyte rather than boiling it down
and trying to separate the sulfate from the hydroxide.
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clearly_not_atara
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It should be possible to purify the KOH product by adding MeOH, which will ppt K2SO4, then distilling off the MeOH to leave behind a hydroxide
solution. KOH is quite valuable and has been a primary target of Hexabromobenzene's electrolytic odyssey.
[Edited on 21-8-2025 by clearly_not_atara]
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Hexabromobenzene
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Potassium hydroxide can be separated very easily from sulfate by evaporation. In concentrated alkali, other salts are practically insoluble
You do not need to make a large volume of catholyte if you only make bisulfate because the main source of alkali is from the anode chamber. You can
make a small cathode chamber for this synthesis
Keep in mind that potassium hydroxide very easily absorbs carbon dioxide from the air. It must be processed quickly and stored in a closed container
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Texium
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Thread Moved
21-8-2025 at 17:35 |
Chemister
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That makes sense. One thing to consider is that if you use a small cathode chamber it will probably decrease the efficiency of your cell because a
high concentration of KOH in the cathode chamber will lead to more OH- ions migrating across the membrane into the acid side neutralizing your
product, which is more or less the issue with a pure sulfuric acid cell but with OH- ions. Remember that KOH is much more soluble than K2SO4 meaning
that you can get into a situation where the concentration of KOH in your cathode solution is so much higher than the concentration of K2SO4/KHSO4 in
your anode solution that almost all of the current passing through the membrane is hydroxide ions rather than potassium ions meaning that your
efficiency is basically 0%.
Ideally to maximize the efficiency of the system the hydroxide concentration should be kept as low as possible to prevent hydroxide from crossing over
the membrane. This presents and interesting trade off if you are trying to produce potassium hydroxide as one of your primary outputs. You can
increase the current efficiency of your system by keeping a low KOH concentration in the cathode chamber, but that also makes it more difficult to
extract the KOH from solution. If you targeted a maximum concentration of 1 mol/L of hydroxide in the cathode solution per 2 mol/L of potassium in the
anode solution you could probably still get something like 75% efficiency while having a high enough concentration of hydroxide for it to be worth
recovering.
Also, for removing the water from the hydroxide solution you definitely have to boil it rather than evaporating it in open air since the amount of air
needed to evaporate the water would almost definitely have enough CO2 to convert most if not all of the hydroxide to carbonate (also potassium
hydroxide is deliquescent, so you can't drive all of the water off by evaporating it anyway).
Also2 if you are trying to make potassium hydroxide from potassium sulfate you might be able to do it by just reacting potassium sulfate
with calcium hydroxide. Calcium hydroxide and sulfate have similar solubility, so if you just mix the calcium hydroxide and potassium sulfate you will
get a mixture of potassium sulfate and hydroxide which would probably be comparable to what you end up with in your cathode solution if you kept the
hydroxide concentration moderate. If you want a higher concentration of hydroxide you could pass the potassium sulfate through a column of calcium
hydroxide in order to create a concentration gradient (see Countercurrent Exchange) which could theoretically reach nearly 100% potassium hydroxide.
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Hexabromobenzene
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Large concentration of potassium hydroxide in cathode chamber will not affect production bisulfate. Hydroxide ions will neutralize and release heat.
There is no point in striving for the maximum concentration of sulfate in the cathode chamber at the first stage. Bisulfate will form due to the
movement of potassium ions to the cathode container. And until the whole sulfate goes to the bisulfate at anode chamber, we will have high current
efficiency
Only diffusion due to different concentrations or salinity can be increased
Potassium salts are interesting that potassium residue can be removed as a persulfate from the anode chamber if we making sulfuric acid
Of course, hydroxides cannot be evaporated in the open air. Only boiling
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Chemister
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The problem is that hydroxide ions can also carry current just like potassium ions. If the hydroxide concentration is too high most of the current
will be taken up moving hydroxide from the cathode chamber to the anode chamber which doesn't produce any product, instead of moving potassium from
the anode chamber to the cathode chamber.
Edit: nevermind i forgot that the rate that hydroxide can come over should limited by the electrolyte concentration in the anode chamber meaning that
worst case you should still get 50% efficiency. That said, it seems like the situation would be similar in a regular sulfuric acid cell which runs on
magnesium sulfate, since hydroxide ions will be continously removed by precipitation. It seems like there should always be sulfate ions coming over
making the sulfuric acid concentration increase, so its weird that the efficiency is so low. Also from what I have seen, magnesium sulfate cells also
typically have trouble getting a sulfuric acid concentration more than a few percent which is also kind of strange.
I guess my question is how would the bisulfate method be more efficient than the magnesium sulfate method? Both methods sequester one of the water
ions (H+ or OH-) to prevent it crossing the membrane, so why would one be any better than the other?
[Edited on 22-8-2025 by Chemister]
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Hexabromobenzene
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But experiments show that one of the chamber can even be empty at the beginning
You can have only sulfate in the cathode chamber and water with a couple of drops of sulfuric acid in the anode, but you will still get sulfuric acid
and alkali, but with 2 times less efficiency than sulfate in 2 chamber
Same is true for sulfate only in the anode chamber.
Magnesium sulfate method is also not bad. But the conductivity of the electrolyte will drop significantly due to the large volume of hydroxide paste
in the cathode chamber and the depletion of ions
Concentration of acid possible during electrolysis is theoretically unlimited. In practice, osmosis forces are probably at work.
[Edited on 23-8-2025 by Hexabromobenzene]
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macckone
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Using a three compartment system will give better results based on extensive studies done. With the sulfate added to the middle chamber and extract
the hydroxide and bisulfate from the electrode chambers. Keeping the middle chamber below the electrode chambers will reduce contamination.
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Hexabromobenzene
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A couple of thought about electrolysis sulfates
Example: electrolysis of sulfate and has exhausted half of sodium sulfate in the cathode compartment. We have 2 hydroxide ions and one sulfate ion. At
first I thought the yield would be 25%, but sulfate Ion has a charge 2. And for us, charges and not the number of ions are important. The output
should be considered how many electrically charged particles will remain with sulfate. Then the yield will be 50%
Also thoughts about hydrosulfate. If we have sodium hydrosulfate, it is beneficial for us that the cathode compartment is larger than anode. But if we
have only sodium sulfate, it is beneficial for us that the anode compartment is more cathode
But the strong difference in volumes is not recommended. Osmos forces will reduce yield
[Edited on 28-9-2025 by Hexabromobenzene]
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Hexabromobenzene
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Quote: Originally posted by macckone  | | Using a three compartment system will give better results based on extensive studies done. With the sulfate added to the middle chamber and extract
the hydroxide and bisulfate from the electrode chambers. Keeping the middle chamber below the electrode chambers will reduce contamination.
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Two chamber cell will work at 5 volts or even 3. Three chamber cell will require much more voltage.
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macckone
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| Quote: Originally posted by Hexabromobenzene | | Quote: Originally posted by macckone | | Using a three compartment system will give better results based on extensive studies done. With the sulfate added to the middle chamber and extract
the hydroxide and bisulfate from the electrode chambers. Keeping the middle chamber below the electrode chambers will reduce contamination.
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Two chamber cell will work at 5 volts or even 3. Three chamber cell will require much more voltage. |
Purity vs voltage. Three chamber cell gets nice product.
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Hexabromobenzene
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So, here we go. The first electrolysis of sodium sulfate. It was discovered that if the sulfate is pure and free of impurities, a small amount of
ozone is released at the anode. This indicates that the lead anode has a high potential. However, the anode's degradation is minor. It is coated with
a thin layer of oxide, but underneath is the metal.
Impurities act as depolarizers and significantly accelerate destruction. High current density also allows for increased potential, which is beneficial
for anode preservation.
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Hexabromobenzene
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2l concentrated sodium sulfate solution (400 grams per liter) were electrolyzed in an apparatus with a diaphragm made of a three-layer vacuum cleaner
filter. Сurrent was 2.2 A for 6 days, and the voltage was 5 V (approximately 300 ampere-hours). Do not use such a concentrated solution. Sodium
sulfate crystallizes in the cathode chamber on the diaphragm and cathode.
After electrolysis, the anode chamber was quickly removed to prevent mixing. Volume of liquid after electrolysis anode chamber was approximately 1
liter, and cathode chamber, 600 ml.
The anode solution was evaporated in a polypropylene container in an oven. This yielded 120 mL of sulfuric acid with a specific gravity of 1.54 and
230 grams of sodium hydrogen sulfate with residual acid after filtration with polypropylene fabric.
Сontents of the cathode chamber were frozen in a refrigerator, and precipitate was separated. 350 ml of sodium hydroxide solution with a density of
1.17 was obtained.
230 grams sodium sulfate was separated from cathode solution after refrigeration. However, another portion crystallized on the diaphragm and
electrodes and was dissolved separately.
Polypropylene diaphragm made from a 3-layer vacuum cleaner bag has significant leakage at different liquid levels but protects against diffusion. It's
better to make a diaphragm from a 6-layer vacuum cleaner bag using the method described in the diaphragm thread.
[Edited on 9-10-2025 by Hexabromobenzene]
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Hexabromobenzene
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Destruction of the lead electrode was insignificant. Not more than 1-2 grams of lead dioxide powder was in the anode solution. Destruction of the lead
anode is only a few milligrams per ampere hour
Anode is covered with a brown layer, but underneath there is metal. Corrosion is noticeable along the boundaries of the crystals (the anode is a lead
ingot). A network of shells has formed
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Hexabromobenzene
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My reasoning in this section is incorrect. Sodium hydrogen sulfate DOES NOT EXIST in acid with a concentration of less than 20%. We are making simple
sulfate electrolysis.
Sodium hydrogen sulfate can be converted into acid by dissolving water and crystallizing sodium sulfate in the cold, followed by evaporation. And
indeed, this is what happens with my sample.
You can see the phase diagram of sulfuric acid sulfate here.
https://beta.iopscience.iop.org/article/10.1149/1.3493888
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