Sciencemadness Discussion Board

Purifying sulfuric acid

TheIdeanator - 8-11-2015 at 19:09

I'm considering getting into chemistry as a hobby and perhaps as a career choice and have been looking into finding the cheapest stuff available. Lowes seems to be the best for sulfuric acid - $10/qt at 93% as drain cleaner. Unfortunately there's a bit of rhodamine B and other "anti-corrosive" shmoo mixed in.

How would I go about removing the stuff that isn't H2SO4 and water?

While you're here, what tips do you have to convince me that I can have fun while still being a ruthless penny-pinching bastard? Also, this is my inaugural post, hi!

[Edited on 9-11-2015 by TheIdeanator]

Upsilon - 8-11-2015 at 19:24

Best bet is distillation. But distilling sulfuric acid is by far not an easy task since it boils at over 300C, which is too hot for all but the highest quality glassware. Vacuum distillation should bring down this temperature.

Or, you could take the easier route and just get a small amount of higher purity stuff and use this dirty crap for everything you can. For example, dirty stuff is perfect for nitric acid production, gas generators, etc.

JJay - 8-11-2015 at 19:35

Some people have suggested adding hydrogen peroxide and boiling off the extra water. That may be pretty effective for removing organics, but I don't think it would remove most minerals.

I've seen name-brand Pyrex flasks crack at 300 C... if you're going to distill sulfuric acid, use equipment you know is reliable, and be prepared for a water-balloon type explosion of boiling sulfuric acid, hot enough to ignite paper or leaves.

TheIdeanator - 8-11-2015 at 19:55

Quote: Originally posted by JJay  
a water-balloon type explosion of boiling sulfuric acid, hot enough to ignite paper or leaves.

The mythbuster in me wants to do that with 3 liters of acid and a bunch of mannequins diving for cover. I hope they do a lab accident special.

Upsilon - 8-11-2015 at 20:35

It's very unlikely that the flask would "explode" - borosilicate glass is very good at resisting splintering upon breaking. A crack is a real possibility, though, which would start leaking boiling sulfuric acid, which as stated above, can cause many substances to spontaneously combust upon contact.

annaandherdad - 8-11-2015 at 21:31

Nurdrage has a video on distilling sulfuric acid.

XeonTheMGPony - 10-1-2016 at 11:41

The drain cleaner variety I get is good 96 or so % from the bottle but dirty as well.

I take a 100Ml sample and dose with 35% food grade peroxide to find the dosage value, then multiply by 5 for my 500Ml batch.

I divide the dosage by two.

Now I take my 500Ml of acid in a Pyrex 1L round bottom flask and chill in the fridge over night, I then remove and add the first dosage while swirling to mix thoroughly, this will be an exothermic reaction releasing ozone and some acid mist. Do out side or in a fume hood, Allow to react under it's own heat till it ceases to bubble, you may need to swirl it a few times.

Prep a heating mantle (I use a put that I filled with wood ash n a bit of water to make an insulated crucible) Add the second dose of peroxide, allow to react in the heating mantle once bubbling has subsided put on low heat, (It will pick up again at the bubbling, remember to swirl to ensure good mix) Again as the bubbling subsides increase heat!

Allow to heat for around 2H to 3H, check every 30M, if you are getting good dense white fumes kill the heat and allow to cool (DO NOT BREATH FUMES!)

Once at room temp place a 100Ml beaker on a sensitive scale and tare it, pour in a precise 100Ml then divide the weight by 182 to yield your concentration in %. If below 85% you can simply reboil.

Your product now should be a nice amber color of good purity.

For my last batch it was 200Ml 35% peroxide per 500Ml of acid, and a boil time of 5H with a low powered heating element. Post treatment concentration was 172g for an exact 100Ml = 85%

!!You must figure out the dosage of peroxide for each and every bottle!!

H = Hour
M = Minute
Acid = H2SO4

aga - 10-1-2016 at 12:22

Recently i acquired a large volume of clean 36% sulphuric acid.

Before that i used the brown 96% stuff sold as drain cleaner and purified it thusly :-

Put 200ml of the brown stuff in a 250ml beaker.

Begin heating.

Add 3% H2O2 (max available here) with stirring until the brown colour has gone. This was usually a small quantity, in the 10ml range.

Boil until white fumes are seen, and a glass briefly placed over the beaker shows no condensation (i.e. no water in the vapour)

Allow to cool.

Every single step of this process is Dangerous !

Done with caution, eye protection and gloves (all should be Standard Procedure) and Outside, away from people and animals, it worked for me many times without mishap..

TheChemiKid - 10-1-2016 at 15:02

Here are some good videos:
Hydrogen Peroxide Method
Distillation Method

AJKOER - 11-1-2016 at 15:34

OK, my quick research suggests the actual impurities in technical grade H2SO4 are primarily heavy metals, and not organic matter (like carbon, see as one reference, ). Some mention of Iron, Nickel and a touch of Arsenic, likely present as sulfides.

So if Carbon is not the visible impurity, but it is indeed heavy metal sulfides, the use of H2O2 may 'appear' to work by transforming the sulfides into soluble sulfates. However, you may still experience heavy metal (more precisely, transition metal) accelerated corrosion issues, for example, with the acid.

[Edited on 11-1-2016 by AJKOER]

Magpie - 11-1-2016 at 15:57

Assuming you have an Ace Hardware nearby I suggest buying a quart of Rooto sulfuric. It's reasonably pure ~96% acid. I've been practicing this art for over 10 years now and have never boiled down sulfuric acid.

feacetech - 11-1-2016 at 16:00

Technical grade isn't the same as drain cleaner?

drain cleaner will have inhibitors and other additives (the brown colour).

I use spent sulphuric acid at work its mainly clear however it sometimes has a slight Fe brown discoloration

we test for purity
reducing test
and iron (mainly since it interferes with one of the processes)

If the acid has come from battery's it will have a lot of Pb

AnalaR describes some basic purity tests

Clear colourless oily liquid 1.84 not less than 98%

Non-volatile matter <0.002%
Chloride <0.00002%
Nitrate <0.00002%
Ammonium <0.0002%
Arsenic <0.000005%
Iron <0.00005%
Heavy metals (Pb) <0.0001%
Substances reducing permanganate <0.0001%

I can detail any of these tests from the AnalaR if you like

[Edited on 12-1-2016 by feacetech]

byko3y - 11-1-2016 at 17:11

I've posted few times on this board instructions for sulfuric acid distillation.
- you should never keep water and boiling sulfuric acid close to each other, ESPECIALLY they should NEVER be separated with a single glass, even if it's quarts glass, otherwise some random crack will lead to an explosion. I use air cooling plus water cooling;
- vacuum distillation is more efficient as well as much more dangerous;
- almost any borosilicate or better glass can handle the distillation. The distillation device is pretty simple;
- don't ground joints should not be oriented horizontally, because acid vapors will leak through them. I was not able to observe any leakage through vertically oriented joints;
- PTFE works well for sealing, although I don't think it's really needed.

careysub - 11-1-2016 at 18:02

If you battery acid that is sold to fill batteries (not from a battery) it should have very good purity, although it will only be 35% H2SO4.

The battery acid/water fill mixture must have very good purity for good battery performance. Instructions recommend using distilled water for filling or make-up water, so I expect that the acid itself will have similar purity.

byko3y - 2-2-2016 at 16:50

I was just going to post a message that I've discovered a hydrogen peroxide to clean the sulfuric acid from organic impurities, when I realized nobody actually proposed another, old and well known way of removing organic - chromic acid.
I was trying to use chromic acid and got some limescale-like deposits on the flask that are extreemely hard to remove, while amount of chromic acid was insufficient and not all the impurities were removed.
Then I've tried to use hydrogen peroxide, concentrated by slow evaporation a 15% solution up to 30%, and no deposits could be seen, while organic were removed in a matter of minute and excess of hydrgen peroxide was destroyed by simple boiling.

Also, my air condenser got a crack again because I've used a foil as a thermal insulation (because I could not get the acid to reach the target flask, because the air condenser was positioned horizontally; I know it's dumb), and my air condenser cracked.... again. Previous time I tried to hold the condenser with clamp to ensure the whole device won't fall apart because of bumping. Now I realize the best thing you can do is to leave it alone and don't let anything to contact with the condenser surface.
You can see at the NurdRage's video that nothing is holding his air condenser (which is just a tube that connects the source vessel and water condenser), while the red labels at the hot part of the devices become black during as the temperature rizes. Evaporationg or decomposition of organic left on that tube can lead to crack, because it's pretty close to placing a water drop on that tube.

Hawkguy - 2-2-2016 at 20:15

Hmm usually it also contains Rodine, an acid inhibitor.

Detonationology - 2-2-2016 at 20:37

Quote: Originally posted by Hawkguy  
Hmm usually it also contains Rodine, an acid inhibitor.

*Rhodamine B*

Hawkguy - 2-2-2016 at 21:03

Quote: Originally posted by Detonationology  
Quote: Originally posted by Hawkguy  
Hmm usually it also contains Rodine, an acid inhibitor.

*Rhodamine B*

Thanks. I don't know how to clean it out, or if it decomposes into what when I distill it. I've gotten discoloured distillate before, which is dissapointing.

AJKOER - 4-2-2016 at 10:32

Assuming any Ferric, Cupric,...or other transition metal impurity, based on Fenton chemistry and appropriate pH level with associated speciation, one may be able to improve on the action of H2O2 by introducing sunlight, that is Photo-Fenton chemistry.

The characteristic product of Fenton chemistry includes hydroxyl radicals which will effectively reduce organics. Also, soluble salts, in the case of Photo-Fenton, coud be converted to possibly less soluble lower valence state products for removal.

[Edited on 5-2-2016 by AJKOER]

UC235 - 4-2-2016 at 10:49

Quote: Originally posted by Detonationology  
Quote: Originally posted by Hawkguy  
Hmm usually it also contains Rodine, an acid inhibitor.

*Rhodamine B*

Actually the additive is called rodine 31A. It's not a dye, but an inhibitor of acid-induced corrosion on steel, brass, and copper. It's a mixture of alkylpyridines and 1,3-diethylthiourea.

kt5000 - 4-2-2016 at 13:23

AutoZone sells quarts of 35% sulfuric acid to fill car batteries. It's crystal clear, and the price isn't too bad.

ave369 - 5-2-2016 at 00:08

An addition to byko3y's suggestion: distillation of chromic-sulfuric acid mixture is called Zintl-Karyakin distillation and known as the best (if dangerous because of possible hexavalent chromium spills) method to purify sulfuric acid.

AJKOER - 5-2-2016 at 14:04

An interesting approach for a small amount of H2SO4 would be treat it with Cl2 and leave in a thin clear plastic container exposed to sunlight. Follow up by heating in a flask to over 150 C to remove volatile products, including HCl, outdoors or in a fume hood.

Some of the possible chemistry:

Cl2 + H2O = HCl + HOCl

HOCl + uv ---) HCl + O

HOCl + uv ---) .OH + .Cl

.Cl + Cl- = .Cl2- (see )

So, the products of photolysis could include hydroxyl radicals (which can degrade organic compounds), active monoatomic oxygen, active chlorine radicals and the dichloride radical anions (producing volatile and easily removable products).

[Edit] An advantage of this approach over H2O2 is that it is cheaper, less dilutive and may even concentrate the H2SO4 by consuming water.

[Edited on 5-2-2016 by AJKOER]

byko3y - 5-2-2016 at 14:27

Too bad chlorine is almost insoluble in sulfuric acid.

AJKOER - 5-2-2016 at 15:03

Quote: Originally posted by byko3y  
Too bad chlorine is almost insoluble in sulfuric acid.

Yes, per this source, , I would guess that low temperatures would be more likely favorable, if at all feasible.

Fluorite - 16-12-2020 at 19:43

I always thought that sulfuric acid will oxidize hydrogen sulfide to sulfur as sulfur dioxide does but after watching nilered zinc sulfide video
Oddly enough there's no reaction between the h2s and h2so4 although sulfuric acid is a stronger oxidizer than sulfur dioxide
Anyways is it possible to dilute sulfuric acid with water and then bubble h2s so it will remove all heavy metals and then concentrating again? This can be useful for purifying sulfuric acid to remove lead when using lead electrodes and the result should be safe sulfuric (toxicitywise) is this correct?

zed - 16-12-2020 at 22:42

Umm. Purifying Sulfuric Drain Cleaner....Acid. What are the chances chromatography will work?

The idea being, to first heat the acid, which by some reports, causes it to badly discolor.

After which, the acid can be run through a short column packed with anhydrous CaSO4.

Is there a chance that the colored material will be slower to elute, than the H2SO4 itself?

Thereby leaving the crud stuck in the CaSO4, while the acid passes though, and is purified?

rockyit98 - 16-12-2020 at 23:51

Concentrating H2SO4 without distillation (with flask ans some PTFE ) up to 98%
i made 98% maybe 99% H2SO4 using 35% acid
you don't need H2O2 because HOT H2SO4 is in it of its self powerful oxidizer.;)

Fyndium - 17-12-2020 at 03:05

I believe that distilling it is the best overall method considering that 1l and smaller flasks are very cheap, owing the sacrificiability if it happens to crack. Generating sufficient heat is trivial with propane burner. Activated carbon, and bubbling air with aquarium air pump and bleeder into the boiling acid are the single most effective methods to prevent bumping, because overhead stirring may be incompatible with ptfe due to extreme heat.

Antigua - 17-12-2020 at 03:07

The cost of flasks would be the last of my worries when dealing with 300°C boiling hot concentrated sulfuric acid. Even thinking that it could possibly crack makes me shiver.

Sulaiman - 17-12-2020 at 04:15

I have distilled sulphuric acid at atmospheric pressure in cheap Chinese borosilicate glass over an open flame,
pay attention to anti-bumping measures as boiling conc. sulphuric acid bumps extremely violently,
potentially expelling boiling hot conc. sulphuric acid.

Just one accident or lapse of sanity can ruin the rest of your life !
extreme care required.

the process of distiling conc. sulphuric acid gives me a tight sphincter :o:P