[teros]

Hi everybody, I'm a physicist trying to do a work about exoplanet atmospheres and stability of several liquids, but I'm having a serious problem to find information about sulfuric acid (H2SO4). I need the range of temperatures where this substance is in liquid state for different atmospheric pressures: 0.1, 0.3, 1, 3 and 10 atm. (at least). I only have the values for 1 atm. but I am taking  for the boiling points approximate values using the Antoine equation. Nevertheless, I've read in the wikipedia that, although the sulfuric acid boiling point is 337 ºC, over 300 ºC it descomposes slowly. So, for my issues of long term liquid stability, I am taking 300º C as the upper limit.

My questions are: a) does this 300º C issue happen at other pressures, for example at 10 atm.? or does this value change with pressure? b) which is the triple point of sulfuric acid? I need this to know which is the smaller atmospheric pressure where liquid H2SO4 can exist (I supposse the temperature will be similar to the melting point at 1 atm. as the melting point curve is almost vertical for small pressure ranges).

Of course, if somebody can give me the experimental values of melting and boiling temperatures for these atmospheric pressures, much better!

Thanks in advance for any help.

[Arkcon]

Hmmm ... tricky.  OK.  What you're looking for is a phase diagram of sulfuric acid.  Its unfortunate, but all I can find is with mixtures with water.  That makes sense, mixtures with water is how its typically used, but someone should have studied the properties neat at some point.  Maybe you can poke around a little bit in physical chemistry journals, like this one: http://pubs.acs.org/doi/abs/10.1021/jp035572v

[Borek]

What you're looking for is a phase diagram of sulfuric acid.

I am not convinced it is 'that simple'. Somehow I doubt existence of planetary atmospheres made of pure sulfuric acid, I would expect them to rather be some variable mixture of H₂O/SO₃/H₂SO₄ (see below), which makes modeling more complicated (and definitely points to the property diagrams of the mixture). Sounds like something chemical engineers working in sulfuric acid production can have good resources for (although they probably work mostly at high temperatures).

Perhaps some resources will be mentioned in the papers/books discussing Venus atmosphere, from what I remember sulfuric acid is abundant there.

I've read in the wikipedia that, although the sulfuric acid boiling point is 337 ºC, over 300 ºC it descomposes slowly.

Sounds a bit strange to me. A lot depends on what the decomposition products are, but the most obvious reaction I would expect is the reversible dissociation into H₂O and SO₃. That means you are dealing with an equilibrium mixture of the compounds mentioned earlier, with all temperature/pressure effects applied (which is also a partial answer to other parts of your question).

[Enthalpy]

Equilibrium gas pressure versus temperature and concentration, up to 1 bar
http://www.dedietrich.com/en/solutions-and-products/mineral-acid-treatment/sulfuric-acid-treatment
you can estimate an activation energy (for Antoine) and extrapolate to somewhat higher pressure. I'd dare 10bar.

Higher pressure, data of unknown origin
http://www.sciencemadness.org/talk/viewthread.php?tid=21281&page=2

Looks like heat is a way to get sulphuric acid half-dry.

All this relates to the equilibrium partial pressure of sulphuric acid, optionally combined with water vapour. At a total atmospheric pressure lower that this, liquid sulphuric acid would boil (and possibly stop after its composition has changed). At a partial pressure lower that this, it would evaporate without boiling.

The decomposition too is an equilibrium that depends on both the temperature and on the pressure of the decomposition products, so no binary answer.

The intuitive perception with sulphuric acid is that many reactions compete with evaporation and decomposition.

[Enthalpy]

I've tried to misuse CPropepShell to evaluate the decomposition of sulphuric acid, but this software couldn't even reach an equilibrium. It's meant for combustion, where all atoms can reorganize freely, and in addition, it probably doesn't expect sulphuric acid as a product of a hot flame, but gaseous sulphur oxides and water instead.

Time for someone to develop a similar equilibrium software but targeted for other uses?
http://www.chemicalforums.com/index.php?topic=80019.msg292226#msg292226