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Author Topic: How is molten potassium hydroxide electrolysis carried out?  (Read 6240 times)

Offline SorryDnoodle

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So I have been trying to perform a electrolysis on some molten potassium hydroxide and I believe I have the wrong materials for my cathode/anode

I have tried steel and copper for both, the copper one seems to make a dark blue substance, I assume it is copper hydroxide. And the steel is odd, at first it makes a really dark color, not black but perhaps a deep purple, hard to tell with a face shield, but after a while it turns green.

So my question is, what should I be using and what is being created here?
« Last Edit: 12/05/2014 19:46:29 by chris »


 

Offline chiralSPO

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Re: Molten potassium hydroxide electrolysis
« Reply #1 on: 09/05/2014 23:29:50 »
You're going to need something much more resistant to corrosion than just steel. Platinum should work with no problem (other than cost). You might be able to get away with nickel.

I'm not sure what is created in your experiments. Most likely some copper salt is responsible for the blue color on the copper electrodes, though copper hydroxide is white, not blue...

Can you be more specific to the conditions of the reaction? In each case, was the color formed at the anode or the cathode? Were you performing the reaction open to the air, or under an inert atmosphere? What voltage was applied? What temperature was the melt? What type of steel (different types have chromium, nickel, molybdenum, vanadium, manganese or other transition metals added as well as different carbon content)?

Sorry for the barrage of questions, but each of them is relevant.
 

Offline SorryDnoodle

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Re: Molten potassium hydroxide electrolysis
« Reply #2 on: 10/05/2014 01:11:40 »
Sorry for the barrage of questions, but each of them is relevant.
Very true, though I am short for answers sorry to say, I pretty much rushed in head first wanting to test it out without pre-planning or actually buying the correct equipment, pretty poorly done but I was bored and wanted to try it.

I have been searching a bit after making this thread and I noticed Nickel being mentioned a couple of times so I will try and get my hands on some of that, but perhaps investing in a platinum electrode is the best option either way.

/fast answers: Open air, Cathode(i think), 12 V 2 A, temperature unknown, I believe it was just regular old carbon steel, though I am not sure at what %.

Thanks for the reply!
 

Offline chiralSPO

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Re: Molten potassium hydroxide electrolysis
« Reply #3 on: 10/05/2014 18:50:12 »
I am surprised that the color shows at the cathode rather than the anode. If it were forming at the anode, it would just mean that the metal was being oxidized preferentially over the hydroxide, which would be expected for both iron and copper (though stainless steel and nickel do survive this because of passivated oxide surface formation).

Given that the reaction is observed at the cathode, it means you are probably reducing something other than the potassium (or that the potassium formed is immediately reacting with your electrode or with the air and the electrode). KOH pulls CO2 out of the air fairly rapidly (though probably not as much of an issue at elevated temperatures), but I could see CO2 reacting with copper and hydroxide to form something similar to the mineral azurite (Cu3(CO3)2(OH)2), which is a very pretty blue (azure). I think it is unlikely that azurite is the product, but it is a possibility. You could also be reducing the CO2 to formate HCO2 and forming copper formate (which is blue), but I'm also not so happy with that answer, as it requires copper2+, which should be unstable at the cathode with a 12V separation from the anode...

12V is also probably overkill for this reaction. I suspect 4V to 6V with a higher current is more likely to give good potassium production.

e + K+ → K     E0 = 2.93 V
2 OH → O2 + 2H+ + 2 e     E0 = 0.40 V

So thermodynamically, 3.33 V is the minimum potential required, but you can probably expect a couple hundred mV overpotential for each reaction to go, so I estimate 4 V as the likely onset of the desired reaction. (with Pt or Ni as the anode, the overpotential on the oxygen production side will drop slightly, but still probably require at least 3.9 V total)
 

Offline SorryDnoodle

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Re: Molten potassium hydroxide electrolysis
« Reply #4 on: 10/05/2014 21:56:33 »
I am surprised that the color shows at the cathode rather than the anode. If it were forming at the anode, it would just mean that the metal was being oxidized preferentially over the hydroxide, which would be expected for both iron and copper (though stainless steel and nickel do survive this because of passivated oxide surface formation).

Given that the reaction is observed at the cathode, it means you are probably reducing something other than the potassium (or that the potassium formed is immediately reacting with your electrode or with the air and the electrode). KOH pulls CO2 out of the air fairly rapidly (though probably not as much of an issue at elevated temperatures), but I could see CO2 reacting with copper and hydroxide to form something similar to the mineral azurite (Cu3(CO3)2(OH)2), which is a very pretty blue (azure). I think it is unlikely that azurite is the product, but it is a possibility. You could also be reducing the CO2 to formate HCO2 and forming copper formate (which is blue), but I'm also not so happy with that answer, as it requires copper2+, which should be unstable at the cathode with a 12V separation from the anode...

12V is also probably overkill for this reaction. I suspect 4V to 6V with a higher current is more likely to give good potassium production.

e + K+ → K     E0 = 2.93 V
2 OH → O2 + 2H+ + 2 e     E0 = 0.40 V

So thermodynamically, 3.33 V is the minimum potential required, but you can probably expect a couple hundred mV overpotential for each reaction to go, so I estimate 4 V as the likely onset of the desired reaction. (with Pt or Ni as the anode, the overpotential on the oxygen production side will drop slightly, but still probably require at least 3.9 V total)

Thanks for the reply, i'll do some adjustments when I can and make a short reply later if it all works out.
 

Offline SorryDnoodle

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Success! After a few attempts I finally managed to make some Sodium from NaOH and Potassium from KOH !
I used Carbon/stainless Steel anode/cathode and lowered the temperature so the K/Na would't get dissolved and it worked.

pretty small amounts because my heating setup is terrible and my container is even worse, but it worked so onto a bigger scale of things!

Thanks for the assist, really appreciate it!
 

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