Sciencemadness Discussion Board

Hydrochloric acid production - and other halides/haloacids and electrolysis

l0k1 - 20-11-2012 at 07:22

I knew that the primary method for producing HCl involved electrolysis but I never really looked into it before. I didn't realise how easy it was but the wikipedia page didn't help by leaving out the most important input into the description of direct production.

The electrolysis is dead simple, a 9v battery with probes stuck in a solution and you get one side making hydrogen and slowly you start to see the yellow-green colour appear in the water. So, first off, I presume that the bath should be hot in order to increase the rate at which the chlorine gas exits the water as it partially dissolves when the water is cold.

Next, once you have these two gases generated, you pipe them together into something to shine bright light on it. I believe unlike soda glass, pyrex does not block nearly as much UV so a nice hot halogen lamp sitting right next to a flask filling with this gas there should be a fairly good conversion to HCl which is then captured in a column of distilled water.

I know that metallic electrodes are not a good idea - just playing around with a 9v battery and a pair of neodymium magnet i made one all ugly and dull, the one that generates the chlorine gas, I had the intention of using graphite and I know that it gradually decomposes releasing a bit of carbon dioxide in the process but that also will leave the cell with only sodium hydroxide in it, which is of course itself useful as well.

So, the reason why I'm posting this is to pick the brains of the folks here as to any details I'm missing such as severe dangers or other such things. I already know HCl and Cl2 are dangerous but in a properly set up rig these never see the outside of the glass.

The other thing is that where I live now, HCl is nearly unobtanium. I mean, it's able to be got (and perfectly legal) but being such a frequently needed material to be able to make it from readily available salt, electricity and light makes it far preferable, and also means that one can avoid having to store it and all the irritations that involves.

Would it be better to simply electrolyse the I2 straight out of the cell, or is it better to generate Cl2 in an NaCl cell and pipe the Cl2 through a solution of iodides? Though it is one more step it seems to me that since gasing KI solution produces KCl and I2 it would be a simpler workup (filter, wash and press). Also, what would be better electrode material for these reactions than graphite or carbon rods? I was thinking something like rhodium plated brass or something like that, or platinum plated carbon, or something.

[Edited on 20-11-2012 by l0k1]

elementcollector1 - 20-11-2012 at 11:22

"A 9V battery with probes stuck in the solution"
Please tell me you don't mean you would actually put the battery in the solution.

"Next, once you have these two gases generated, you pipe them together into something to shine bright light on it..."
And then you rush to the hospital, as the resulting explosive reaction will have destroyed all your glassware and probably a few parts of you as well.

"...but that also will leave the cell with only sodium hydroxide in it"
This is actually a personal pet-peeve of mine, so I'll restate it. Unless you divide this setup into two half-cells with a salt bridge, this setup will never get you sodium hydroxide. Ever.

It's better to just buy the HCl, I get mine for $3.99 per 32 fl.oz. or so.

"Would it be better to electrolyse the I2 straight out of solution"
When did we start talking about iodine?

"What would be better electrode material for these reactions than graphite or carbon rods?"
Pure platinum works excellently.

So, why the inability to use the search engine? Because I'm absolutely positive that threads on HCl (aq) and I2 have been done to death already.

tetrahedron - 20-11-2012 at 12:48

besides, the typical 9V battery stores less than 1000mAh of charge (1200mAh for the lithium type), which corresponds to 1.36g (37.3mmol) of hydrogen chloride at hypothetical 100% efficiency.

experimenter_ - 18-2-2016 at 11:41

It is a pity to ridicule this method (described in the first post) for HCl production through electrolysis. There is at least one advantage it can offer; a HCl acid of high purity. Distilling HCl acid from commercial sources will not give a good purity because iron chloride (and possibly other chlorides) distill together with it. Some people also may find it difficult to acquire reagent grade acid. So, this method worths to be discussed. Some practical questions:

1) I suppose that electrodes could be made from titanium (they use it in commercial production) instead of dirty carbon rods or expensive platinum?

2) Is it necessary to divide the electrolysis cell? If not, what problem would arise? I think that some Cl2 will react with NaOH to form bleach. This means that the output gas will not be H2 / Cl2 in equal amounts, thus the reaction between those two towards HCl will not consume the gas completely, some H2 will be escaping. Is this a serious reason to use a split cell, or it doesn't matter?

3) Someone mentioned the danger of explosion when H2 and Cl2 combine. However, do they really explode if we only light them mildly? I think explosion only happens when strong UV light is used. Correct me if wrong.

For the time being, tried a small prototype with carbon rods .. it can produce the gasses. Tried to capture them in a test tube inverted in water in order to check if they are explosive but with no luck, the Cl2 dissolves in the water.

Any other guys interested on this project?

[ Commenting on tetrahedron's criticism .. I have to say that we could always use electricity from a wall plug to overcome the issue with the limited capacity of the 9V battery :P ]

hissingnoise - 18-2-2016 at 12:41

Quote:
It is a pity to ridicule this method (described in the first post) for HCl production through electrolysis. There is at least one advantage it can offer; a HCl acid of high purity. Distilling HCl acid from commercial sources will not give a good purity because iron chloride (and possibly other chlorides) distill together with it.

You should ignore that old thread!

HCl is best purified by distillation!

Being non-volatile, metal salts cannot come over as long as the adapter outlet is some inches above the liquid . . .

The NaCl/H2SO4 method is cheap, quick, convenient and gives high purity and concentration!

I've never heard of it being prepared in the lab by electrolysis!

Oh, and welcome to scimad . . .

experimenter_ - 18-2-2016 at 13:31

Quote: Originally posted by hissingnoise  


Oh, and welcome to scimad . . .


Thank you for the welcome and your detailed answer.

After some more searching, it seems that guys here have already tried this method!
http://www.sciencemadness.org/talk/viewthread.php?tid=2154
https://www.sciencemadness.org/whisper/viewthread.php?tid=95...

Reading those two threads made me to change my mind of course ..


My problem is that when trying to purify commercial muriatic acid by distillation, I find in the distillate a greenish salt (I can see it after steam/ evaporating the distilled HCl, very low heat). Maybe I'll try outgasing HCl in distilled water, seems much safer way compared to this thread (even if it is lengthier in time).

So, the brine electrolysis method is good only for "preppers" who like to make HCl acid using their solar panels and salt only.

ave369 - 21-2-2016 at 00:58

Sulfuric acid and table salt, no?

Even the unboiled Bat works. No iron, the azeotropic HCl can be distilled clear as water. Yeah, this "unobtainium" can be made from these two OTC reagents with a simple distillation setup. If you use stronger H2SO4, you get HCl outgassing which can be dissolved in distilled water (or, better still, azeotropic HCl).


[Edited on 21-2-2016 by ave369]