On 23/11/16 19:21, Chris Hogg wrote:
On Wed, 23 Nov 2016 14:26:54 -0000 (UTC), Cursitor Doom
wrote:

"Concentrated" nitric acid is only 70% pure. Obtaining anything stronger
than that becomes costly and the shelf-life declines markedly.
Sulphuric acid is commonly available (god knows why given the fact it's
used as a weapon nowadays in the black community) from BnQ., plumbers'
merchants and such like at concentrations of about 98% and you don't
really need anything stronger than that for general use.
Hydrochloric acid seems to top-out at only 36% purity and it's tough to
obtain any solution of it stronger than that.
Do we have any chemistry aces here who can explain the wide disparity in
the maximum strengths of these commonly available acids?

This, from way back in the memory banks. It may not be quite correct
in detail, but the gist of it is OK:

If you take some dilute nitric acid, and try to concentrate it by
boiling to drive off the water, the temperature slowly rises as water
is driven off until it reaches 121°C and a concentration of 68%, when
it forms what is known as a constant boiling mixture* (CBM), in simple
terms the nitric acid (HNO3)is boiling off as fast as the water, and
the vapour above the liquid has the same composition as the boiling
liquid, so the boiling liquid never gets more concentrated. If you
want stronger nitric acid, known as fuming nitric acid, you have to
distil it from a mixture of sulphuric acid and potassium nitrate, but
even then you can only get a maximum of 99%, because the nitric acid
itself dissociates into N2O5 and H2O and forms another CBM.
2HNO3 - N2O5 + H2O.

Pretty much the same explanation goes for hydrochloric acid, although
here the CBM at atmospheric pressure is at 110°C and a concentration
of about 20% HCl. But because HCl gas is very soluble in water, if you
distil a solution under reduced pressure, the water boils off in
preference to the HCl, and you get higher concentrations; lab grade
conc. HCl is about 37%, as you say. Whiffy stuff!

But H2SO4 doesn't easily evaporate, unlike the two above, and H2SO4
forms a CBM with water of 98% that boils as 330°C, so can't be made
any more concentrated by distillation.

* https://en.wikipedia.org/wiki/Azeotrope

Very good explanation of acid strengths. Just to gild the lily, also see:
https://en.wikipedia.org/wiki/Oleum

--

Jeff