I am trying to build a system to generate dry HCl gas. I was wondering if you bubble air through diluted HCl will it produce a gas with mostly water vapor or HCL vapor, can we vary the proportions of each by varying the temp of the inlet air and the acid, like hot air bubbling through cold acid or vice versa giving more acid or water vapor.

I am aware of tried and tested techniques to generate dry HCl gas, but this question came to my mind exploring techniques if one does not have access to conc. sulfuric acid. Surely something similar has been studied and researched before where the rate of evaporation has been measured.

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    $\begingroup$ Basically you are trying to find something that can dry the air without reacting with the acid. This will rule out the common ones such as $\ce{CaCl2, KOH}$. $\endgroup$ Commented Jan 29 at 14:04
  • $\begingroup$ patents.google.com/patent/US5958356A/en - Check out this patent...! $\endgroup$ Commented Jan 29 at 14:06
  • $\begingroup$ And bubbling air through $\ce{HCl}$ would not be advisable, rather heating it to form the vapours and passing the vapours through $\ce{MgCl2}$ supported on an activated carbon or silica gel substrate activated by heating to a temperature between 150° C. (302° F.) and 300° C. (572° F.) under vacuum will remove water at partial pressures of below 0.5 torr. (according to the patent above) $\endgroup$ Commented Jan 29 at 14:08
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    $\begingroup$ The problem you face is that HCl is very soluble in water and blowing air through dilute HCl will not be very effective. At the very least you will have to spend a huge effort to dry the resulting gas stream which is going to be very annoying and time consuming versus almost any other method of generating dry HCl gas. $\endgroup$
    – matt_black
    Commented Jan 29 at 20:24
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    $\begingroup$ Whats wrong with calcium chloride, it generates relatively dry HCl for gassing salts. $\endgroup$
    – The Entity
    Commented Jan 30 at 15:09

2 Answers 2


HCl forms a high boiling azeotrope at 110 Celsius and 20% HCl. This azeotrope has been well studied as a gravitational standard for acidimetric titrations. Low concentrations will concentrate on distillation until the azeotrope is reached. More concentrated will lose HCl. Distilling neither will give dry HCl gas but concentrated 37%HCl should come close. The recommended drying agent is concentrated H2SO4 so a better method would be to react NaCl with sulfuric acid directly. Molecular sieves might work; check with the manufacturers. Other reactions such as PCl3 or PCl5 with water are also complicated involving hard to obtain and nasty chemicals These reactions could possibly be engineered to give dry HCl. Tetra chlorides of tin, titanium or silicon or AlCl3 might also work. Look them up for details.


For the first question:

will it produce a gas with mostly water vapor or HCL vapor

The gas would be mostly water vapour because dilute HCl is present as ions so it's concentration as whole (unbroken) HCl would be low.

For second question:

can we vary the proportions of each by varying the temp

If you bubbled gas that is 200°C then entire water along with HCl would get evaporated. This is the only way to achieve large amount of HCl by bubbling but mole fraction of HCl in our gas would be still very low.


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