Evaporating sulfuric acid

[gregory-d]

After a little experimentation I am curious.
In an attempt to remove some of the water I slowly warmed my sulphuric acid on a heat controlled hot plate at 250c.
The hotplate being set to 250c I very much doubt that the acid got up to that temperature in that time.
I have a IR thermometer on it way.
It was only there for an hour but as it cooled I covered it with a piece of plastic.
Coming back later there was indeed water condensed on the plastic.

My question is simple really
Has anyone tried heating sulphuric slowly to 300c.
By slowly I mean so that it never actually boils.
My thinking is that as the boiling point increases with the concentration and as the liquid gets close to it's boiling point the water will evaporate and the concentration will increase.

As the boiling point is 300c at 95% I am thinking that this could be a safe way to concentrate sulphuric acid.
Instead of trying to go all the way to 98% as many horror stories I have read here suggest it's not much of a clever idea

Or is this idea just another one of my "Don't be an idiot Gregory" ideas?

 

[anachronism]

Haha you already answered your own question Gregory. I'm going to go with your "don't be an idiot Gregory" option.

 

[gregory-d]

Haha you already answered your own question Gregory. I'm going to go with your "don't be an idiot Gregory" option.

But why it won't boil.
It wont get to the volatile phase.
The water will be well over it's own boiling point and thus should be trying to become a gas.
If done slowly this should collect near the surface and evaporate off gently.
I don't understand what is wrong with the idea.

 

[Yggdrasil]

But why it won't boil.
It wont get to the volatile phase.
The water will be well over it's own boiling point and thus should be trying to become a gas.
If done slowly this should collect near the surface and evaporate off gently.
I don't understand what is wrong with the idea.

The water is too tightly bound to the Sulfuric acid to separate in layers, some will evaporate in time as it achieves equilibrium at that temperature.
Hot Sulfuric is also much more prone to drag water out of the air/(surroundings)
which in turn also makes it much more dangerous if something goes wrong.
Think about boiling water 100C, triple this and add something that will even drag the moisture out of your skin/eyes/lungs additionally to the heat.
Concentrated Sulfuric hotter than touch warm should not be treated unless you are properly set out to deal with it.
And that means in particular the vapors/steam/droplets that will be suspended in the air if something go south.

It can be done, but at what cost?
I'll vouch to buy it!

 

[anachronism]

To answer your question as to what's wrong with the idea:

I don't know how well kitted out you are from a safety perspective however concentrated Sulphuric is nasty stuff even at room temperature because it is so hungry for water. I'm also unsure as to what you understand about the relationship between temperature and reaction speeds so please bear with me:

For every ten degrees C you increase the temperature of that acid, it doubles the speed that it reacts. That doesn't sound like a lot however the chart below puts what that means into context when heating this to 250 degrees C

Temperature	Reaction speed
10	1
20	2
30	4
40	8
50	16
60	32
70	64
80	128
90	256
100	512
110	1024
120	2048
130	4096
140	8192
150	16384
160	32768
170	65536
180	131072
190	262144
200	524288
210	1048576
220	2097152
230	4194304
240	8388608
250	16777216

In essence at 250 degrees C that acid will "work" 16 million times faster than when it's at 10 degrees C. It's a concept that is used in wet ashing of organics but no way heaven made would (edit) I want to be anywhere remotely in the vicinity of that. I'm no shrinking violet and I do take managed risks however I think this one goes beyond risk management.

I hope that's of some help Gregory.

Jon

 

[gregory-d]

The water is too tightly bound to the Sulfuric acid to separate in layers, some will evaporate in time as it achieves equilibrium at that temperature.
Hot Sulfuric is also much more prone to drag water out of the air/(surroundings)
which in turn also makes it much more dangerous if something goes wrong.
Think about boiling water 100C, triple this and add something that will even drag the moisture out of your skin/eyes/lungs additionally to the heat.
Concentrated Sulfuric hotter than touch warm should not be treated unless you are properly set out to deal with it.
And that means in particular the vapors/steam/droplets that will be suspended in the air if something go south.

It can be done, but at what cost?
I'll vouch to buy it!

Ok that was the reason for a slow controlled heating and as the temperature is below the azeotropic point of 98.3% then the water would be gradually driven off.
This could easily be done in a semi enclosed area with adequate ventilation and safety barriers and as the sulphuric is not breaking down at that temperature it could be done way more safely than actually boiling the liquid.
This is somewhat akin to roasting something at 200c in the oven.
200c is way higher than the boiling point of water but still considered safe because it is enclosed and slow. The chicken only blows up in the microwave.
Actually a high temperature oven (ie reflow type oven) could be a good environment, then the air would stay dry until the temperature reduced to room temperature.

 

[gregory-d]

To answer your question as to what's wrong with the idea:

I don't know how well kitted out you are from a safety perspective however concentrated Sulphuric is nasty stuff even at room temperature because it is so hungry for water. I'm also unsure as to what you understand about the relationship between temperature and reaction speeds so please bear with me:

For every ten degrees C you increase the temperature of that acid, it doubles the speed that it reacts. That doesn't sound like a lot however the chart below puts what that means into context when heating this to 250 degrees C

Temperature	Reaction speed
10	1
20	2
30	4
40	8
50	16
60	32
70	64
80	128
90	256
100	512
110	1024
120	2048
130	4096
140	8192
150	16384
160	32768
170	65536
180	131072
190	262144
200	524288
210	1048576
220	2097152
230	4194304
240	8388608
250	16777216

In essence at 250 degrees C that acid will "work" 16 million times faster than when it's at 10 degrees C. It's a concept that is used in wet ashing of organics but no way heaven made would (edit) I want to be anywhere remotely in the vicinity of that. I'm no shrinking violet and I do take managed risks however I think this one goes beyond risk management.

I hope that's of some help Gregory.

Jon

Thanks Jon that fills in a big hole in my understanding.

 

[gregory-d]

Thanks guys.
Yes Yggdrasil it does seem that Occam's razor has struck once again.
Much cheaper to buy than blow up the house.
But don't worry guys I am sure to come up with more crazy ideas.
I believe it is good to discuss these things in a place like this so that as others wonder the answer will already be there.

 

[4metals]

Are you evaporating in an open vessel, like a beaker, or have you invested in a small glass distillation setup?

 

[gregory-d]

Are you evaporating in an open vessel, like a beaker, or have you invested in a small glass distillation setup?

No just a beaker at the moment.
I am going to do some lower temperature experiments just to see what I can get out of the sulphuric if I take it to something above a bit the boiling point of water.
Just taking it up slowly and seeing how it goes with constant temperature monitoring and holding it steady for a while.
One video I saw suggested doing this to help pull the residue together. I haven't succeeded in doing this yet but I'm being pretty cautious.
I did get some water to evaporate from my bath so something does get release.
Even if it's just a bit it each time it may be enough to just keep the batch in a consistent concentration.
It would be hard to justify a distillation setup.

 

[4metals]

Well one of the things about sulfuric acid, and especially concentrated sulfuric acid, is it is hydroscopic, which means it just loves to suck moisture in from the surrounding air. A still will minimize that effect because the pool of acid being heated is much more contained. But be certain to store the acid, however you concentrate it, in a tightly sealed bottle once it cools.

 

[gregory-d]

Well one of the things about sulfuric acid, and especially concentrated sulfuric acid, is it is hydroscopic, which means it just loves to suck moisture in from the surrounding air. A still will minimize that effect because the pool of acid being heated is much more contained. But be certain to store the acid, however you concentrate it, in a tightly sealed bottle once it cools.

Thanks 4metals
I'm going to modify an oven to control the temperature at higher levels and thus create a large reflow oven for dropping components off PCBs and I was thinking I would try that one day too. The high temperatures will naturally be drier. But that is a little while off.

 

[Darkthirty]

I had a recipe for plastic explosive that involved heating a substance on a hotplate while adding another until it started to sparkle. At that point you shut the hot plate off and ran like the devil was chasing you...not worth the risk. H2SO4 is cheap when you consider the worst case scenario,
Edited to remove unnecessary image - moderator

 

[gregory-d]

Yes my ideas can be pretty crazy sometimes.
I know.
But you don't know unless you either swallow your pride and ask or try and blow yourself up with your ego intact.

 

[Yggdrasil]

Thanks 4metals
I'm going to modify an oven to control the temperature at higher levels and thus create a large reflow oven for dropping components off PCBs and I was thinking I would try that one day too. The high temperatures will naturally be drier. But that is a little while off.

That is where you are mistaken, hotter air can hold more water.
So if you want dry you need to cool the air and let it condense out and then heat it in a dry environment.

 

[gregory-d]

That is where you are mistaken, hotter air can hold more water.
So if you want dry you need to cool the air and let it condense out and then heat it in a dry environment.

Oh yeah. Every day's a school day.
So the stil is still the best idea. Ok thanks everyone.

 

[Yggdrasil]

Oh yeah. Every day's a school day.
So the stil is still the best idea. Ok thanks everyone.

I think the buy new Acid is the best idea.
Sulfuric is relatively cheap, and you do not need very much.

 

[Ayham Hafez]

If you are evaporating water from sulphuric acid using beaker, be CAREFUL, once all water evaporated (maybe the sulphuric acid concentration be above 98%), the beaker will be broke, and the hot sulphuric acid will destroy your lab, I have a bad experience with this before.

 

[gregory-d]

If you are evaporating water from sulphuric acid using beaker, be CAREFUL, once all water evaporated (maybe the sulphuric acid concentration be above 98%), the beaker will be broke, and the hot sulphuric acid will destroy your lab, I have a bad experience with this before.

Yes Ayham I was never trying to go to 98%.
Not trying to be rude but I am curious. What temperature did you run your mistake at or did you just add heat?

 

[Ayham Hafez]

Yes Ayham I was never trying to go to 98%.
Not trying to be rude but I am curious. What temperature did you run your mistake at or did you just add heat?

Heat about 100 c only, but I forgot it till all water evaporated