Sciencemadness Discussion Board

Distilling fuming hydrochloric acid

ampakine - 13-6-2011 at 08:03

If I was to produce a concentrated HCl solution (lets say 70% w/w) by reacting NaCl with H2SO4 then distilling this solution would I be right in assuming I'll end up with a constant boiling HCl solution of 20% (or whatever its azeotropic ratio is) w/w?

DJF90 - 13-6-2011 at 08:42

You're never going to reach a concentration of 70% w/w. It'll only dissolve so much - 37% is about the point where it is saturated. Distillation of 37% will first drive off hydrogen chloride gas, and then the azeotrope of 20ish% will come over. Personally I wouldn't bother; concentrated hydrochloric acid fumes enough without trying to distil it!

mr.crow - 13-6-2011 at 10:06

You may be thinking of HCl the same way as nitric acid. Hydrochloric acid is HCl gas dissolved in water, where as nitric acid is a liquid mixed with water.

You can make HCl gas (nasty!) with NaCl and H2SO4, then dissolve it in water with an inverted funnel. Keep the water in an ice bath too.

If you really want to distill it (like hardware store acid) dilute it to 20% first then distill the azeotrope.

ampakine - 13-6-2011 at 12:35

mr.crow: I assumed the HCl would be dissolved in the water as soon as its formed. Can you elaborate on this inverted funnel setup a bit? Sounds like there'd be no need for distillation if the only thing you're dissolving in the water is HCl gas.

entropy51 - 13-6-2011 at 13:32

Quote: Originally posted by ampakine  
mr.crow: Can you elaborate on this inverted funnel setup a bit? Sounds like there'd be no need for distillation if the only thing you're dissolving in the water is HCl gas.

Quote: Originally posted by woelen  
A suckback IS bad in your case. The melt is hot and contains a lot of H2SO4. If you get the water in this hot melt, then you'll severely regret what you have done! BE CAREFUL!

A safe way of preventing suckback is the use of a small inverted funnel, which you immerse just a mm into the liquid, which must be in a fairly tall beaker (total surface of liquid must not be much larger than surface liquid, covered by the inverted funnel opening). If HCl gas reaches the surface of the liquid, and liquid is sucked into the inverted funnel, then the level of the liquid will go down, and the funnel is no longer immersed in the liquid. Some tweaking may be needed to get the level of immersion adjusted well, but this method definitely works and is safe. At any cost should suckback be prevented with your hot NaCl/H2SO4 melt.