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Author: Subject: Make Potassium (from versuchschemie.de)
blogfast25
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[*] posted on 20-12-2010 at 06:48


Quote: Originally posted by len1  


{big snip}

the overall reaction would be

Mg + 2KOH -> Mg(OH)2 + 2K (wrong) (2)

meaning no hydrogen is evolved (and using up 1/2 less magnesium for the same amount of K generated), contrary to experiment, which shows hydrogen to be evolved at a rate close to (1) above.

[Edited on 20-12-2010 by len1]


And I cannot, for dear life, find any unreacted Mg in my reaction product mix. None. And it shouldn't be too hard to find. But everytime I sieve some sludge off, wash off the off white MgO, the metal looks like K only amd reacts accordingly. I've even reacted some of the sludge without washing, containing also some very fine K, cautiously with water only to obtain in the end an off white sludge, no Mg to be found: in these mild, cool and alkaline conditions remaining Mg would not react with water or any remaining t-butanol.

I believe that not so much this process but the reaction itself may have interesting other uses in organic/inorganic chemistry...
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[*] posted on 20-12-2010 at 07:03


@ sedit:

Personally I’m now convinced the grade of Mg is far less important than we originally thought: it seems that prior to anyone here actually replicating pok’s results we kind of ‘talked ourselves’ into possible Goldilocks effects pertaining to the magnesium..

But in the mean time we have five experimenters (pok, len1, nurdrage, woelen and me) who’ve all obtained very similar results with differently sourced magnesium. I think the vague term ‘good grade’ probably suffices as a specification. The patent seems to confirm that too.

But it could be worth examining what a very fine grade could do for reaction speed: since as we now believe at the heart lies a redox reaction: 2 KOR (solution) + Mg (solid) < == > 2 K (liquid) + Mg(OR)2 (???), increased surface area of the solid reagent Mg may have significant effect on the overall rate, unless that step is not rate-determining…


[Edited on 20-12-2010 by blogfast25]
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[*] posted on 20-12-2010 at 08:35


I'd like to report that the "crust" i got from the paraffin solvent run has a gravel like consistency. It forms large chunks that break up when poked but the smaller pieces themselves are rock-hard.

So there is some condition that favors the formation of crusts vs. Sand.

Anyway, Pok's results with crusts i now believe are authentic.

In case anyone is wondering, the Paraffin i used was "IR Spectroscopy grade" i got from Fluka. Probably too pure for our purposes but at least it gives evidence that D70 isn't a magic solvent.


@blogfast

I agree with the magnesium assessment. I have since tried two different sources of magnesium in different forms. Magnesium turnings (99.98%) from sigma aldrich about 1mm in size. And magnesium powder (99%) from Riedel-de Haen and both gave similar results. I like the turnings more though since they seem to coalasce better.

[Edited on 20-12-2010 by NurdRage]
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[*] posted on 20-12-2010 at 08:43


Quote: Originally posted by NurdRage  
I'd like to report that the "crust" i got from the paraffin solvent run has a gravel like consistency. It forms large chunks that break up when poked but the smaller pieces themselves are rock-hard.

So there is some condition that favors the formation of crusts vs. Sand.

[Edited on 20-12-2010 by NurdRage]


In both my tests I obtained extremely fine ‘sand’ (clearly the majority of the reaction by-product) but also some hard, white chunks. I thought they might be residual KOH but I think now I may be wrong on that. I’ll see if I can investigate…
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[*] posted on 20-12-2010 at 11:42


@len1: Your remarks have been incorporated into the webpage and I have modified the equations. When MgO is formed instead of Mg(OH)2, then there is no excess of magnesium anymore, just have a look at my analysis. Taking into account the loss of K through the addition of the t-butanol, I find a yield of isolated K-metal between 75% and 80%, which is acceptable. Please have a look at the end of the web page (for your convenience I post the same link here again).

http://woelen.homescience.net/science/chem/exps/synthesis_K/...

The reaction is somewhat inefficient, because half of the magnesium is used simply for making H2, which escapes from the system and also a considerable part of the magnesium is used up in the initial phase where water from the KOH reacts with the magnesium. But even with these inefficiencies it remains a VERY NICE find. One can almost have a gram for gram conversion of magnesium to potassium and for the home chemists that of course is great fun!

@Jor: I now cleaned up the erlenmeyer and all the grey gunk. The glass of the erlenmeyer is not etched at all! I first added the Mg, then the KOH. The KOH hence was lying in a bed of Mg-powder and did not touch the glass.

[Edited on 20-12-10 by woelen]




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[*] posted on 20-12-2010 at 12:35


Quote: Originally posted by blogfast25  
develop a work up that can coalesce the crude, formed metal really quickly, less than 1/2 hour or so... I rinsed the latest batch with kerosene and it's all potassium, once the byproduct has been washed away.

[Edited on 19-12-2010 by blogfast25]


Wasn't this very thing mentioned in the patent on how to purify the resulting Potassium? They mention the use of Dioxane to clean the Potassium and cause it to rise from the reaction mixture IIRC. I feel this was employed due to its density and if my theory is correct then any non reactive solvent with a density greater then that of Potassium and boiling point above Potassiums melting point may work.

[Edited on 20-12-2010 by Sedit]





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[*] posted on 20-12-2010 at 13:22


Apparently purification of potassium metal should be described in Brauer. I wonder, if Mg(OtBu)2 really is the crux of this reaction if it would be worthwile to synthesize and purify it first. It might also have other uses in organic chemistry.
It's even commercially available from Aldrich, but it's rather expensive.

[Edited on 20-12-2010 by vulture]




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[*] posted on 20-12-2010 at 13:49


Since as I was a little impatient and ended up with two batches of 1 – 2 mm K balls instead of a few larger ones, I tried Garage Chemist’s isopropanol (IPA) method (see above), to see if I can get some coalescence in kerosene at about 100C.

I have no IPA on my shelves and used salted out windscreen de-icer (aerosol grade): it may still contain some water and was slightly cloudy (salt?) I’ll distil it tomorrow (or get some rubbing alcohol).

I pipetted some 50 K globules into a test tube and replaced the supernatant solvent with a few charges of kerosene so it was clear enough to observe and then heated the tube au bain marie. Nothing happened up to that point and then I dropped a couple of drops of the IPA into it: the balls started ‘dancing’ up and down and up and down while also becoming very silvery. And there was coalescence, albeit slowly. Perhaps there was still some water in the IPA: some dross also formed (KOH or K 2-propanoxyde?)

After about half an hour I stopped the test and noticed something unusual: while cooling the tube and swirling it in cold water much coalescence took place all AT ONCE, with nearly all the balls coming together in one rather oddly shaped ball. This must have happened when the K was in the process of solidifying.

Beneath (another really bad shot – focus on background, not object! How do I do it??) after replacing the cloudy kerosene and remelting/resolidifying:



I wonder if this may be the basis for a shorter procedure: cook for about 1 ½ hour, replace dirty Shellsol with another clean solvent and coalesce with IPA?


[Edited on 20-12-2010 by blogfast25]
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[*] posted on 20-12-2010 at 17:30


I had thought Mg would still be in excess on an MgO basis given I found it was in more than 2-fold excess on an Mg(OH)2 basis further up this thread. Ill have to recheck that. Thanks Wilco.

Yes I am sorry, on an MgO basis there is a 0.3gm KOH deficit in the way I mixed the reagents - or almost stoichiometric amounts, so the yield is about the same with respect to both Mg and KOH

6.4 Mg to 12.6 KOH

Mg + KOH -> MgO + K + 1/2 H2

24 39

Mg + 2H2O

24 36

0.84 1.26


(6.4-0.84)*56/24 = 12.97

[Edited on 21-12-2010 by len1]
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[*] posted on 21-12-2010 at 01:10


I've been going over the patent and example 3 caught my eye:

In the reactor described in Example 1, a mixture of 74.8 g potassium tertiary amylate (0.59 mole), 7.3 magnesium chips (0.3 mole) and 400 ml decalin is heated with stirring at 150° C. for 3 hours. A regulus consisting of 15.1 g (0.38 mole) potassium is recovered from the cold reaction product mixture. Yield: 65.5%.

In this example I notice they use 0.3 moles of mg and end up with 0.38 moles of Potassium. How's that possible?
And also, where does the resulting t-amyl alcohol go from this reaction? Does it end up as magnesium tertiary-amylate?, thus the products of this being mg t-amylate rather than MgO?
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[*] posted on 21-12-2010 at 02:37


What they did in the experiment is add 1 mole of Mg for each 2 moles of potassium tert amylate. Theoretically the reaction should go like this:

Mg + 2 t-PeOK --> 2K + (t-PeO)2Mg (here t-Pe stands for the tert-pentyl group)

Probably the patent writers had the above reaction in mind.

In this situation, I also cannot see any other possible reaction. There is no KOH in the mix, nor water, so there is no option of producing hydrogen, as opposed to the reaction we have done with t-BuOH and KOH.

So, indeed, I see no mechanism of formation of MgO, simply because there is no hydroxide in the system. There also is no water in the system, which indirectly could lead to formation of hydroxide and oxide. Decalin is just the solvent, it is an inert cycloalkane with a boiling point just below 200 C.

So, in this reaction, 1 mole of Mg can form 2 moles of potassium (theoretically). In the original reaction from Pok, one mole of Mg forms one mole of potassium and one mole of hydrogen atoms (hence half a mole of hydrogen molecules H2).

Still, Pok's original reaction is more interesting for the home chemist, because tert-butanol (or tert-amylalcohol) plus KOH is easier to obtain than the potassium salt of these alcohols.

For people who cannot find Shellsol D70, the use of decalin may indeed be an option. It's boiling point may be a tad low, but it comes quite close to the lower part of the boiling range of Shellsol D70.




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[*] posted on 21-12-2010 at 03:20


@woelen If I read your site correctly, you stirred only when adding butanol and still got 85% yield - is that correct?

Another interesting observation I made today while trying out various ideas. You can simmer the mixture all you like,but until the addition of alcohol you only see flurries of Mg flakes thrown up to the surface of the D70 by random H2 bubbles coming out of the reagents on the bottom of the flask. But within 5mins of adding the alcohol tiny balls of potassium start darting on the surface. There seems to be some force attracting them to each other as they never separate by far, and bump into each other many times until they coalesce.

I wonder if tiliting the flask slightly will help coalesce the potassium by graviating it all towards one part of the flask.

[Edited on 21-12-2010 by len1]
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[*] posted on 21-12-2010 at 05:52


Yes, I only swirled the flask somewhat when adding the tert-butanol, I would not even call it stirring, just gently rocking it forward and backward (remember, the flask had the big cooler on it, the cooler was loosely attached to a clamp and it was very hot, so real stirring and swirling is not an option).

I doubt whether tilting really helps. The crud also moves to the lower part of the flask and forms a horizontal bed again. One thing which might help is the use of a spherical flask or even a pear-shaped with the 'sharp' side pointing downwards. This, however, might introduce another problem, because it might be difficult for the tert-butanol to reach the lowest parts of the pear-shaped flask.

Actually, having the K-metal coalesce into a single big ball is not really advantageous. This may cause difficulties taking it out of the flask. I already had some difficulty getting the bigger balls out, because there hardly was room for the balls and the thickness of the spatula in which the balls was laying while being moved out of the flask.

My preferred size would be balls of 4 to 5 mm diameter. A single ball then is ideal for experiments, no further cutting needed.




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[*] posted on 21-12-2010 at 06:42


Quote: Originally posted by len1  

I wonder if tiliting the flask slightly will help coalesce the potassium by graviating it all towards one part of the flask.

[Edited on 21-12-2010 by len1]


That's why I mentioned much earlier to maybe use a round flask. Or even one of these with a conical bottom... I really think it might help just a little to keep the reagents together and promote coalescence.

In the mean time I'm still surprised that coalecsence of liquid K is so hard to achieve (takes so long): mercury it ain't!

[Edited on 21-12-2010 by blogfast25]
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[*] posted on 21-12-2010 at 06:43


I will do the experiment in January. I have been thinking about stirring it slowly with a Teflon stir bar on the magnetic stirrer, but I am afraid that at the temperatures involved, the Teflon might decompose or even dissolve in the very hot oil. These decomposition products are very harmful, so I'd like to avoid it. How significant is decomposition at 220C for 3 hours?

[Edited on 21-12-2010 by Jor]
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[*] posted on 21-12-2010 at 06:45


Quote: Originally posted by woelen  
Actually, having the K-metal coalesce into a single big ball is not really advantageous. This may cause difficulties taking it out of the flask. I already had some difficulty getting the bigger balls out, because there hardly was room for the balls and the thickness of the spatula in which the balls was laying while being moved out of the flask.



Why not use a wooden or clean SS skewer (a lomg needle): prick into the ball and lift it out?
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[*] posted on 21-12-2010 at 06:56


Quote:
I have been thinking about stirring it slowly with a Teflon stir bar


Is Teflon compatible with molten alkali metal? I know that at some temperature there is a violent reaction, as the fluorine in the PTFE combines with the alkali. I think in this case you would be better off with occasional manual swirling.
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[*] posted on 21-12-2010 at 06:58


Quote: Originally posted by Jor  
I will do the experiment in January. I have been thinking about stirring it slowly with a Teflon stir bar on the magnetic stirrer, but I am afraid that at the temperatures involved, the Teflon might decompose or even dissolve in the very hot oil. These decomposition products are very harmful, so I'd like to avoid it. How significant is decomposition at 220C for 3 hours?
Don't make it too fancy and don't risk your equipment. If you want to introduce a new variable, then try indeed with a round-bottom flask or a pear-shaped flask and see whether that works.

I personally think that stirring is not good, just some simmering assures that there is circulation of liquid and the simmering causes globules of metal to move around and eventually coalesce, but the process is slow.

I also think that it is not that interesting to investigate the results with all kinds of mechanical agitation and methods of stirring. Whether it coalesces in 2 hours, 3 hours or 4 hours is not that relevant, as long as it does coalesce.

More interesting is to find out whether other chemicals also can be used, such as amyl alcohols or other solvents like decalin. The fact that up to now only Shellsol D70 seems to give good results makes me feel still somewhat uncomfortable. Shellsol D70 is not a precisely described chemical entity, it just is a brand of solvent. If Shell for whatever reason decides to stop selling this stuff, then there is a problem. It may even be that 5 years from now the material marketed under the name 'Shellsol D70' has very different properties from what it has now. So, a better thing would be if we find a well-defined chemical entity which can be used as solvent and which is readily available for the general public.




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[*] posted on 21-12-2010 at 07:12


Quote: Originally posted by Jor  
I have been thinking about stirring it slowly with a Teflon stir bar


I also recall warnings about Teflon (or any other halocarbon) with K or the Na alloy.




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[*] posted on 21-12-2010 at 07:22


Quote: Originally posted by woelen  
More interesting is to find out whether other chemicals also can be used, such as amyl alcohols or other solvents like decalin. The fact that up to now only Shellsol D70 seems to give good results makes me feel still somewhat uncomfortable. Shellsol D70 is not a precisely described chemical entity, it just is a brand of solvent. If Shell for whatever reason decides to stop selling this stuff, then there is a problem. It may even be that 5 years from now the material marketed under the name 'Shellsol D70' has very different properties from what it has now. So, a better thing would be if we find a well-defined chemical entity which can be used as solvent and which is readily available for the general public.



I have already reported t-amyl alcohol is usable as well as candle wax and paraffin oils.

[Edited on 21-12-2010 by NurdRage]
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[*] posted on 21-12-2010 at 08:36


Yes, you're right. I missed the report of 5 mm sized potassium balls, I only read about the 1 ... 3 mm globules, which to my taste is too much of a hassle to isolate. But if you get most of the metal in 5 mm balls using other mineral oils than Shellsol D70, then indeed that is good news.

I think we should try to summarize all things up about this process and see whether we can make a compact thread with all kinds of recommendations.

Our experiences also should be coalesced into a single ball of useful information ;). I'll think about this and carefully read all of this thread again and see if I can make a start with this one of the next upcoming days.




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[*] posted on 21-12-2010 at 08:52


But why do you really want to have small pieces? They will be harder to store for a longer time as they will more quickly oxidise.
And if it really is impossible to get it out of the flask, you can always try to cut it with a spatula inside the flask.
But I agree it is a very convenient size to use in experiments, and you won't have to cut it wich may be somewhat dangerous when the material gets oxidised.

But it seems the solvent is not really critical, as long as it contains alkanes/cycloalkanes and has a high boiling point. I think the viscosity determines the size of the balls.

As quite some people are about to have potassium, it may be a good idea to note that it somewhat problematic to store, because as seen int he link by Formatik, it forms a crust of KO2 and KOH.H2O, wich can lead to a fire when the material is cut. I would personally never cut it under a flammable solvent, because the damage done when something goes wrong is much larger than when something goes wrong when you cut K in the air, only leading to ignition of the K and not the solvent.
The KOH.H2O is problematic as when you melt or heat the K (also when you cut?) it reacts with the potassium, and ignites. The KO2 is less of a problem I think. So I will store it in my dessicator, maybe inside another container, containing a sand wetted with a solution of hydroquinone in dilute NaOH, to keep away oxygen.
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[*] posted on 21-12-2010 at 09:51


I have mine stored "classically" in parafin oil (boiled before to eliminate the water). It stores very well. I have some pieces that must be 20-30 years old and they only have a very thin dark blue, almost black crust. I never saw the red oxides some would report. It is dangerous to handle however since a 1 gram cleaned piece left in air it will eventually catch fire. Na never does this; it just slowly "dies away".

I personally find sodium is more troblesome to store and Li even worse (for Li the best way is an airtight container, with no oil - ideally flushed with Ar as it happily reacts with N).
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[*] posted on 21-12-2010 at 13:55
Coalescence experiment…


Based again on garage chemist’s coalescence of K with the help of some isopropanol (IPA) I tested this idea again. I gathered all my small balls of K (including the one larger one) of both previous experiments on a suitable SS sieve, thereby getting rid of most of the MgO ‘mud’ (but not the crusty bits) and losing some of the fine K and transferred them into a 100 ml round flask and rinsed them with a few more washes of clean kerosene (lamp oil).

The round flask was then mounted as follows on a water bath:



(I know: ‘hot water + potassium + kerosene + open flame = not a great idea’ but armed with the knowledge of mild danger and a CO2 extinguisher I took my chances).

On reaching 70C inside the flask a few drops of (the now distilled IPA) were added and coalescence started but really quite slowly, I doubt if it coalesced much faster than the long reflux times needed in pok’s, len1’s and woelen’s experiments. So I quit after about 20 mins…

As indicated above, I had noticed considerable agglomeration with the test tube test when it went through the melting point of K and decided to test this again. So I dismantled the set up and allowed the round flask to cool down, swirling gently and monitoring the temperature carefully. And yes! Going past the 68C MP of K, probably about 25 – 50 % percent of the K coalesced into 4 – 5 mm balls all at once. And then again and again: by toggling the flask temperature between 70 and 65 C, each time on cooling the balls grew and more large ones appeared. It fully confirms the earlier observation. Do the balls somehow get ‘sticky’ during solidification? Are ‘seed crystals’ on the surface of the globules promoting ‘sticky collisions’? It appears thus…

This potentially paves the way to a much reduced total time: use a round flask and cook (1 ½ - 2 hours) until you’ve got 1 – 2 mm balls or larger. Allow to cool and toggle between 65 and 70C for coalescence. Alternatively, decant off dirty solvent first, wash with a few aliquots of clean solvent and toggle between 65 and 70C. IPA may be optional but won’t harm either…

Pix tomorrow, my balls are now sleeping...;)

[Edited on 21-12-2010 by blogfast25]
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[*] posted on 21-12-2010 at 13:59


PTFE + hot K = danger:

(CF2)n + 2n K === > n C + 2n KF + much heat!

No need for stirrer bars. Or for stirring...
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