Sciencemadness Discussion Board - Can I purify sulphuric acid by electrolyse? 2 H2O + H2SO4= O2 + 2 H2 + H2SO4 ??

Can I purify sulphuric acid by electrolyse? 2 H2O + H2SO4= O2 + 2 H2 + H2SO4 ??

CrEaTiVePyroScience - 14-4-2012 at 11:01

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

Pulverulescent - 14-4-2012 at 11:17

Here's a patent on the subject . . .

Poppy - 14-4-2012 at 11:25

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...

meaniac - 14-4-2012 at 19:05

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.

99chemicals - 25-5-2012 at 15:07

Quote: Originally posted by CrEaTiVePyroScience

2 H2O+H2SO4 --> 2 H2+ O2+H2SO4?
CrEaTiVePyroScience

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.

liquidlightning - 25-5-2012 at 22:55

What creative is asking is if the sulfuric acid will be affected by the electrolysis.

weiming1998 - 26-5-2012 at 02:03

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]

Lambda-Eyde - 26-5-2012 at 02:26

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]

hissingnoise - 26-5-2012 at 04:10

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.

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 . . .

Lambda-Eyde - 26-5-2012 at 04:32

Quote: Originally posted by hissingnoise

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"

AJKOER - 26-5-2012 at 04:35

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.

99chemicals - 26-5-2012 at 05:30

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.

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?

weiming1998 - 26-5-2012 at 06:00

Quote: Originally posted by 99chemicals

Quote:

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).