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starman
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[*] posted on 3-3-2010 at 06:13
Problem boiling down sulfuric acid


Today I attempted to concentrate some old 31% w/v sulfuric acid I had in the shed.This was done in a large crystallising dish on a heating mantle outside with boiling chips added.A thermometer was added though only to a depth of about 10mm instead of required 75mm for accuracy.
All went well with a nice regular boil starting at just over 100C through to about 210C.At only 220C boiling stopped and I got an intense(call the fire department type)fume off.
Suspecting the thermometer was giving a very low reading I terminated(dont want the neighbours calling aforesaid depatment)
Measuring density this evening gave about only 1.54(maybe 64%)

Anybody any idea? Does H2SO4 degrade over time?Any comment including detailing my own ignorance welcome.




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hissingnoise
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[*] posted on 3-3-2010 at 06:32


I do my boiling in a shed and have had my neighbour inform me that my shed is on fire.
I get a blueish smoke but it isn't really intense; the shed can't be entered until it clears somewhat.
If the air was saturated with moisture I'd expect a more intense mist.
It's possible though, that your acid contains some contaminent which caused the fume-off.
Properly stored H2SO4 shouldn't deteriorate over time, though.



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aonomus
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[*] posted on 3-3-2010 at 06:45


Test the fumes with litmus to see if its acidic? Doesn't 65% H2SO4 boil at 300degC+++?
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hissingnoise
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[*] posted on 3-3-2010 at 07:14


-Starman, you don't need boiling-chips in a shallow dish.
A shallow dish though, exposes a large surface-area of H2SO4 to moist air and dilution may begin once the temperature reaches a certain minimum.
If your solution was battery acid it may have had some additive which fumed once it got hot enough.
I'd try heating it again to see if the fuming reoccurs

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UnintentionalChaos
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[*] posted on 3-3-2010 at 07:17


The high boiling point doesn't mean that it doesn't have an appreciable vapor pressure before that temperature. I'd move it outside (sulfuric acid mists are a known human carcinogen (larynx and to a lesser extent, lungs)) and crank the heat.

[Edited on 3-3-10 by UnintentionalChaos]




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hissingnoise
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[*] posted on 3-3-2010 at 08:02


When sulphuric acid reaches its boiling point SO3 is evolved as a thick white vapour.
It's possible that your acid did boil and then reabsorbed a certain amount of moisture as it cooled.
It might be worth considering heating your acid in a RBF with a vacuum connection to an aspirator.
Boiling-chips would be needed in that set-up!


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[*] posted on 3-3-2010 at 08:34


After a certain concentration, sulfuric acid emits more SO3 gas that water vapor. The SO3 looks like smoke. What you experienced is absolutely normal. If I need to boil down H2SO4, I use a flatbottom boiling flask with a 24/40 joint filled with boiling chips and do it outside on a corning ceramic hotplate with an extension cord. Once the volume of the flask reaches the volume I calculated for 98% concentration, I pull the plug and let the unit cool for at least 30 minutes. The extension cord is there to eliminate the need to approach the boiling acid setup for any reason.

When it is cool enough that I can safely stopper it, I go ahead and stopper the flask and bring it inside. Be careful with stoppering as the cooling liquid will form a vacuum and you will never get the stopper off again if you put it on the flask too early.

I'm sure you already know this but don't handle the flask while it is hot. Any thermal stress could cause the bottom to crack (even borosilicate at these temperatures) and hot concentrated sulfuric acid is not fun.

[Edited on 3-3-2010 by rrkss]
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hissingnoise
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[*] posted on 3-3-2010 at 10:40


i've never used boiling-chips in H2SO4 because I don't know what would resist the hot acid!
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[*] posted on 3-3-2010 at 11:07


Most of the boiling chips I have seen were made from glass; bits of silica gel work too.
Incidentally, while there's not much need for boiling chips in a shallow pan they don't do any real harm and it's a good habit to get into.
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[*] posted on 3-3-2010 at 12:24


I've never had bothersome bumping anyway and thought plain glass didn't have enough nucleation points to be very effective.
Heating flasks slowly and evenly is always a good idea though!
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[*] posted on 3-3-2010 at 15:17


I've always had horrible bumping when directly boiling sulfuric acid. I once had ~100mL of partially concentrated acid with some broken glass in the bottom of a 1000mL erlenmeyer flask. It bumped so violently at one point that it ejected one of the pieces of glass :o. Glass is the only thing I could come up with that would resist the acid, and it did absolutely nothing to reduce bumping.

I went back to the drawing board. The whole point of boiling stones is to introduce small bubbles into the solution. The small bubbles grow into big bubbles via evaporation and cool the solution. If the solution is not cooled in this manner, no evaporation occurs until the sulfuric acid is well above its boiling point. At this point it will bump randomly and violently. The bubbles don't seem to form on their own in sulfuric acid. The simplest solution is to pump bubbles into the solution with an aquarium pump, silicone tubing and a glass tube. The silicone tubing appears to hold up to the acid vapors. I've been able to concentrate 900mL of battery acid in a 1000mL flask in a few hours.

IIRC fresh battery acid has very few impurities. The impurities would poison the battery. As far as additives, what could one possibly add to the electrolyte that would improve the functioning of the battery?

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[*] posted on 3-3-2010 at 16:10


In his Treatise on Chemistry, Sir Henry Roscoe cautions about violent bumping of H2SO4. He also recommends small pieces of scrap platinum to prevent bumping. That has worked for me, but hissingnoise says he's never observed bumping without boiling chips. So who knows but I certainly wouldn't take chances with this stuff.
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[*] posted on 3-3-2010 at 16:34


I've ever only used pyrex jugs on a hotplate and have never seen acid being ejected - slight bumping, yes, but nothing to worry about.
I like the 'scrap' platinum idea though not everyone has that stuff lying around.
I'll certainly use it when using a RBF for heating H2SO4.
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[*] posted on 3-3-2010 at 16:38


Well thanks for the help guys.I have had trouble with bumping before even in a 5 litre jennaglass bowl which is why I added what I referred to as "boiling chips" but was in fact activated carbon.Great for bubbles but maybe contributing to my problem?(Reduction? Naah,surely not)
I'll stick the lot in an erlenmeyer with some broken borosilicate,if that fails I'll try the vacuum strip.BTW only H2SO4 here is "no fume" acid for pools.Haven't been able to source electrolyte.It is clear at 31% and straw yellow at 64%.
Thanks again for the replies.




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[*] posted on 3-3-2010 at 18:17


I use graphite boiling chips in my acid to prevent bumping. Activated carbon should do the same thing and should not contaminate the acid.
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[*] posted on 4-3-2010 at 07:28


For boiling anything the front-glasses of old washing-machines are useful, heated with the content in an old microwave ....
==> The Power is limited, everything is mechanically encapsulated ...
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[*] posted on 4-3-2010 at 08:11


Quote: Originally posted by rrkss  
I use graphite boiling chips in my acid to prevent bumping. Activated carbon should do the same thing and should not contaminate the acid.
It can react with acid. Graphite is oxidized by hot concentrated sulphuric acid, giving so-called graphite oxide (a.k.a. graphene oxide), a not well defined compound which has oxygen inserted in the graphite lattice.

http://en.wikipedia.org/wiki/Graphene_oxide

With sulphuric acid alone, the oxidation will not be fast nor complete, but there definitely is formed some of this material, and the sulphuric acid is contaminated with diverse products.




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[*] posted on 4-3-2010 at 09:05


Quote: Originally posted by hissingnoise  
i've never used boiling-chips in H2SO4 because I don't know what would resist the hot acid!


Have you tried pieces of broken unglazed pottery? I've never tried to concentrate sulfuric acid, but the pottery shards seem to work well for everything else I've boiled.




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hissingnoise
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[*] posted on 4-3-2010 at 09:11


Thanks woelen, you sure do the homework where it counts - and I had wondered if oxidation would occur despite graphite's inertness.
I'm guessing though, that amorphous carbon or charcoal would undergo a slow oxidation to CO2. . .
-Magpie, I've considered pottery shards but I have some bits of Pt wire to use as entropy51 suggested.
But as I said, I've never had problem bumping - not even when distilling HNO3 from dilute HNO3 in H2SO4 or from KNO3/H2SO4.
With KNO3/H2SO4 I'd be wary of chloride contamination of KNO3 which could lead to dissolution of Pt.


[Edited on 4-3-2010 by hissingnoise]
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[*] posted on 4-3-2010 at 10:29


I once concentrated battery-acid in an erlenmeyer-flask ...
--> When I wanted to test the pH with indicator-paper it was dissolved within maybe 1/2 second (acid was hot), and the acid became colored brown from the dispersed residual carbon ...

Thats how agressive H2SO4 can be; any help would come late: Main/major damage is done within the first 2 or 3 seconds ...
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[*] posted on 4-3-2010 at 12:11


Quote:
Graphite oxide exfoliates and decomposes when rapidly heated at moderately high temperatures (~280–300 °C) with formation of finely dispersed amorphous carbon, somewhat similar to activated carbon.


If it forms, it quickly decomposes at the boiling point of the acid. I've done this technique numerous times and always got good oily colorless sulfuric acid which always performed the way I wanted it too. I've tried boiling without boiling chips and had a severe bumping problem.
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[*] posted on 18-3-2010 at 22:11


Just to follow up on this first timers might find this linkhttp://www.amazingrust.com/Experiments/how_to/Concentrating_H2SO4.html useful.Also has a density/% chart.
I also found when using a large dish a fan running on high blowing across the dish delays the onset of heavy SOx fumes until the acid reaches an 80% concentration and disperses same so the neighbours may be less likely to call in the fire department.

[Edited on 19-3-2010 by starman]




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[*] posted on 19-3-2010 at 00:29


Some (dangerous) physical experiment comes to my mind:
==> Heating H2SO4 close to the boiling point
==> as well water

> Then quickly inject one into the other, eg. the water (only 100 [Cels]) into a chamber, where the H2SO4 is ...
==> Must give quite an steam-explosion ... ; how much water would be boiled off this way ? Achievable pressure-rising ?

Maybe the hot water could be saturated with Na2CO3, in an amount that neutralizes the acid ...
==> Even more gas, now CO2 ...
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[*] posted on 8-3-2016 at 13:09


Quote: Originally posted by per.y.ohlin  
I've always had horrible bumping when directly boiling sulfuric acid. I once had ~100mL of partially concentrated acid with some broken glass in the bottom of a 1000mL erlenmeyer flask. It bumped so violently at one point that it ejected one of the pieces of glass :o. Glass is the only thing I could come up with that would resist the acid, and it did absolutely nothing to reduce bumping.

I went back to the drawing board. The whole point of boiling stones is to introduce small bubbles into the solution. The small bubbles grow into big bubbles via evaporation and cool the solution. If the solution is not cooled in this manner, no evaporation occurs until the sulfuric acid is well above its boiling point. At this point it will bump randomly and violently. The bubbles don't seem to form on their own in sulfuric acid. The simplest solution is to pump bubbles into the solution with an aquarium pump, silicone tubing and a glass tube. The silicone tubing appears to hold up to the acid vapors. I've been able to concentrate 900mL of battery acid in a 1000mL flask in a few hours.

IIRC fresh battery acid has very few impurities. The impurities would poison the battery. As far as additives, what could one possibly add to the electrolyte that would improve the functioning of the battery?



Ok, I am on my second attempt at distilling sulfuric acid. I purchased a 500ml retort just for this because there's no joints to get stuck at high heat. I have glass chips inside. And the bumping has so far stopped me from proceeding.

I am going to try the platinum thing, since I have one gram of platinum in my 'collection' and see if it works. Also I noticed that I can prevent bumping if I tap the table under the distillation with a hammer to induce nucleation . Maybe I could leave a vibro-massage on the table while the distillation goes on.

[EDIT]

I tried putting my jagged 1 gram platinum bar ( they bend these off a sheet ) into the mix of glass shards and it maybe helped a little, but not enough.

Using an electric sander next to it while it boils seems to work ok, but I'm not doing that.

So now I have got a steel brad encased in glass tubing that I made stirring the mix while it distills. That seems to work pretty good.

I will try just the glass shards and stirrer next batch.



[Edited on 8-3-2016 by Agent Smith]

[EDIT2]

I finally used a regular teflon coated stirrer and turned it on. No bumping. Doesn't seem to hurt the stirrer.

[Edited on 9-3-2016 by Agent Smith]
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[*] posted on 9-3-2016 at 00:51


Do not use glass chips. I tried to use them, once. The acid bumped like gunfire, like there was a war going on inside - I was afraid that my retort will explode.

Then I used brick pieces as boiling chips - and it boiled peacefully, like I was boiling an egg. Use porous stuff like brick or, better, pumice.




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