Why does sulfuric acid concentration top-out at 98%?

I’m sitting here looking at my mad scientist supplies and noticed that my sulfuric acid is a 98% concentration, or 18 molar. What’s the other 2%? (not water, surely!) Is there such a thing as 100% sulfuric acid? My acetic acid is also 98%.

You can get close - 99.999%


My WAG is that 98% is a legal fiction to cover their asses if some other contaminant is there.

The same site offers acetic acid at ≥99.7%.

If anyone has a bottle from Aldrich, we can check the certificate of analysis from that lot.

If you merrily dissolve sulphur trioxide in conc sulphuric, you can have what is regarded as 102% concentration. See Oleum:


Very handy for making TNT etc.

Sulfuric acid and phosphoric acid solutions with water don’t really behave like how we typically think about an aqueous solution, especially at high concentrations. There’s a lot of hydrogen and oxygen around, but it doesn’t really stay in static, discrete molecules of H20 or H2SO4. You get molecules of acid joining up into stuff like disulfuric acid. Phosphoric acid in particular likes to form all sorts of fun polymeric species at high concentrations. Both of them can be dehydrated such that there is less hydrogen and oxygen relative to sulfur or phosphorus than you would “expect” for 100% H2SO4 or H3PO4.

It appears a little more complicated than that. The spec sheet states “99.999% Based On Trace Metals Analysis”, but it also states “Purity (Titration by NaOH) 95.0 - 98.0 %”.

Some chemicals are hygroscopic (they absorb water). Sometimes that means you can’t remove the water – you can only get 95% ethanol by distillation alone – and other times it means the chemical will absorb moisture from the air. A quick glance at the wiki page on azeotropes(mixtures that cannot be separated by distillation) indicates that sulfuric acid forms a 98% azeotrope with water, which neatly matches the titration purity figure above.

When I get back to lab I’ll try to look up lot-specific certificates of analysis, to see if they offer more details about what is actually in the last few percent.

Water, some trace metals either from the pipes or from the sulphur oxide.

One of the biggest costs of H2SO4 factories comes from the expense of getting pipes and valves that won’t get eaten away by their own product. I had a project where the factory wanted to treat certain spare parts as financial assets and the mother company did not; the maintenance manager showed us a valve that had cost more than my team manager’s yearly salary - it eventually did get created as a financial asset.

Chemical engineer here - this is basic chemical engineering we learn in undergraduate school.

This has to do with the vapor pressure of SO3 over H2O. In simple words - above 98%, the SO3 vapor pressure is high and you’ll have SO3 loss to the vapor space . Below 98%, the water vapor pressure is higher so that the water mist will carry SO3 out with it.

It is explained very well here :

See figure 3 on page 5.

I did my college internship in a Plant that made sulfuric acid and then used the sulfuric acid (sold some of it too) to make phosphoric acid and then used the phosphoric acid (sold some of it too) to make fertilizers and detergents. It was fun :slight_smile:

Oh and you need to respect sulfuric acid :slight_smile: and phosphric acid :slight_smile: but you must revere HF (A lot of Hydrofluoric acid is produced as a side product of making Phosphoric acid - and you can virtually see it eating up all the steel over the course of a short time - a good lesson in metallurgy too.)

Little know fact.

Atheletic sweat is 110%.

110% sulphuric acid? I think I’ll cancel that gentle walk.

If by “revere” you mean “fuxache keep that stuff away from me!” then yes. I’ve never mucked about with HF but I’ve read about it and it seems like I’d be at less risk of harm if I fed my own arm into a bandsaw. :eek:

To continue the hijack : To handle Sulfuric acid and H2SO4, we use carbon pumps & condensers, PTFE (Teflon) lined piping, and glass lined reactors. For HF, the opposite is the case. NO glass (silica) ; and mild steel (low Si) is the norm. HF doesn’t typically harm (most) steels.

The acidic nature is really no big deal to humans, just rinse it off when contact is made. The real problem occurs when H2SO4 escapes… It makes one hell of a toxic H2SO3 vapor cloud, hence the name : Fuming Sulfuric acid.

HF is indeed some nasty stuff, but it’s nature is insidious. Get it on you, and you may not notice right away. It is NOT like “feeding your arm into a bandsaw”, unless you consider the pain encountered a few days after the fact, “bandsaw”-like. It attacks the calcium in your bones, not the skin, and usually its effect is relatively slow to manifest. Boric Acid is the antidote I use after exposure.

Phosphoric acid is no big deal either, we feed it to our bugs in the waste treatment facility in our plant as a nutrient. They love it! Get it on you at appx. 90%, rinse it off, and you’ll be OK.

The point of my rant is this: Contact with the acids mentioned in this post (and many others) is not a life altering event. With the exception of H2SO4 (that I mentioned), if you get a little bit on you, rinse it off with water… You’ll be fine.

When I was at school I was allowed to prepare HF in a fume cupboard.

I doubt if that’s encouraged nowadays.

The rest of it is water. And the reason is the same reason you can’t easily get pure ethanol. The substances are purified from aqueous solution by distilling (the vapor is enriched in the compound with a lower boiling point). However, liquids can form azeotropes which boil with an unchanged composition, so that distillation does not enrich the mixture in either component. Sulfuric acid forms an azeotrope with water of 98% H2SO4 (by weight). Ethanol forms an azeotrope with water of 95% ethanol. So those are normally the highest concentration you can (cheaply) get.

Wikipedia lists some azeotropes: