CrEaTiVePyroScience
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Can I purify sulphuric acid by electrolyse? 2 H2O + H2SO4= O2 + 2 H2 + H2SO4 ??
Hello,
So I've made sulfuric acid by electrolysis of copper sulfate.
I have made a video about the experiment ( https://www.youtube.com/watch?v=vem4COJtihY )
So I find out that I can't get any higher concentration of sulfuric acid then 40% with the copper sulfate method. Even if the reaction was done and I
added more copper sulfate I wouldn't be able to get any higher then that 40%. So I boiled it down to about 95%. But I am wondering if I also can get a
higher concentration by just normal electrolysis of diluted sulfuric acid. Let's say I got 200ml of 40% sulfuric acid and add two platinum electrodes
and run about 6v current through the sulfuric acid for let's say a week. Will all the water get splitted up in hydrogen and oxygen and will I be left
with like almost 99% sulfuric acid so it will be 2 H2O+H2SO4 --> 2 H2+ O2+H2SO4? Or will the sulfuric acid also react?
I would love to try it about but would prefer first a confirmation before wasting too much money on the current=)
What do you think , will it work?
Regards,
CrEaTiVePyroScience
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Pulverulescent
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Here's a patent on the subject . . .
"I know not with what weapons World War III will be fought, but World War IV will be fought with sticks and stones"
A Einstein
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Poppy
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Buy some Vanadium oxide and try the contact process route, if you want planty acid. I dont believe it is prohibited in most countrys
Think about your poor electric bill...
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meaniac
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The patent posted by Pulverulecent is instructive. It implies that the acid can be concentrated to ~75% without electroreducing the sulfate ion.
You may be able to use Lead electrodes (instead of Pt) up to at least that concentration.
There may also be more info to be found in the chemistry of Lead Acid cells (lots of patents on them). There's also lots of interesting chemistry
involved in recovering lead compounds from old batteries, but I digress.
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99chemicals
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You cannot have a reactant going into and equation and the exact same chemical coming out unchanged. The 2 H2SO4s cancel and leave you with 2 H2O
--> 2 H2 + O2. Or simpily just electrolisis of water.
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liquidlightning
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What creative is asking is if the sulfuric acid will be affected by the electrolysis.
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weiming1998
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Maybe, you can use this to concentrate H2SO4, but the amount of electricity needed is tremendous.
According to this site (http://www.kentchemistry.com/links/Kinetics/BondEnergy.htm)
481kj of energy is produced when 2 mol of hydrogen and 1 mol oxygen combusts, forming 2 mols H2O. According to the law of conservation of energy, the
reverse reaction has to cost as much energy as the forward reaction. So for every 36g of H2O removed, you need 481kj of energy. Now, say if you have
500g of dilute sulfuric acid that is 100g sulfuric acid and 400g water, and you want to remove the water, you will need 400 divided by 36, or roughly
11 multiplied by 481kj of energy, which is 5291kj of energy.
According to Wikipedia, http://en.wikipedia.org/wiki/Electric_power, 1 watt is a joule of energy per second, so using a power supply with 1kW of power, this feat of
drying the acid can be done in about an hour and a half (not counting energy lost as heat or electric resistance), so maybe good for an occasional
experiment or two, but definitely not good in the long-term or a large amount of acids (electricity bill!)
Let's calculate the energy needed to boil the mix. Using the same 500g of acid. (The real amount of energy will most likely to be slightly more, due
to some of the heat energy dissipating, and also, I used the calculation of 500g sulfuric acid/water mix as if all of it has the same amount of heat
capacity, which is not true, although the heat capacity for H2SO4 is likely to be a little lower, because water does not conduct heat very well). Ok,
so water's heat capacity is roughly 4.18J/g, which 500g will be about 2.9kj to raise it about one degree. So, to rise the temperature from 20 degrees
celsius to about 300 degrees celsius (boiling point of azeotropic sulfuric acid) , you will need only about 900kj of energy, and this is greatly
exaggerated since mass will constantly be lost as water vapour, reducing the amount of energy needed to rise the temperature up, with a bit of product
loss. So definitely not an economic way of concentrating H2SO4. If my calculations are far off, please forgive me, as I am not very good at physics.
Anyway, (Wikipedia, again, http://en.wikipedia.org/wiki/Hydrogen_peroxide ) states that hydrogen peroxide is made by the electrolysis of H2SO4, so, over time, the sulfuric
acid will convert to H2O2 instead. But this source might not be very reliable.
[Edited on 26-5-2012 by weiming1998]
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Lambda-Eyde
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You have obviously never paid an electricity bill. Off the top of my head, I pay somewhere along the lines of 0,2-0,5 $ per kWh. I expect the prices
outside of Norway to be even lower.
More related to the subject, I can't understand why the OP wants a more concentrated acid. Practically all the sulfuric acid you find in chemistry
labs is in the range of 94-98% by weight. Why he would further purify his from 95% to 99% seems like a waste of time. I can't think of any situation
where you'd want 99% sulfuric acid, although they do exist. Considering that sulfuric acid has an affinity for moisture (in air), special precautions
need to be taken when purifying it. A closed vessel with a calcium chloride guard tube, for example. If you were to electrolyze sulfuric acid in an
open beaker, it would probably absorb moisture from the air just as fast as it was electrolyzed. Oh, and if you do succeed in going from 95%
to 99% - good luck keeping it that way.
Edit: Disregard that, I read the OP a little bit too quick.
[Edited on 26-5-2012 by Lambda-Eyde]
This just in: 95,5 % of the world population lives outside the USA
Please drop by our IRC channel: #sciencemadness @ irc.efnet.org
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hissingnoise
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| Quote: | | Off the top of my head, I pay somewhere along the lines of 0,2-0,5 $ per kWh. I expect the prices outside of Norway to be even lower.
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What? Isn't Norway famous for its cheap hydroelectric power ─ I mean Birkeland and Eyde did their NA from air experiments in Norway because
power costs were so low there . . .
I'm told I pay €0.17 per unit . . .
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Lambda-Eyde
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Quote: Originally posted by hissingnoise  |
What? Isn't Norway famous for its cheap hydroelectric power ─ I mean Birkeland and Eyde did their NA from air experiments in Norway because
power costs were so low there . . .
I'm told I pay €0.17 per unit . . .
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The reason I said that is because everything is more expensive in Norway (but we have even better salaries). Most stuff from the US costs roughly half
of what I would pay here. In my part of the country, power is cheap in the summer, but usually increases in the winter because the companies sell
electricity to the continent - thus depleting the water magazines for the winter...
Birkeland and Eyde didn't really pay for their electricity like us normal consumers; Sam Eyde owned the rights to some waterfalls where he had a power
plant built IIRC. This eventually culminated in the development of the Birkeland-Eyde process and Norsk Hydro. The second part of my user name is
taken from Sam Eydes surname. Someone had already taken "Lambda" 
This just in: 95,5 % of the world population lives outside the USA
Please drop by our IRC channel: #sciencemadness @ irc.efnet.org
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AJKOER
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I would use the impure H2SO4 to make NaHSO4. Wash and recrystallize as needed to remove impurities.
Then, to quote Garage Chemist in a prior Sciencemadness thread ( http://www.sciencemadness.org/talk/viewthread.php?tid=5495 ), "From 680-880°C, plain sodium pyrosulfate (from NaHSO4 at 480°C) gives off all its
SO3".
Also to quote, in the same thread, AlphaCheese, for more details: "According to US patent 6767528 sodium bisulfate decomposes to produce sodium
pyrosulfate and water at about 240° to 250° C. Sodium pyrosulfate then decomposes to give sodium sulfate and sulfur trioxide at close to 460° C. It
doesn’t discuss the need for the H2SO4 catalyst because the SO3 is mixed with the H2O from the decomposing bisulfate in this patented process."
Then, having generated pure SO3, combine with water:
SO3 + H2O --> H2SO4
all obviously much easier said than done.
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99chemicals
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| Quote: |
Anyway, (Wikipedia, again, http://en.wikipedia.org/wiki/Hydrogen_peroxide ) states that hydrogen peroxide is made by the electrolysis of H2SO4, so, over time, the sulfuric
acid will convert to H2O2 instead. But this source might not be very reliable.
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What would be the equation for the electrolisis or H2SO4(If it converting to H2O2 is true?).
Would it give or hydrogen, oxygen, SO3, SO2, H20?
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weiming1998
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| Quote: Originally posted by 99chemicals |
| Quote: |
Anyway, (Wikipedia, again, http://en.wikipedia.org/wiki/Hydrogen_peroxide ) states that hydrogen peroxide is made by the electrolysis of H2SO4, so, over time, the sulfuric
acid will convert to H2O2 instead. But this source might not be very reliable.
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What would be the equation for the electrolisis or H2SO4(If it converting to H2O2 is true?).
Would it give or hydrogen, oxygen, SO3, SO2, H20? |
If Wikipedia is correct, then electrolysis will oxidize the SO4(2-) ion into S2O8(2-) ions according to the following equation:
2SO4(2-)==>S2O8(2-)+2e-. The S2O8- ions would hydrolyze, according to this site (http://www.chem.unep.ch/irptc/sids/oecdsids/Persulfates.pdf) into O2, H2O, SO4(2-) and H+ ions, but I suppose H2O2 could be an intermediate stage.
Anyway, if this is what happens, then there would be no net loss of H2SO4, as long as you heat the acid slight at the end (peroxydisulfuric acid
decomposes easily at elevated temperatures into molecular oxygen and a small amount of ozone (I can tell by the smell).
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