Sciencemadness Discussion Board

Dilute conc. HCl to az. HCl for ease of storage?

Sulaiman - 31-5-2022 at 20:53

Because I think that 20% HCl solution will store better than 36% as bought,
I intend to dilute my conc. HCl to azeotropic.
(at least I would be confident that the concentration will remain constant)

Will this significantly reduce the problem of corrosion of nearby metals due to HCl outgassing?

When is az. HCl not a replacement for conc. HCl?

woelen - 31-5-2022 at 22:47

Azeotropic HCl indeed is much easier on storage than 36% HCl, it does not fume noticeably. Even 30% HCl already is much easier on storage than 36% HCl.

But... azeotropic HCl is not a replacement for 36% HCl for many experiments. The 36% acid contains quite some undissociated HCl (that is why it fumes so much) and that gives properties quite different from that of much more dilute solutions. Certain organic reactions (e.g. replacing hydroxyl groups with Cl on ncome alcohols) only work with highly concentrated HCl, but formation of certain metal complexes and isolation of them also can only be done in concentrated HCl (e.g. formation of cesium salts of copper(II) chloro complexes, reactions of antimony in oxidation state +3 in solution).

I store my 36% HCl in a glass bottle with a teflon-lined cap (GL45 red cap).

j_sum1 - 1-6-2022 at 01:42

I buy hardware store grade HCl which is around 32-33%.
It looks clean and clear but has proved to be full of crap - at least the last few purchases.
(Weirdly, Ti seems to be a main contaminant. Purple colouration appearing when acid is used, then becoming cloudy as the mixture hydrolyses. Formation of an orange / beer-bottle complex on addition of peroxide.)
I dilute and distil to azeotropic as a matter of course. Several advantages.

* No fuming. I store the bulk acid in its original container out of the way and, because of reasonable quality caps I don't have much problem with these containers.
* The acid is a lot cleaner. And sufficiently strong for most purposes.
* I end up with a product with a consistent concentration, unlike the starting material. And that concentration stays stable because of the lack of fuming.

I can still use the original acid when I need higher concentration and am unconcerned about purity.
On the rare occasions I need both high purity and high concentration, I can generate HCl gas to fortify a small amount of the azeotropic acid. So far, this gas been quite rare and I have only needed small amounts.

So, in my view, making azeotropic HCl is something of a win and I recommend the practice.

(The only other thing I will say is that it is bad practice to let your distillation boil dry if it contains titanium. The TiO2 sticks to the glass and is troublesome to clean off.)

unionised - 1-6-2022 at 02:00

Unless you are boiling it (at 760mmHg) there's nothing magical about the 20 % or so azeotrope.
The "fumes" vs concentration graph doesn't have a kink in it at 20% or anything like that.

Adding a little water will reduce the fuming a bit.

Putting a suitable lid on it will reduce them completely.

Having said that, it might be worth storing the bottle of acid inside a plastic box with some sodium carbonate. (In my experience, there is no such thing as a "suitable lid").

Sulaiman - 1-6-2022 at 03:08

Quote: Originally posted by j_sum1  
I buy hardware store grade HCl which is around 32-33%.
I dilute and distil to azeotropic as a matter of course. Several advantages.

* No fuming. I store the bulk acid in its original container out of the way and, because of reasonable quality caps I don't have much problem with these containers.
* The acid is a lot cleaner. And sufficiently strong for most purposes.
* I end up with a product with a consistent concentration, unlike the starting material. And that concentration stays stable because of the lack of fuming.

I can still use the original acid when I need higher concentration and am unconcerned about purity.

My HCl is also hardware store grade, diluting and distilling sounds like a good idea.

RustyShackleford - 1-6-2022 at 05:08

Quote: Originally posted by j_sum1  
(Weirdly, Ti seems to be a main contaminant. Purple colouration appearing when acid is used, then becoming cloudy as the mixture hydrolyses. Formation of an orange / beer-bottle complex on addition of peroxide.)


Interesting it would have Ti as a main contaminant, from my experience its usually Iron. Seems likely with Ti as contaminant the origin of the HCl is from titanium refining (where they convert TiO2 from ore into TiCl4, distill that off then produce Ti products from that.

[Edited on 1-6-2022 by RustyShackleford]