zephler - 15-2-2011 at 20:13
I did UTFSE, but did not really come up with a lean answer to this question - can anyone help? - H2SO4 is not on hand, but I would like to make some
HCl gas. I heard that it can be used, although it is not as good, and the H3PO4 has to be heated to around 70C or so - comments?
[Edited on 16-2-2011 by zephler]
blogfast25 - 16-2-2011 at 11:30
The first dissociation constant of H3PO4 is 7.25 x E-3, so it’s not even a really strong acid, as is needed for the displacement reaction NaCl + HA
=== > NaA + HCl. Heating salt with pure H3PO4 will probably yield a bit of HCl because it’s a gas. Phosphoric acid isn’t used in HCl
generators.
ScienceSquirrel - 16-2-2011 at 12:10
I think you might have more luck if you heat ammonium chloride with phosphoric acid.
I believe that ammonium chloride can be made to react with sodium hydrogen sulphate to yield hydrogen chloride.
In this way you using the equilibrium between a very volatile ammonium salt and an involatile ammonium salt.
not_important - 16-2-2011 at 14:36
NaCl + H3PO4 will work, using reasonably concentrated H3PO4 and heating. As H3PO4 is effectively non-volatile, the equilibrium for H3PO4 + NaCL
<=> H2NaPO4 + HCl can be forced to the right by driving off the HCl. But you're likely looking at temperatures above 130 C to get good yields,
with the possibility of distilling off the HCl-H2O azeotrope when using standard 85% H3PO4; might be useful dehydrating the acid to 95-97 percent
first by heating it for awhile, it's mentioned elsewhere on the board.
OTOH, ammonium sulfate (NH4)2SO4 readily loses NH3 to give ammonium hydrogen sulfate (NH4HSO4 - ammonium bisultfate), which loses ammonia fairly
readily, as well as decomposing with reduction of the SO4<sup>(-2)</sup> and oxidation of the NH3/NH4(+). You can smell ammonia above a
boiling solution of ammonium sulfate, and the solution's pH will slowly drop. It's the higher volatility of HCl that allows the reaction to give
decent yields of HCl, not any involatility of ammonium sulfate.
metalresearcher - 17-2-2011 at 05:19
In aqueous solution it will work too (but not as good) as the solubility of Na3PO4 is much lower than NaCl, so adding NaCl to H3PO4aq will precipitate
Na3PO4 and the HCl will dissolve in the water.
See: http://en.wikipedia.org/wiki/H3PO4