WTF happened to my H2SO4?

[mongo blongo]

Something really strange happened to my H2SO4!
I get my acid from a car battery top up. It is 34% and I boil down it to concentrate it.
I have been using the same batch for about a year where this has never happened before! I keep it in my shed.
The other day, when making some TNP, I picked up my bottle of conc acid and noticed that there was what seemed to be a huge chunk of ice in it!
Now, it couldn't be ice, because it was 9 degrees in my shed and I also had a water bath in there too which was still liquid. There should have been no water in it because the density checked out and it turns tissue paper black instantly.
I poured my acid out into another bottle to leave this "solid" in there. I took it inside my house and it slowly melted.
This liquid also turns tissue paper black instantly and has the same density as 98% H2SO4!
I took it back outside again and it froze back into a solid so it must have a melting point between 9 and 23 degrees. I think H2SO4 has a freezing point of -3 degrees.
I haven't a fucking clue what this is! It looks exactly like ice! but it's not!
Does anyone know what has happened?

[kingspaz]

the H2SO4 must have frozen then. like all liquids its freezible. i got this data from a website:
Melting Point : 10.3 degC
Boiling Point : 338 degC
Formula weight 98.08
Specific gravity or density 1.94

from here:

<a href="http://www.ucc.ie/ucc/depts/chem/dolchem/html/comp/h2so4.html" target="_blank">http://www.ucc.ie/ucc/depts/chem/dolchem/html/comp/h2so4.html</a>

[mongo blongo]

Yea it seems to be that but cheak this. The acid I poured out did not freeze when at the same temp the other "stuff" did.

[kingspaz]

thats fucking weird!
come to think of it, i had a batch of acid i purified once which when i looked at the next day had what looked like bits of slightly white coloured ice crystals floating in it. i thought i must have split somthing into it and not realised so i chucked it away.

[nbk2000]

Perhaps what you're seeing is crystallized sulfur trioxide, since you concentrated your acid by boiling it down.

[Rhadon]

@NBK2000: This should be SO3? I don't think that you're correct: SO3 hydrolizes immediately if there is any water present, and once hydrolized to H2SO4 it won't "separate" back to H20 and SO3. Okay, perhaps this will happen at very high temperatures, but not at 0 - 10° C.

Rhadon

[EventHorizon]

This is what happens to mine when I boil it down. At several hundred degrees C and at the end of the boil down cycle there are crystals floating around in it that looks like little bits of puffy cotton, but once it cools down it becomes crystal clear.

[nbk2000]

The whole point of the boiling is to remove the water.

And once the water is gone, you're left with an essentially anhydrous liquid that, if boiled long enough, will have SO3 dissolved in it, much like sugar in water.

Then, when sufficiently cooled, the solution will precipitate out the dissolved solids.

Test: Seperate out the crystals and add water drop by drop to them. You'll hear a hissing sound if it's SO3 crystals because of the intense affinity of the crystals for water. You'd then have a concentrated acid solution left.

[Rhadon]

@NBK2000: Yes, you're right: When boiling down your battery acid you get it (at best almost) free from water. But not more! You won't be able to decompose H2SO4 to H2O + SO3 and evaporate the water. Would be nice if this could work, but I'm really sure it won't since SO3 has a very high affinity to H2O, like you said yourself.
==> H2SO4 which is concentrated by boiling down battery acid will never contain free SO3.
But let's assume that the solid is SO3. Then it would have to react with the humidity in the air what would cause it to produce much fumes <expression?>.

Rhadon

[Lagen]

My guess is that the fluff is some sparingly soluble salt, might come from the batteries, or you name it. If it is slightly water soluble, when almost all the water is taken away it precipitates. Then, after the acid absorbs a fraction of a percent of water (which it will do quite quickly if not sealed off immediately), the salt redissolves.

Yes, it cannot be SO3 as that boils at 44.7°C (melts at 16.8°C). It is not possible to make SO3 by boiling sulfuric acid, otherwise it would be also possible to make oleum this way, which is not the case. At 330 something a constant boiling point mixture, or azeotrope, forms and everything will boil off at that point. Also, SO3 is soluble in H2SO4 by about 30%, and if the solution contained 30% SO3 (and a little undissolved fluff) you could easily detect it as the density would be over 1.92.

[mongo blongo]

NBK. I just did the test but nothing happened.
This stuff also seems to be insoluable in water.
edit: There was no crystals just after boiling it.

[ January 04, 2002: Message edited by: mongo blongo ]</p>

[nbk2000]

Yes, I know these facts, but I'm just throwing ideas out in the wind to see what blows back. After all, assumption is the mother of all fuck ups.

Mmmm....if you took the acid from a battery, then maybe it's a lead salt. Is the salt very dense? Crystal shape (rhombic, needles, etc), hardness, etc.

Also, it may be informative to take a small crystal of it and blast it with a propane torch to see if it vaporizes, fuses, etc. Also, note the color change to the flame. That's indicative too.

[mongo blongo]

Sorry it IS soluble but it dissolves very slowly.

I am not sure if it has been in a battery before because I just bought 5L of it of this dude from a battery shop.
It doesn't have much of a crystal shape, it just looks like ice.
It couldn't be PbSO4 because PbSO4 is a white crystalline substance with a specific gravity of 6.2 and a melting point of 1170 C and the solubility is negligible.
This stuff reacts with metals when warm.
No flame colour change with a gas burner.
It vaporizes when heated.

[kingspaz]

mongo, what did you use to concentrate the acid in? if you tell us exactly what you did then we could have a better chance of working out what went wrong. like what containers has it been contained in prior to concentration, etc.

[Lagen]

Mongo, are you referring to the same icy stuff as in your original post? The properties you describe suggest that it could be sulfuric acid. The melting point is strange, it would have to be pretty damn close to 100% H2SO4! By boiling it you could only get to 98.3%, but the fact that the liquid portion didn't freeze at the same temp. indicates that it contained more water. (100%: m.p. 10.3°, 98.3%: m.p. 4°C) After melting it you say that based on density it checks out to be 98%, but remember that the densities are so similar in that range that it is difficult to check even with a very very precise hydrometer! So it looks as if 100% acid separated out of solution. This concept is not new, however I was convinced it could only be done below -70°C! You should concentrate on verifying if the stuff could be sulfuric acid, if this separation method worked it would be great news for us all.

Heres a quick kill way of doing it without any chemicals: Dissolve some in a little water. The temperature should rise noticeably. Now the interesting part: To another sample add some cracked ice or snow at a temperature above zero. The two should slowly melt together and the temp. of the liquid portion should decrease below zero! Then your substance is almost certainly pure sulfuric acid.

Today I will try to duplicate your results and take some m.p. measurements with my acids. Have an ice day!

[mongo blongo]

Lagen! looks like you have solved the problem!
This stuff is 100% H2SO4!
As I stated in one of the above posts this stuff has many of the same properties as H2SO4 like turning tissue paper black (well it melted as well!)etc.
This explains the stuff freezing when the liquid acid did not. The acid I poured off was at a lower conc than the solid acid and as Lagen said, a small change in the conc can have a much larger effect on the melting point! Also this small change in conc would not show up in the density.
NICE ONE LAGEN!!! :)
Something else I did was to a large chunk of the 100% acid, was to add only a few drops of water to it (at 9 degrees) and after about 30 mins it completely turned back to a liquid!
So I'm convinced it's 100% (or near) H2SO4.

Well now if you want duplicate what I did, I would advise, when boiling the acid, don't just boil it until you see thick fumes. I let it fume for quite some time (about 10 mins) until the fumes get very very thick! Then let it cool slowly until it stops fuming (this will let water, produced when the H2SO4 decomposes, to evaporate with no further decomposition of the acid, thus obtaining conc acid with minimum water in it)

Next thing now is to see if we can work out the best temp to freeze it to separate out some 100% H2SO4. Then boil the acid again to freeze out some more.
This may be a good method to get 100% H2SO4! :)
What do you think?

Kingspaz- I keep the 34% acid in a plastic (acid resistant) container, which is what it was in when i bought it. I boiled it in a pyrex coffe jar with a gas stove.

[Lagen]

I just tried and failed. I rebuilt the still I normally use for HNO3, the thick fumes were liquefying in the condenser and some acid actually distilled over, and I thought the remainder would be concentrated enough. It froze at about 0-1°C, so it was below azeotropic concentration. I suspect some water might have refluxed in the distillation flask, the small still is not that effective at these high temperatures. Now I will either get No. 29 fittings or boil the acid outside, the way you have suggested. Seems that the trick can only be done at or above the azeotrope. Another thing we should check is what is the atmospheric pressure when the separation works.

[mongo blongo]

Just some more points to support the conclusion.

I added some NaNO3 to it in a test tube and heated it. It evolved HNO3 fumes.

The solid H2SO4 was added to some conc H2SO4 (liquid) and it SLOWLY sank to the bottom (showing that it had a slightly higher density).

I think that the right temp to freeze the H2SO4 would be somewhere between 9.5 and 10 degrees. this should freeze out almost 100% pure H2SO4!
I will try to experiment with this but I need to boil down some more acid first. I will do this as soon as it stops fucking raining outside! :(
I will post results.

Lagen- What about boiling it under a vacuum? It could be possible to remove the water without decomposing the H2SO4?

[ January 07, 2002: Message edited by: mongo blongo ]

[ January 07, 2002: Message edited by: mongo blongo ]</p>

[Lagen]

The vacuum won't help much in this case. You distill HNO3 under a vacuum as it has a decomposition point lower than its boiling point at the standard pressure. The vacuum lowers the boiling point, while the decomposition point remains the same. H2SO4 decomposes at 340°C, but the boiling point of the azeotropic mixture is just below that, 338°C. And unlike with HNO3, the decomposition of H2SO4 is reversible. So decomposition is not a problem here.

The problem is, that some of the less concentrated stuff boiling off will always condense on the colder parts of the vessel you use for the boiling, and flow back (this is called "reflux"). The last time I distilled the reflux was visible and obviously I didn't get much over 97%. To counter this you need either very good insulation, a heating mantle, or you can experiment with different glassware setups to lower the heat dissipation. I will be buying the larger fittings tomorrow, but I've got one more idea I'll be experimenting with tonight.