-jeffB
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Old School Potassium
Many years ago, my family got a complete 1959 set of the Encyclopedia Britannica, and I spent many happy hours reading the entries for individual
elements, reaction classes, "Liquefaction of Air", and so on. Many were the nights I spent dreaming of reproducing some of the processes described
there. Few were the ones I actually attempted, which was probably just as well -- I tended toward clumsiness and absent-mindedness, not good survival
characteristics for an amateur chemist.
One that particularly impressed me was the entry for potassium, which detailed the original isolation of the element. The 1919 edition, now available
online, also presents it:
| Quote: | | To Sir Humphry Davy belongs the merit of isolating this element from potash, which itself had previously been considered an element. On placing a
piece of potash on a platinum plate, connected to the negative of a powerful electric battery, and bringing a platinum wire, connected to the positive
of the battery, to the surface of the potassium a vivid action was observed: gas was evolved at the upper surface of the fused globule of potash,
whilst at the lower surface, adjacent to the platinum plate, minute metallic globules were formed, some of which immediately inflamed, whilst others
merely tarnished. |
I eventually found an OTC bottle of KOH, but never quite got around to trying this.
I've recently gotten the bug again, and, inspired by the work of BromicAcid and others here, decided to take a swing at it. I had a few
disadvantages: I'm a bit short of platinum servingware, and significantly more cautious than I was as a teenager. But I also had a few advantages:
the postings here, a nice plexiglas shield, that extra caution I mentioned, and a surplus variable-output power supply that can be set to current or
voltage regulation. This last seemed like a good thing; with 300V or more compliance, it could start current flowing even through a relatively high
resistance, then automatically back down the voltage once a conductive channel was established, minimizing the chances of (a) no result or (b) potash
popcorn.
Since I have no platinumware, I used a thin sheet of steel (purchased long ago from a hobby-store metal kiosk) as the base/cathode, and a thin strip
cut from that sheet as the probe/anode. I found a bottle of KOH as weird little Hershey-kiss pellets, 3-6mm in diameter and 2-3mm tall. The pellets
had whitish rims (hinting that they'd absorbed some CO2?), but weren't caked together at all; apparently the bottle had been well-sealed.
I placed one pellet with its flat base on the steel sheet. I pushed it around a little bit, and noticed that it left a bit of a trail -- already
trying to deliquesce? We're in a warm and humid spell here, and the dewpoint was probably well above 10 C, so it wouldn't surprise me. I breathed
across the sheet, and it fogged, except for a teardrop-shaped area surrounding and trailing the pellet. (I'd never seen that particular demonstration
of aggressive dessication before!) After a few more breaths, I figured it was probably moist enough to start conducting.
I set the power supply to constant-current mode at its minimum power, around 50mA, and touched the probe to the top of the pellet. The voltage
quickly dropped to 10V or less, a channel melted/fused from the contact point to the base, and a small amount of gas started to effervesce. The steel
discolored slightly, and gray balls began to form around the bottom of the pellet. At this point, I turned up the current to 100mA. After a few
minutes, I started to notice some purple streaking (ferrate?), and occasional wisps of white smoke. I saw a few very faint sparks. I saw larger
sparks when I pulled away the probe, as the PS shot up to 300V to try to maintain a constant current. The sparks were mostly lavender, but that's not
surprising if you think of them as small arcs drawn from a potassium compound -- I don't think they indicated the presence of metal, especially since
they seemed to originate from the anode.
I was hoping for Davy's "minute metallic globules", but if they formed, they were too minute for me to spot. But something was certainly
happening.
After ten or fifteen minutes, I decided to quit and see what I had. I took the plate to a basin, ran water over it, and was rewarded with a FAT
purple spark. So, yep, there was some metal there!
Things to do next:
Gloat -- it's nothing compared to Bromic's performances, but it was a nice step for me, and I didn't damage anything. 
Figure out the theoretical yield of potassium in micrograms per milliamp-minute, to avoid unrealistic expectations or unwelcome over-achieving
surprises.
Try much higher current levels. This PS only goes up to 300mA, I think, but if I only need 10V or less, I might try something with a 12V supply and a
light bulb as ballast.
Think a lot about further steps, possibly including modifications that would let me isolate recognizable quantities of metal.
It's been awhile since I did my in-depth forum search. Have many of you tried this particular method? Any other helpful suggestions or warnings?
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MadHatter
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Congratulations !
Sometimes the old school methods are the best for experimenters not interested
in any sort of mass production. Thanks for posting Davy's experiment. I've copied it to the FTP.
From opening of NCIS New Orleans - It goes a BOOM ! BOOM ! BOOM ! MUHAHAHAHAHAHAHA !
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BromicAcid
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Glad that I could be of inspiration. This reaction was also inspiration for my first real preparation of sodium (though I never isolated any). I
started off with some NaOH pellets and let them absorb some water from the air and then attacked them with nickel rods connected to a battery charger.
I didn't get much of an effect though until I added some more water which ended up putting the NaOH into solution and as the electrolysis continued
it drove off the water and I ended up with molten NaOH and small little skittering globules of sodium that made me giggle somewhat disturbingly.
Best of luck on your future work with this.
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microcosmicus
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| Quote: |
Figure out the theoretical yield of potassium in micrograms per milliamp-minute, to avoid unrealistic expectations or unwelcome over-achieving
surprises.
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1 milliamp-minute = 0.06 Coulombs = 0.62 micomoles of electrons
0.62 micromoles of K weighs in at 24 micrograms
[Edited on 10-1-2008 by microcosmicus]
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hashashan
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Say guys .. this might be stupid but did anyone try to electrolyze NaOH under oil?
just molten NaOH under mineral oil... that way the O and H will bubble away through the oil and we will end up with plain Na
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-jeffB
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| Quote: | Originally posted by hashashan
Say guys .. this might be stupid but did anyone try to electrolyze NaOH under oil?
just molten NaOH under mineral oil... that way the O and H will bubble away through the oil and we will end up with plain Na |
I've considered that, but I'm concerned that the oil would (a) prevent good electrical contact and/or (b) conduct away heat too rapidly. I thought
about running under a blanket of butane, but I don't know how to do it safely (hah) in view of the evolved oxygen and potential for sparking.
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-jeffB
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| Quote: | Originally posted by microcosmicus
1 milliamp-minute = 0.06 Coulombs = 0.62 micomoles of electrons
0.62 micromoles of K weighs in at 24 micrograms
[Edited on 10-1-2008 by microcosmicus] |
That's about what I figured:
96500 C/mole = 26.8 Ah/mole
(26.8Ah/mole)/(39 g K/mole) = 0.69 Ah/g K
= 41.2 A-min/g K
= 2474 A-sec/g K
= 2.5 A-sec/mg K
= 25 s/mg K at 100mA
I did some more trials last night. Short summary: mostly disappointing, but intriguing, and I think I know why platinum wasn't just an extravagance.
More details when I have more time to write them up.
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microcosmicus
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| Quote: |
Say guys .. this might be stupid but did anyone try to electrolyze NaOH under oil?
just molten NaOH under mineral oil...
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KOH melts at 330C, NaOH at 318C
Mineral oil is as mixture of hydrocarbons which boil at
around the same temperature --- I saw the figure of
260-330C cited. You would want to use
heavier hydrocarbons to keep from having your protective
fluid from vaporizing away, especially since the temperature
of the electrolyte is not likely to be maintained right at its
melting point. Paraffin wax, whose components boil at
400C and above, might be a better choice.
[Edited on 10-1-2008 by microcosmicus]
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BromicAcid
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I tried it once before in a u-tube to keep the anode and cathode separate, it was horrible. See the picture below but the oil spattered everywhere
and turned brown then black, I guess molten NaOH and hot liquid sodium metal was too much for it.
[Edited on 1/10/2008 by BromicAcid]
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-jeffB
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| Quote: | Originally posted by BromicAcid
I tried it once before in a u-tube to keep the anode and cathode separate, it was horrible. See the picture below but the oil spattered everywhere
and turned brown then black, I guess molten NaOH and hot liquid sodium metal was too much for it. |
I couldn't quite figure out what hot alkali or alkali metal could do to paraffins, but I've got a newfound appreciation for the oxidative power of an
anode in molten KOH. 
I was definitely making ferrate. After running the power for a few minutes, purple streaks started to build outward from the anode; when they reached
the cathode, I think potassium generation ceased.
I ran with a pair of steel nails for electrodes, and higher current, and got jets coming out with purple/yellow flame at the end -- potassium vapor,
maybe, or just finely divided potassium riding along on the hydrogen. Again, though, when the purple discoloration got to the cathode, the jets
stopped.
When I dumped the purple mass into water, it formed a purple solution, but it also fizzed gently. I thought perhaps it was finely dispersed potassium
generating hydrogen, but now I think it was oxygen from the ferrate decomposing. The purple solution fairly quickly went to a ferric-oxide brown.
Tonight, I tried again, this time using carbon electrodes. I found a jar of one-inch carbon planchets (apparently used for EM sample prep) at our
local surplus-junk dealer -- I put the KOH pellet on one as the cathode, and broke another in half to use as the anode. I tried running 200mA, and
got the jets again; I backed down to 100mA, and got fairly steady current/voltage, with some occasional spitting. At times, a silver-greyish material
would bubble out of the molten part, and turn more grey. I suspected this might be potassium, but I would've expected it to burn in the air, not just
tarnish.
After five or ten minutes, I stopped, chipped the mass off the planchet, and dropped it into a bucket of water. BIG pop and purple flash. Woohoo!
I repeated the run, this time dropping the mass, planchet and all into the water. Big flash and pop when it hit, with one little purple
ball-lightning zipping across the water to the right, and another hitting and sticking to the side of the bucket on the left. The planchet, with most
of the unreacted pellet, sank to the bottom, where the pellet dissolved; after a few seconds, a small purple light came chirping up from it.
This is fun.  I'm not sure what I'll try next -- probably doing it under
paraffin. I'll also think more about setting up to record video.
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-jeffB
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Sodium with carbon electrodes, or lack thereof
I tried again last night with the carbon planchets, but this time with some sodium hydroxide hand-segregated from the metal bits and other detritus in
a can of drain opener. The NaOH was much harder to start; it wasn't as hygroscopic as the KOH, and it seemed less prone to fuse. I got frequent
yellow sparking, but nothing that looked like a buildup of metal, and no reaction when I quenched the mass in water. I actually got better results
when I tried doing NaOH with steel electrodes. Of course, the humidity was higher then, too, so that might have been more of a factor.
We've got a spell of dry and cold weather coming up, so it may be that rerunning the KOH reaction will be tougher, too. I'll post results, if any,
later in the week.
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Fleaker
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I think I will be attempting this soon with a rewound MOT at 30-40A. I have a large platinum electrode that would do well for this application.
I am going to look over len's design and see if I can't run his cell with KOH.
Neither flask nor beaker.
"Kid, you don't even know just what you don't know. "
--The Dark Lord Sauron
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garage chemist
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Don't forget the distillative method for potassium (potassium carbonate + magnesium), it is superior to any electrolytic method.
It has been discussed here some time ago, and a german reference was posted of which I translated the relevant parts.
https://sciencemadness.org/talk/viewthread.php?tid=2105&...
See S.C.Wack's and my posting.
This does require red heat, reachable with a good bunsen burner, and (preferably) vacuum or inert gas so the potassium can be distilled off.
Can surely be done in a pyrex or better quartz test tube or steel pipe (if you can get it hot enough) under vacuum/hydrogen/inert gas.
There is a reason why electrolytical methods for potassium production did not have any future in industry- they pose more problems than the analogous
sodium production.
All industrial K production processes are some kind of reduction combined with distillation.
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-jeffB
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| Quote: | Originally posted by garage chemist
Don't forget the distillative method for potassium (potassium carbonate + magnesium), it is superior to any electrolytic method.
It has been discussed here some time ago, and a german reference was posted of which I translated the relevant parts.
https://sciencemadness.org/talk/viewthread.php?tid=2105&...
See S.C.Wack's and my posting.
This does require red heat, reachable with a good bunsen burner, and (preferably) vacuum or inert gas so the potassium can be distilled off.
Can surely be done in a pyrex or better quartz test tube or steel pipe (if you can get it hot enough) under vacuum/hydrogen/inert gas.
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I did see this post. I paid particular attention to the part about Kohlenoxydkalium. If I were to try the procedure, I would probably start with KOH instead of K2CO3, both for safety and because I already have it on hand.
I'm concerned about safety anywhere above the submicro scale, though. KOH tends to attack glass anyhow, and running the vessel up to red heat would
surely accelerate the process.
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garage chemist
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The article says that no Kohlenoxydkalium forms when the stochiometric composition of the mixture is correct.
Only when too much K2CO3 is used the danger of (COK)2/(COK )6 formation exists.
So it would be prudent to use a little excess Mg to compensate for weighing errors, also the mix should be homogenized well and fine Mg be used.
COK forms when carbon monoxide is present during condensation of potassium vapor. Mg reduces CO at the reaction temperature, so enough Mg will provide
safety.
If you use KOH, better dehydrate it in a crucible at red heat before use, commercial KOH is often something like 86% pure, the rest being water!
[Edited on 15-1-2008 by garage chemist]
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-jeffB
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| Quote: | Originally posted by garage chemist
If you use KOH, better dehydrate it in a crucible at red heat before use, commercial KOH is often something like 86% pure, the rest being water!
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I wonder what would happen if I didn't? I assume that I would simply need additional Mg to react with any water that wasn't driven off before the
reaction commenced. Throwing in some extra Mg is more appealing than fusing and roasting KOH, then breaking it up and powdering it again without
letting it absorb more moisture. The roasting and pulverizing is probably easier and cheaper for an industrial scale, but not for me.
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garage chemist
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You could pour out the molten KOH on a metal sheet and crush it up while still hot.
But you're right, you could just use more Mg instead.
This will make more hydrogen gas during the reaction then, but that could actually be a good thing since it would act as a protective gas.
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-jeffB
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| Quote: | Originally posted by garage chemist
You could pour out the molten KOH on a metal sheet and crush it up while still hot.
But you're right, you could just use more Mg instead.
This will make more hydrogen gas during the reaction then, but that could actually be a good thing since it would act as a protective gas.
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Yeah, and I'm not too concerned about the safety issues of hydrogen coming off the mixture, given that potassium vapor is the desired product.
Given that the KOH and Mg will both be molten at the reaction temperature, I wonder how finely they need to be ground and mixed? I don't have any Mg
turnings or powder, but I do have some thin sheet. My KOH is in large prills (4-8mm diameter). I may try this initially with one or two prills and a
corresponding amount of Mg, considerably less than a gram total.
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garage chemist
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The Mg will have to be in fine turnings or powder form. It doesnt really mix with the KOH upon melting since it has a MgO layer on its surface which
will only grow when reaction sets in.
Grind down some of your Mg with a file to make some powder (I did that myself a long time ago, when I only had massive Mg as well), or look around for
a supplier of Mg powder.
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Fleaker
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Interesting. I recall trying something similar with lithium carbonate...
I'll look into it, I have a nice stainless steel setup for it.
Neither flask nor beaker.
"Kid, you don't even know just what you don't know. "
--The Dark Lord Sauron
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