no1uno
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Sub-boiling distillation/purification of HCl
IMPORTANT - THIS IS NOT MY WORK
Take note of the opening warning, I am not trying to take kudos for some other person(s) work, I am merely posting something which I regard as a
solution to a question I have seen many times here and elsewhere.
Check this out (somebody might want to grab it, IIRC this site used to have a good writeup on making lead acetate which is no longer available): http://www.crscientific.com/article-hcl.html
It provides for the sub-boiling distillation of HCl to purify it (leaving the contaminants behind in the larger beaker). It might be possible to
optimize the setup somewhat (I can't, I am currently devoid of glass, the police have all of mine ). As pool acid is cheap and OTC a means of purifying it simply is of some interest (I would have thought anyway),
especially this one which does not require forming HCl gas and passing it into water.
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Panache
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i disagree that the method posted will work. Firstly the author didn't get the method to produce even muratic acid (i believe 30%) let alone 33 or
37%, he only supposes it will. Secondly it is patently obvious that the highest possible theoretical concentration of HCl solution achievable by the
method would be the same as the starting concentration of the acid used. Thirdly if one wants to make any more than 100mL you need an apparatus the
size of a small car (yes i am exaggerating, but the point remains that scaling up to provide even a decent amount is not feasible. Fourthly yes it
will remove impurities with no vapour pressure but so will a simple distillation that can scale up indefinitely if you modify it to be
pseudo-continuous. Fifthly if you require a standard solution from purchased HCl solution you could just titrate it against a standard solution, this
would have the same accuracy with or without the Fe impurities as they are miniscule anyways and barely barely affect the solutions H+ concentration.
That said the site looks interesting and i have bookmarked it for a further look so thank-you for sharing it, but also the general technique
described, with minor modifications is very good for a number of other purifications, i had not thought of it previously.
Sorry for being negative largely but it raining and weather drives my objectivity 
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12AX7
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The main point I got was it minimines aerosols from boiling, since it's not boiling. And those aerosols carry stuff. So you really do get only
whatever was on the glass, which is probably pretty clean. Other volatiles are another matter of course.
Tim
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watson.fawkes
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| Quote: | Originally posted by Panache
i disagree that the method posted will work. Firstly the author didn't get the method to produce even muratic acid (i believe 30%) let alone 33 or
37%, he only supposes it will. Secondly it is patently obvious that the highest possible theoretical concentration of HCl solution achievable by the
method would be the same as the starting concentration of the acid used. |
He started with "10 to 12 M" and
got 7.1 - 7.3 M or so. He never claimed this was a concentration technique. | Quote: | | Thirdly if one wants to make any more than 100mL you need an apparatus the size of a small car (yes i am exaggerating, but the point remains that
scaling up to provide even a decent amount is not feasible. |
Huh? This is just a simple way of making a cold
finger. In this case his collection vessel isn't distant from his source vessel. If the tactic looks familiar, it's essentially the same as a solar
still, the desert survivalist's friend. | Quote: | | Fourthly yes it will remove impurities with no vapour pressure but so will a simple distillation that can scale up indefinitely if you modify it to
be pseudo-continuous. |
This is a simple distillation. | Quote: | | Fifthly if you require a standard solution from purchased HCl solution you could just titrate it against a standard solution, this would have the same
accuracy with or without the Fe impurities as they are miniscule anyways and barely barely affect the solutions H+ concentration.
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He was titrating to find how successful he was, not to use it as a titration standard.
What I find interesting about this is that if you put the whole apparatus under pressure, you can still suppress boiling, but the partial pressures
will go up, thus increasing the distillation rate. This is the real way to consider scaling up this idea--keep it at sub-boiling, but use a more
typical configuration with a cold finger above a separated collection vessel.
As for concentrations, what distillation always does is to create a product closer to the azeotrope. The pseudo-compound of the azeotrope always has
lower partial pressures than other mixtures (including the pure substances), so its distillation rate is always faster. Distilling alcohol
concentrates it ordinarily because there's a lot of excess water with respect to the azeotrope. On the other, completely ludicrous, side, if you had
an almost anhydrous alcohol-mixture, distilling it would lower the alcohol content, at least until it went to total completion.
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12AX7
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If you start with muriatic acid (31.45%, ~10M), isn't that superazeotropic (i.e., excess HCl) and mostly HCl should distill first? Which is
hygroscopic so, I assume, high concentration HCl should result.
Incidentially, if you wanted to put the work into the apparatus, you can easily make this "pseudo-continuous". Just put hoses into the reciever and
flask. In fact the only difference between this and a regular distillation apparatus is the unusually intimate relation between the condenser,
reciever and still pot's atmospheres, and the lack of cooling at the reciever (which could be fixed with some more hoses).
It is quite slow and "wasteful" of space, but it is extremely simple to set up. Hah, if you care to drudge through the archives, you'll even find *I*
was distilling in this manner several years before I got a still per se! It's so simple even I can do it
Tim
[Edited on 9-11-2008 by 12AX7]
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Panache
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i assumed the poster, no1uno (difficult to pronouce) was the same person as posted the method on the other site, so when i said 'i disagree' i meant
with the suppositions in the article. perhaps i spend too much time making subtle jokes i see them at times when they do not exist and i thought the
'i am not the author of this material' disclaimer was a joke and hence i responded to the thread post and the link author as one and the same
oops
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DJF90
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This is useful in the fact that if alterations can be made to make it much more efficient (for lack of a better word), which could provde us with a
method of cleaning dirty acid. This would be especially useful if we could apply this method to cleaning sulphuric acid, as this is more commonly in a
more impure state (I have never seen HCl sold in shops that have colourants/corrosion inhibitors/load of other crap in it). An I'm gonna take a guess
and say that no1uno should be pronounced "no one you know"?
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watson.fawkes
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| Quote: | Originally posted by 12AX7
If you start with muriatic acid (31.45%, ~10M), isn't that superazeotropic (i.e., excess HCl) and mostly HCl should distill first? Which is
hygroscopic so, I assume, high concentration HCl should result. |
Nope. The existence of an azeotrope implies
a deviation from Raoult's law, which is the linear relationship between concentrations in liquid and vapor. In an excess-HCl solution, the partial
pressure of H<sub>2</sub>O is raised relative to the partial pressure of HCl as they would otherwise be from pure substances. The
wikipedia page on azeotropes has a good treatment of this, and also reminds me to qualify that previously I was talking about positive azeotropes, which was the
case in question.
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unionised
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If you open a bottle of conc HCl and blow across the top you get fumes. This is because the acid loses HCl(g) readily. If you heat it you get nearly
pure HCl(g) off first untill the concentration is near the azeotrope.
So yes, as 12AX7 says, mostly HCl distills off first.
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tapira1
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Azeotropic HCl
The HCl azeotrope has a concentration of about 6N in HCl; muriatic acid and/or concentyrated HCl are 12 N in HCl. Muriatic acid is usually conc. HCl
contaminated (on purpose) with iron (as the chloride).
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unionised
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Muriatic is a very old term derived, IIRC from the latin for brine. It just means hydrochloric acid and it is not specific to any given concentration
or purity. The word should be allowed to die and its soul to rest in peace among crosswords and quiz questions where it won't cause any confusion to
novice chemists.
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Picric-A
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unionised is righ, murictic acid is just an old name used for HCl however this name is a commonly used name for store bought HCl. This HCl is aorund
32% and is heavily contaminated with Fe ions, Cl ions and Na ions.
Distillaiton of this yealds decnt HCl that can be used on the Lab.
i dont see why you would want to use the method described in this thread as it is much easier and more effective to simply distill in quickfit
apparatus.
[Edited on 22-9-2008 by Picric-A]
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S.C. Wack
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You need not heat the HCl at all. Placing open containers of HCl in a jar such as a desiccator with pure water and letting it stand will give pure
HCl, as long as the contaminants are not volatile. Also works for ammonia.
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watson.fawkes
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| Quote: | Originally posted by unionised
If you open a bottle of conc HCl and blow across the top you get fumes. This is because the acid loses HCl(g) readily. If you heat it you get nearly
pure HCl(g) off first untill the concentration is near the azeotrope.
So yes, as 12AX7 says, mostly HCl distills off first. |
Somehow I forgot to check on the sign of the
HCl+H<sub>2</sub>O azeotrope; it's negative, which means the azeotrope point is a local maximum. I had thoughtlessly assumed it was a
positive azeotrope.
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unionised
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| Quote: | Originally posted by Picric-A
unionised is righ, murictic acid is just an old name used for HCl however this name is a commonly used name for store bought HCl. This HCl is aorund
32% and is heavily contaminated with Fe ions, Cl ions and Na ions.
Distillaiton of this yealds decnt HCl that can be used on the Lab.
i dont see why you would want to use the method described in this thread as it is much easier and more effective to simply distill in quickfit
apparatus.
[Edited on 22-9-2008 by Picric-A] |
Well, I don't think I have ever seen a bottle of HCl labelled as muriatic acid.
The reason for sub-distilling acid has already been given a couple of times and it's that you get a better product.
If you don't have nice quickfit glassware then not only is it not easier or more effective to use it, it's not possible.
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