Sciencemadness Discussion Board - Dirty 96% Sulfuric Acid

Dirty 96% Sulfuric Acid

moontheloon - 27-12-2016 at 07:58

Hey guys!

I have some sulfuric acid that is 96% w/w but it is tainted as it is a black colour. All I want to do with this at the moment is make copper (II) sulfate with Copper (II) oxide. Is this still okay to use?

Tsjerk - 27-12-2016 at 08:03

Does the color dissolve in water? What happens with it when make a 1% solution? Maybe the color doesn't incorporate in copper sulfate crystals, try it with a small amount.

Dan Vizine - 28-12-2016 at 05:39

It is not uncommon for old bottles of relatively concentrated sulfuric acid to exhibit a black color. The color usually results from attack on some component of the cap. The actual amount is small, the color is intense and it's water soluble. The only ways that would probably work are distillation or decolorizing carbon.

In industry, this type of issue is often dealt with by using the discolored acid to make various metal salts and then decolorizing these salts in aqueous solution using decolorizing carbon.

The identity of the black material is acid-degraded organics.

AJKOER - 31-12-2016 at 15:27

I have a suggestion that may work. Prepare a test amount of the organic tainted H2SO4 by adding a trace amount of say cobalt metal or oxide. Then, pass air/O2 through the acid until it is clear (or at least this is my speculation).

Logic: The generation of powerful sulfate and hydroxyl radicals may breakdown the organic contaminant. See my cited references and background material with links at http://www.sciencemadness.org/talk/viewthread.php?tid=71477 .

[Edited on 31-12-2016 by AJKOER]

macckone - 31-12-2016 at 18:25

When all else fails, convert to iron sulfate and extract sulfur trioxide. Sulfur trioxide can readily be converted to sulfuric acid using a sealed container and putting a separate cup of water in the cup. This avoids the nasty vapor condesation issue.

lab-equip - 2-1-2017 at 11:03

Very old sulfuric acid turns brown to black, do not worry. Perhaps cooking elementary grinded copper in pure sulfuric acid will also bring coppersulfate

ave369 - 19-1-2017 at 13:23

I've recently completely decolorized my H2SO4 from the color of soy sauce to completely colorless. I've added a few milliliters of 36% H2O2 and left it to sit on a shelf for two days. Some gas was evolved, and the acid turned completely colorless. I've lost some concentration, though; it was something like 93% and now it dropped to 90%.

Varmint - 19-1-2017 at 13:53

Ave369, this of course means you made piranha solution to some degree, the only question is one of stoichiometry.

I've seen many references to using H2O2 to "clean" H2SO4, but not once have I seen any cautionary note about the hyper-increased oxidation potential of the result.

Additionally, I've not seen any studies on what these targeted compounds end up oxidized into. Point being, the after product of the cleaning is certainly no cleaner chemistry wise, is just has a lighter shade or water clear. The only exception to this would be pure carbon contamination, which one can safely assume was oxidized to CO2 escaping to the air.

Unless you know the contaminant and have a means to address it and its byproducts selectively, you end up with still tainted acid, and in your case, diluted with H2O. That dilution might cost more in time, energy and yield to get back to the original concentration than the side effects of having just used it straight away.

DAS

Maroboduus - 19-1-2017 at 16:22

I like Dan Vizine's proposal of just making the salt and purifying afterwards.

Re-crystallization is a powerful purification tool for getting the organics out of an ionic salt, and the decolorizing carbon is great for getting that last bit of tint out.

You might even be able to get most of those impurities out of the salt you're making by leaching it out with a solvent that won't dissolve the salt itself. (good chance to break out your Soxhlet apparatus)

EDIT: Forget the last paragraph. My brain must have slipped out of gear there for a moment. Not likely any common solvent is gonna dissolve crud like that.

[Edited on 20-1-2017 by Maroboduus]

chornedsnorkack - 20-1-2017 at 05:41

Quote: Originally posted by Maroboduus

Re-crystallization is a powerful purification tool for getting the organics out of an ionic salt,

Sulphuric acid freezing point is +10 degrees at 100 %, and -35 degrees around 94 % of eutectic H₂SO₄ and H₂SO₄.H₂O.
So 96 % sulphuric acid should freeze around -10 degrees.
But sulphuric acid is fairly viscous, which might hamper nucleation of sulphuric acid crystals and growth of clean crystals rejecting (organic or inorganic) impurities into the mother liquor.
How efficient is cleaning sulphuric acid by freezing out pure 100 % sulphuric acid crystal? Like exposing 96 % sulphuric acid, freezing point -10 degrees, to -20 degrees for a time?

Praxichys - 20-1-2017 at 10:23

The easiest way is to add hydrogen peroxide and reconcentrate by boiling. The heat decomposes the extra H2O2 (you must do this or your acid becomes strongly oxidizing which will ruin a lot of reactions) and also brings the concentration back up to 93-95%.

Do this outside since H2SO4 fumes are nasty and the solution will reach about 300C. It's an easy procedure though. Just add about 0.5-1% H2O2 by weight (stoicheometrically - any concentration will work), put the container on a cheap Walmart single burner, and crank it up to max. If you know how much acid you had to start with, the concentration will be back to normal when it boils down to approximately that level. Let cool and titrate if necessary. Again, do it outside and away from things you like because as the concentration approaches 95% a significant fog of H2SO4 is emitted.

Melgar - 20-1-2017 at 11:24

I've always just heated until I got the first whiff of sulfuric acid fumes, then shut the heat off. In a full bottle of H2SO4 drain cleaner, filling the empty space up halfway with 30% H2O2 and then immediately capping the jug, ensuring that the lid is on properly, and shaking it up really well (wearing gloves and protective gear, of course!) is enough to destroy all the black and brown crap that's in it, and has a negligible effect on its strength as an acid and as a dehydration agent.

Or if you want to stick to the "always add acid" rule, and not have to deal with potential mixtures of decomposing H2O2 and H2SO4, pour about 25 mL per liter of 30% H2O2 into a large enough beaker to hold it, then add your acid while stirring, then pour everything back.

Amos - 20-1-2017 at 11:50

Quote: Originally posted by chornedsnorkack

Quote: Originally posted by Maroboduus

Re-crystallization is a powerful purification tool for getting the organics out of an ionic salt,

They were referring to recrystallizing the copper sulfate that the OP wants to make using the acid; not recrystallizing the acid itself.

aga - 20-1-2017 at 11:58

3% peroxide also works.

Heat & stirr and the colour will be gone.

Needs more boiling due to the excess water, but works.