Sciencemadness Discussion Board

Producing and handling solid sulphuric acid?

chornedsnorkack - 9-8-2019 at 09:57

1) 100 % sulphuric acid has a freezing point of 10,3 Celsius. However, sulphuric acid is fairly viscous.
Does liquid concentrated sulphuric acid easily undercool on cooling below its equilibrium freezing point? Is solid sulphuric acid easy to produce?

2) Liquid sulphuric acid has a big heat of dilution with water. It is a common safety precaution to avoid adding water to sulphuric acid due to heating.
But solid sulphuric acid necessarily has a latent heat of melting. How does this compare to the heat of dilution? Is adding water to solid sulphuric acid safe?

Ubya - 9-8-2019 at 14:28

i've never handled 100% sulphuric acid, but you can be sure 98% doesn't freeze even at -10°C

Tsjerk - 9-8-2019 at 14:37

100% sulfuric contains SO3, as H2SO4 by itself disproportionates into H2O and SO3. The only way to eliminate water from concentrated sulfuric is by adding SO3.
I
https://www.google.com/search?q=freezing+graph+concentration...

chornedsnorkack - 10-8-2019 at 01:04

Quote: Originally posted by Ubya  
i've never handled 100% sulphuric acid, but you can be sure 98% doesn't freeze even at -10°C


I cannot. See the graph linked below.
A freezing point minimum is at -35 Celsius and 94 %, or 93 %. The graph to 100 % is nearly linear - slightly convex.

Assuming 94 % and no convexity, that would give freezing point of -5 Celsius at 98 %.
Your acid might supercool. Or it might be 97 % not 98 %, in which case equilibrium freezing point might be as low as -12 Celsius.

True, the reaction
H2SO4<->H2O+SO3 is reversible, so 100 % H2SO4 contains both water and SO3, as well as H2S2O7. But freezing out of H2SO4 molecules into the crystal framework would drive the equilibrium towards H2SO4.
What does glacial sulphuric acid look like, if, say, freezing 98 % sulphuric acid gives you crystals of 100 % sulphuric acid and mother liquor of 97 % sulphuric acid?

vmelkon - 10-8-2019 at 12:42

There is this page:
http://www.sulphuric-acid.com/techmanual/Properties/properti...
Quote:
A freezing point minimum is at -35 Celsius and 94 %, or 93 %. The graph to 100 % is nearly linear - slightly convex.


The minimum seems to be -35 °C at 94%
Quote:

Assuming 94 % and no convexity, that would give freezing point of -5 Celsius at 98 %.


Looks to me that it is 0 °C.

unionised - 10-8-2019 at 13:05

If you managed to get some solid H2SO4 it would be an absolute pig to work with.
It's hard to imagine anything more hygroscopic, and (in most places) it would be cold enough to condense water directly from the air anyway.

chornedsnorkack - 10-8-2019 at 22:08

Quote: Originally posted by unionised  
If you managed to get some solid H2SO4 it would be an absolute pig to work with.
It's hard to imagine anything more hygroscopic, and (in most places) it would be cold enough to condense water directly from the air anyway.


P4O10 is specifically more hygroscopic because it is used to dehydrate, inter alia, H2SO4.
And once separated from mother liquor, solid H2SO4 should be stable to +10 Celsius.

Tsjerk - 11-8-2019 at 02:33

The only most practical way to produce 100% H2SO4 from 96% H2SO4 and P4O10 is by distilling SO3 from the reaction mixture and dissolve that in 96% sulfuric again. Otherwise you will have to distill of the 100% sulfuric acid at 337, not a pleasant endeavor I would say.


Quote: Originally posted by chornedsnorkack  
Quote: Originally posted by unionised  
If you managed to get some solid H2SO4 it would be an absolute pig to work with.
It's hard to imagine anything more hygroscopic, and (in most places) it would be cold enough to condense water directly from the air anyway.


P4O10 is specifically more hygroscopic because it is used to dehydrate, inter alia, H2SO4.
And once separated from mother liquor, solid H2SO4 should be stable to +10 Celsius.


I think what unionised means is that the solid sulfuric acid will turn liquid again upon absorption of the tiniest bit of water as the freezing point would drop like a brick with that tiny bit of water. The first % already drops the freezing point with 5-6 degrees.

P4O10 stays solid untill the H3PO4 itself will start to take up moisture (at RT).

Assured Fish - 11-8-2019 at 14:19

This would be a cool idea an all but if the goal is to simply make solid state sulfuric acid then why not just prepare oleum.
Apparently 30% oleum has a melting point of 21°C according to this.
http://www.inchem.org/documents/icsc/icsc/eics1447.htm

chornedsnorkack - 12-8-2019 at 07:08

Quote: Originally posted by Assured Fish  
This would be a cool idea an all but if the goal is to simply make solid state sulfuric acid then why not just prepare oleum.
Apparently 30% oleum has a melting point of 21°C according to this.
http://www.inchem.org/documents/icsc/icsc/eics1447.htm


At that concentration, the solid phase is not H2SO4, which melts at +10 Celsius. It is H2S2O7, which melts at +35 Celsius.