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Author: Subject: KOH from KCl using electrolysis
Keras
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[*] posted on 2-5-2020 at 10:26
KOH from KCl using electrolysis


Folks,
since I’m still under lockdown, I was planning this time to get some flakes of caustic potash out of potassium chloride (diet salt) using electrolysis.

Problem is: apparently electrolysing KCl gives potassium chlorate and not potassium hydroxide. It’s not that I’m spurning potassium chlorate, but it's not my goal here.

I think the trick is to avoid the chlorine gas to react with the hydroxide, and thus I was thinking about dunking only a short bit of the anode into solution, while, on the other hand, plunging the cathode as low as possible.

Does that make sense?
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elementcollector1
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[*] posted on 2-5-2020 at 10:48


It won't work. Lower surface area of the anode just means it has a higher current density, not that less charged ions will react.

What you could do is get a salt bridge between two separated cells, one for each electrode - this will (very slowly!) get you KOH and H2 gas at the cathode and Cl2 and O2 gases at the anode (the chlorine will also dissolve in the anolyte to a limited extent, coloring the solution yellowish). Salt bridges are tricky, though, because they limit the overall current that can be passed through unless they're very short (ions have to diffuse across/through the bridge, which is much slower than migrating through solution).

One salt bridge I've wanted to try (but never had the time/means) was a vinyl or other resistant polymer tube/hose plugged with cotton on both ends, with salt solution (KCl solution in your case) inside. No idea if it would be effective - it'd probably have to be very short. The thing I'm worried about is that this won't prevent aqueous mixing (the thing that gives chlorate in the first place), and gelling the solution inside the tube with agar is actually necessary.




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Keras
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[*] posted on 2-5-2020 at 11:40


Quote: Originally posted by elementcollector1  
It won't work. Lower surface area of the anode just means it has a higher current density, not that less charged ions will react.


Well, I agree but potassium chlorate results from the bubbling of the Cl₂ formed at the anode in the KOH that is gradually made. I surmised that if I can actually decrease the amount of liquid the bubbles of chlorine have to travel before reaching the air, I will also reduce the amount of KClO₃ produced. But I might be wrong.

I had thought about adding NaOH to the mix, in order to capture the chlorine and form sodium chlorate, while KOH would remain untouched. But I’m pretty sure KOH is more reactive than NaOH, so it would amount to none. Maybe Ba(OH)₂ would work, but I have none here.

Using a salt bridge isn't really an option to me given I'm forced to jury-rig about anything I use. I could make one with a straw, or I could you a shoelace as a wick (as I do for my makeshift alcohol burner). Maybe I'll try the latter.

Thanks for your quick answer!

[Edited on 2-5-2020 by Keras]
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mackolol
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[*] posted on 2-5-2020 at 14:03


If you want to make chlorine and lye from alkali salts, you must purchase nafion membrane and put it between electrodes in such way, that it isolates them.
AFAIK nafion doesn't allow OH- ions to go through it, but allows cations.
It should work with such setup. These membranes are quite expensive but look at aliexpress, they have plenty of it.

Although you have to figure out how your cell will be built in such way, for now I have problems with getting chlorine by electrolysis of plain HCl...

[Edited on 2-5-2020 by mackolol]
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Keras
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[*] posted on 3-5-2020 at 01:53


While I agree this is the best way to do it, getting that membrane, then getting it to fit into my beaker, etc. seems at best impractical. And unrealistic these days.

Besides, I dislike ordering things from China. As far as I can, I like to keep my supply sources in Europe. It might be slightly more expensive, but I feel better supporting European companies.

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mackolol
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[*] posted on 3-5-2020 at 02:25


How you wish... But I strongly doubt that you can do it other way
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morganbw
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[*] posted on 3-5-2020 at 09:44


I am not really suggesting this but you could possibly look into using a mercury cathode.

To be in the real world though, just buy some. It is not that costly and will probably be of a higher quality than you could possibly produce.
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[*] posted on 3-5-2020 at 12:29


You could try a clay terra cotta pot, they're pretty cation permeable but not so much anion.

I've made a short salt bridge with PVC pipe, two elbows, and cotton balls holding in a saturated NaCl solution and it works pretty well to avoid solution mixing.
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[*] posted on 4-5-2020 at 08:47


Mercury cathode is unreachable these days. Besides, call me a wimp, but I refuse to mess with mercury. Terra cotta might be feasible, and I was thinking about transforming another plastic straw into a salt bridge, so why not?

Thanks anyway to everyone for your suggestions!
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