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blogfast25
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Quote: Originally posted by NurdRage ![](images/xpblue/lastpost.gif) | Because the temperature at which drying takes place is well above the boiling point of the alcohols.
The alkoxides don't form until the system is dry so by the time it reaches that temperature most of the alcohol will be in the vapor phase. When
drying starts the fast hydrogen production pushes the alcohol vapor out of the condensor and is gone.
The condensor won't recapture all of it since the vapor pressure is obviously less than 100% (since it's being carried by the hydrogen).
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No, that doesn’t convince me. Add the volatile alcohol at the start. Now temperature is raised and the alcohol/solvent mixture starts boiling. The
vapour phase is of course much richer in alcohol than the liquid phase but in terms of moles there’s far more liquid phase than vapour phase. Now
the chemical drying process starts and independently alcohol in the liquid phase starts reacting with the KOH, drawing more alcohol into the solvent
as alcoholate.
In the case of separate, post drying addition of the alcohol, introduced at higher temperature, some/much of it stays in the vapour phase, also to be
drawn into the solvent by KOH converting it to alcoholate.
I can’t see it make one iota of difference, provided reflux is total (apart from the permanently volatile hydrogen, of course) and the drying
reaction and alcoholate formation do not interfere with each other… The equilibria, reactions and liquid/vapour, are what they are.
Personally I think the separate addition of alcohol is a bit of a ‘chemist’s superstition’. All to play for, I guess…
The other theory I have is that assuming reflux is total and the alcohol doesn’t chemically degrade (or separate out as solid alcoholate), then
separating off the solvent from the K and MgO should make it directly reusable with a fresh batch of KOH/Mg, no extra catalyst needed.
Conservation of mass! That some or all of the alcohol is present as alkoxide shouldn’t make a blinding bit of difference. You could even
see it as the ultimate empirical test of the proposed mechanism… This would start to really play when we're using 'super catalysts' of higher cost.
[Edited on 6-1-2011 by blogfast25]
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NurdRage
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I'm pretty sure i included the statement where i acknowledged i didn't rigorously test where alcohol additions were best. I also believed I stated
that this should be tested experimentally.
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blogfast25
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Quote: Originally posted by NurdRage ![](images/xpblue/lastpost.gif) | I'm pretty sure i included the statement where i acknowledged i didn't rigorously test where alcohol additions were best. I also believed I stated
that this should be tested experimentally. |
I wasn't criticising Nurd ...
What do you think of my second theory?
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NurdRage
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As Homer Simpson would say: "Less yapping more zapping!"
A thorough experiment to test if alcohols can be used in a "one-pot" reaction vs. "post drying" reaction would be a good idea. I personally would like
to be wrong on my vapor loss theory and things would be so much easier if we could do this one-pot.
As for the alkoxide, an issue is that alkoxide might solidify into the MgO crust/sand when the solvent cools to room temperature. Since working with
200 degree solvent is rather dangerous i think the alcohol experiment should be done first.
A more controlled experiment is to add pure alkoxide to a fresh reaction directly.
I think we've had enough theory, lets go back to lab and gets some data now that we know what to test for.
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blogfast25
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Fair enough. And I say: 'It doesn't hurt to theorise!'
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MagicJigPipe
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I forgot to mention that I did swirl occasionally throughout the "reaction". About every 5 minutes or so. Also, there was no reaction upon addition
of water to the flask after rxn. (and I didn't expect there to be because the Mg didn't even seem to react at all). However, the white 'precipitate'
at the bottom was extremely insoluble in water. Mg(OH)2?
Since I don't think anyone has had this result before, could anyone offer any insight? I definitely saw alcohol refluxing throughout the solvent
after I added a bit more later on in the reaction so something tells me it's probably not that...
Ah screw it. I'll just suck it up and buy some Shellsol. But I would really like to find a solvent and Mg source that people could very easily
obtain and would still work (like what I tried). I suppose I could try a few more times and eliminate some unknowns like the age of the KOH (and
whether or not it is mostly carbonate). I guess a titration could be in order.
I don't have internet at the moment so it might take me a few days to respond. Good luck to everyone.
"There must be no barriers to freedom of inquiry ... There is no place for dogma in science. The scientist is free, and must be free to ask any
question, to doubt any assertion, to seek for any evidence, to correct any errors. ... We know that the only way to avoid error is to detect it and
that the only way to detect it is to be free to inquire. And we know that as long as men are free to ask what they must, free to say what they think,
free to think what they will, freedom can never be lost, and science can never regress." -J. Robert Oppenheimer
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garage chemist
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I made some turnings today from a magnesium printing/stamping plate that I got from ebay a long time ago, using a drill press with a 4mm drill.
The turnings that resulted were 1-3mm in particle size, mostly 3mm. They burn fine when lit.
I realize that printing plates aren't pure magnesium. One alloy that I found via google is AZ31. Ullmann lists this as being 3% Al, 1% Zn and 0,3% Mn.
Other Mg based alloys contain 6 to 9% Al.
Just a few grams of turnings were made, enough for at least one experiment, starting tomorrow.
In order to stay true to the principle of changing one variable at a time, I will stay with the t-BuOH for my experiments with different sources and
forms of magnesium.
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NurdRage
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More Negative results
Quick update:
Since aromatics are back on the table (a limited number of them anyway) for solvents. I thought i'd also check out some simple aromatic alcohols:
Phenol - failure
Butylated hydroxytoluene, also known as "BHT" - Failure
So i guess phenols (at least these ones) aren't very promising.
I should also update that while cleaning up i smashed my only ground glass flask. So it looks like i'll be outta the Potassium Race until further
notice.
Take care. ![;)](./images/smilies/wink.gif)
============================================
Since i'm outta the race until i get another flask (this could be awhile), i thought I'd summarize all my findings on this topic:
Notable contributions:
"Floating coalescence" - when high density solvent is used (>0.86g/mL) the resulting molten potassium floats as it forms. The gentle agitation of
the hydrogen bubbling knocks the potassium together and causes coalescence into very large balls of potassium. This negates the need for a second
coalescence procedure.
Aromatic solvents can work - Successful results with 1,2,3,4-tetrahydronaphthalene ("tetralin") disprove the conjecture that only aliphatic solvents
may be used.
Magnesium quality is an issue - Probably obvious but i thought gathering some direct data is better than just going with anecdotal. Magnesium treated
with water and heating to produce a thick layer of passivated magnesium oxide did not work. This same type of magnesium works perfectly if untreated.
So surface quality is a big factor in this reaction.
Solvents I've personally tested:
Candle Wax - Successful - but produces K-sand that solidifies into the wax, the resulting wax is extremely dangerous in my opinion
since it isn't deactivated by alcohol and can spontaneously catch fire during cleaning or handling. Only extremely careful heating with alcohol will
properly destroy it. I do not recommend using candle wax if alternatives are available.
Paraffin Oil - IR Grade - Successful - high density (0.86g/mL) IR grade paraffin produces high quality potassium with an approximate run time of 4hr.
"Floating coalescence" enabled.
1,2,3,4-tetrahydronaphthalene - "Tetralin" - VERY successful - FAST run time of ~1hr when used with t-amyl alcohol. "Floating coalescence" enabled.
Toluene - Failure - I was hoping to try a low-temperature approach.
PEG-3350 - Failure - catches fire! do not repeat.
catalysts i've tried:
(i'm calling these catalysts in case we find a non-alcohol that works)
t-butanol - confirmed successful - Has a tendency to freeze into my condenser since I use very cold water.
t-amyl alcohol - Successful - wide liquid range and slightly higher boiling point (102C) make this a better alcohol from a more practical perspective.
PEG-3350 - Failure
Phenol - Failure
butylated hydroxytoluene - failure
I think those are all my noteworthy findings.
Hopefully i'll get back in the race eventually.
But it seems i won't be making a video for my youtube channel on this until i replace my broken apparatus. ![:(](./images/smilies/sad.gif)
Who else is still in the race and what aspects are you pursuing?
[Edited on 7-1-2011 by NurdRage]
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smuv
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NurdRage I am glad you tested both of those, but you have really left me scratching my head. No way you can do a birch reduction on a phenolate! So
the only conclusion is that the phenolates have poor solubility in the solvent system? Or something about the difference in solubility between the Mg
and K salts is preventing the reaction from going to the production of K.
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NurdRage
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Quote: Originally posted by smuv ![](images/xpblue/lastpost.gif) | NurdRage I am glad you tested both of those, but you have really left me scratching my head. No way you can do a birch reduction on a phenolate! So
the only conclusion is that the phenolates have poor solubility in the solvent system? Or something about the difference in solubility between the Mg
and K salts is preventing the reaction from going to the production of K. |
good questions.
The mixture rapidly becomes too cloudy to see whats happening so i don't really know what's up. Just that in both cases hydrogen evolution stops (as
if drying is all it does and goes no further). I let the mixtures cool and then scoop out some of the solid crud and drop it into water to test for
potassium flashes - there are none.
Whatever the reason, they don't work.
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garage chemist
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I started the batch with the selfmade scrap Mg turnings.
On initial heatup, there was some mild hydrogen evolution, the KOH seemed to melt and solidify again after it had given off its water.
After adding the t-BuOH, there was no further reaction. It's been boiling for one hour now and the solvent is still clear, with no sign of potassium
production. This is a striking difference to the first successful run with pure Mg powder where the solvent soon became very turbid.
t-BuOH is again solidifying in the condenser, a Liebig this time.
The batch is looking exactly like MagicJigPipe's failed attempt on page 23 of this thread.
If this batch fails, then we have one more reason to assume that the purity of the magnesium is important, since my turnings were absolutely clean and
oxide-free.
[Edited on 7-1-2011 by garage chemist]
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blogfast25
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Quote: Originally posted by garage chemist ![](images/xpblue/lastpost.gif) | If this batch fails, then we have one more reason to assume that the purity of the magnesium is important, since my turnings were absolutely clean and
oxide-free.
[Edited on 7-1-2011 by garage chemist] |
Strange indeed!
Perhaps you could try a qualitative test on both grades of magnesium by subjecting equal amounts of them to quite dilute HCl and see which of the two
reacts the most vigorously? Test tube test…
It’s well known that the reactivity of a metal can be altered significantly by small amounts of adjuvant metals to form corrosion resistant alloys.
Perhaps your printing Mg alloy has been formulated to be corrosion resistant? Considering the application field that’s perhaps not such a far
fetched guess…
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blogfast25
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@Nurdrage:
Your contribution here has been invaluable but I do have question marks over the Tetralin result. No, not the result itself but rather what caused the
extremely short reaction time: was it the solvent or did you measure out (‘eyeballed’ was the term you used) the alcohol in such a way that
alcohol concentration was higher than usual? If the proposed reaction mechanism is correct then lower reaction times (but also lower yields) should
result from increased catalyst concentration.
I’m on a knife’s edge to go and buy some Tetralin from Sigma but it would prove rather a waste of money if it turned out the alcohol concentration
was mainly responsible for your fantastic result with Tetralin…
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NurdRage
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Quote: Originally posted by blogfast25 ![](images/xpblue/lastpost.gif) | @Nurdrage:
Your contribution here has been invaluable but I do have question marks over the Tetralin result. No, not the result itself but rather what caused the
extremely short reaction time: was it the solvent or did you measure out (‘eyeballed’ was the term you used) the alcohol in such a way that
alcohol concentration was higher than usual? If the proposed reaction mechanism is correct then lower reaction times (but also lower yields) should
result from increased catalyst concentration.
I’m on a knife’s edge to go and buy some Tetralin from Sigma but it would prove rather a waste of money if it turned out the alcohol concentration
was mainly responsible for your fantastic result with Tetralin…
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(can you explain to me, even in u2u, why you add three periods at the end of your posts? are you planning to edit the post later? is your keyboard
broken? is it an unfinished thought that you wish to follow up on in a later edit?)
Since i can no longer perform the experiments i can't test how alcohol loading effects the rate. but you can, just start your reaction in whatever
solvent you're currently using and add in more alcohol. If indeed the rate is wholly dependent on the alcohol then it shouldn't matter what solvent
you use. No need to go out and buy expensive tetralin to test my particular conditions.
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Jor
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Maybe one of the reactants or intermediates are more soluble in tetralin?
Or it's density... It has a high density than molten potassium, meaning any formed K will go to the surface, meaning no K will coat the Mg preventing
further reaction?
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blogfast25
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Quote: Originally posted by MagicJigPipe ![](images/xpblue/lastpost.gif) | Since I don't think anyone has had this result before, could anyone offer any insight? I definitely saw alcohol refluxing throughout the solvent
after I added a bit more later on in the reaction so something tells me it's probably not that...
Ah screw it. I'll just suck it up and buy some Shellsol. But I would really like to find a solvent and Mg source that people could very easily
obtain and would still work (like what I tried). I suppose I could try a few more times and eliminate some unknowns like the age of the KOH (and
whether or not it is mostly carbonate). I guess a titration could be in order.
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There’s quite a choice of solvents now, thanks to Nurdrage’s work. I’m convinced a clean grade of lamp oil (without adjuvants like
‘citronella’ or vitamins!) should work with a ‘good’ KOH and clean Mg (try a pyro grade, not extremely fine – perhaps a 100 mesh).
On the KOH, most of us have been using flakes, not moulded hemispheres (pellets). Although I’ve not tested both back-to-back, the flakes may have
the advantage of being somewhat porous and hence of greater surface area (m2/g). High K2CO3 content would be detrimental because it gives the catalyst
nothing to react with.
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blogfast25
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Quote: Originally posted by NurdRage ![](images/xpblue/lastpost.gif) | (can you explain to me, even in u2u, why you add three periods at the end of your posts? are you planning to edit the post later? is your keyboard
broken? is it an unfinished thought that you wish to follow up on in a later edit?)
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Nurdrage, you seem a little hung up over my three periods! ;-) It’s a valid punctuation mark, you know, implying ‘more to follow’, a ‘pregnant
pause’, ‘food for thought’ or even a bit of a cliff hanger. Don’t stare yourself blind on them… we need your observational skills! I hope that explains it.
Your other point is completely accepted. I should be able to make another run on Sunday, this one a one-pot with 50 % increased catalyst
(2-methyl-2-butanol) concentration.
I'm also ordering a sample of the tetrahydro myrcenol longer chain alcohol, discussed in the organics thread.
[Edited on 7-1-2011 by blogfast25]
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woelen
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Quote: Originally posted by blogfast25 ![](images/xpblue/lastpost.gif) | On the KOH, most of us have been using flakes, not moulded hemispheres (pellets). Although I’ve not tested both back-to-back, the flakes may have
the advantage of being somewhat porous and hence of greater surface area (m2/g). High K2CO3 content would be detrimental because it gives the catalyst
nothing to react with. | I can say that my successful attempt was done with hemispheres of KOH, which are more
than 20 years old and which are covered with some K2CO3. So, apparently the KOH is not that critical.
-------------------------------------------------
An off-topic request: Please try to combine your posts into a single one. I have some complaints of people about the double posting. This is a long
thread, let's try to make things as compact as possible (of course without sacrificing any useful content, and yes, you post useful things)
[Edited on 7-1-11 by woelen]
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not_important
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At elevated temperature, >150 C I believe, you will see
K2CO3 + ROH <=> KHCO3 + ROK
KHCO3 <=> H2O (g) + KOH
and to a small degree
K2CO3 + ROH <=> KOH + RO(C=O)OK
RO(C=O)OK + ROH <=> KOH + RO(C=O)OR
RO(C=O)OK + ROH <=> RO(C=O)OH + ROK
RO(C=O)OH => ROH + CO2(g)
It would generally be expected to be slower to react at first, as the K2CO3 likely has low solubility in the solvent. It can be accelerated by
starting off with a small amount of water present, and slowing raising the temperature. Works with Na2CO3 as well, again with high boiling alcohols so
that the water can be driven off as the conversion progresses. SIf you can get the hydroxide, it's much quicker to convert to the alkoxide.
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garage chemist
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Results of the test batch with scrap Mg turnings:
After 4 hours of boiling, the surface of the turnings had become uniformly dark grey. Some ominous clumps of material had collected, but upon
examination, they consisted of porous solid KOH. No potassium could be found in the mass, and upon adding it to water there was no reaction.
The turnings were only very superficially attacked and had seemingly become passivated, possibly by Al2O3.
The mixture still smelled strongly of tert-butanol, in contrast to the successful batch, which was practically odorless after the reaction.
So this, in conclusion, was a total failure, and further hints towards a necessity to use pure magnesium. Perhaps even firestarter magnesium is too
impure.
The next step will be to make turnings from an ingot of pure magnesium, using the same drill as was used for the Mg scrap, to produce turnings of pure
magnesium with the same particle size as the unreactive Mg scrap.
This will show whether the impure Mg was really the culprit, or if the turnings were simply too coarse.
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blogfast25
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Woelen:
Agreed on the KOH: it’s probably not very critical. Interesting suggestions also by not_important.
Re. the ‘double posting’, there is none from my side. You do get people asking the same questions over and over again, typical of long
threads. Cramming multiple responses into a single post provides only a negligible saving on the actual thread length (but sacrifices readability
somewhat). This thread is long because the subject fascinates many. Having said that, I’ll try and ‘compact’ as much as possible. ![:)](./images/smilies/smile.gif)
Of your complainants at least one showed a mean-spiritedness that doesn't belong in a science forum.
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blogfast25
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Quote: Originally posted by garage chemist ![](images/xpblue/lastpost.gif) | The next step will be to make turnings from an ingot of pure magnesium, using the same drill as was used for the Mg scrap, to produce turnings of pure
magnesium with the same particle size as the unreactive Mg scrap.
This will show whether the impure Mg was really the culprit, or if the turnings were simply too coarse. |
garage chemist:
Going back to your posts, so far you had one successful test with fine reagent Mg (here) and one unsuccessful one using turnings from a Mg rich alloy (here).
While I definitely have to commend your scientific diligence for wanting to now test pure Mg turnings to complete the picture, I wonder if it’s
really necessary (considering the effort involved): most people can get hold of magnesium powder or filings of reasonable purity and there’s not
much to be gained from size reducing pure Mg ingots at home, at least not for most of us.
Personally I think an experiment with home brewed 2-methyl-2-hexanol (2-methyl hexan-2-ol) as catalyst (as you suggested) and using the reagent grade
Mg used in your first attempt would contribute more to our expanding knowledge base. Don't use too much head space and cool moderately in any event.
Longer chain t-alcohols have lower MPs, t-butanol is a bit 'the odd one out'...
[Edited on 7-1-2011 by blogfast25]
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woelen
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I also can add that using impure Mg does not give any results. Some time ago I already posted results with using magnalium which is a mix of at least
50% Mg and at most 50% Al and at most 70% Mg and at least 30% Al. With this material I did have a violent reaction at the start of the experiment, but
finally, I had no potassium at all.
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Nicodem
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Quote: Originally posted by NurdRage ![](images/xpblue/lastpost.gif) | Quote: Originally posted by smuv ![](images/xpblue/lastpost.gif) | NurdRage I am glad you tested both of those, but you have really left me scratching my head. No way you can do a birch reduction on a phenolate! So
the only conclusion is that the phenolates have poor solubility in the solvent system? Or something about the difference in solubility between the Mg
and K salts is preventing the reaction from going to the production of K. |
good questions.
The mixture rapidly becomes too cloudy to see whats happening so i don't really know what's up. Just that in both cases hydrogen evolution stops (as
if drying is all it does and goes no further). I let the mixtures cool and then scoop out some of the solid crud and drop it into water to test for
potassium flashes - there are none.
Whatever the reason, they don't work. |
Interesting experiment. Metal phenolates are stable at such high temperatures (no beta-hydride elimination or other decomposition reactions can
occur), though I'm not sure they are inert toward potassium. Phenol is about a million times more acidic than t-BuOH (though this value is for water
as solvent it can nevertheless be indicative). It is also more acidic than water to a similar degree. This means that magnesium phenolate does not
hydrolyse irreversibly with water (in contrast to magnesium t-butoxide). The magnesium phenolate is also less likely to irreversibly enough form
MgO/Mg(OH)2 and potassium phenolate in its reaction with KOH, because both metal phenolates should be thermodynamically stable to comparable levels. I
might be wrong though.
Though this fits to the proposal of Mg(II) removal from the solution being the driving force for the equilibrium shift in the redox reaction, it still
does not prove it. I wish someone would be interested enough in the mechanism of the reaction to do some scientific work on it.
…there is a human touch of the cultist “believer” in every theorist that he must struggle against as being
unworthy of the scientist. Some of the greatest men of science have publicly repudiated a theory which earlier they hotly defended. In this lies their
scientific temper, not in the scientific defense of the theory. - Weston La Barre (Ghost Dance, 1972)
Read the The ScienceMadness Guidelines!
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NurdRage
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Quote: Originally posted by Nicodem ![](images/xpblue/lastpost.gif) | I wish someone would be interested enough in the mechanism of the reaction to do some scientific work on it. |
I wish it were that easy. But i'm sure you know that real scientific work requires time, equipment and money. Most of us have one, some of us have
two, but none of us have all three.
If there are any particular experiments you think would be most useful for the mechanistic research on this topic then please post them and maybe one
of us will perform it if we happen to have everything needed.
But unless i get a sizeable research grant from the Amateur Chemical Society (pun very much intended) I for one don't have the resources to put into
such research.
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@Garage chemist
This is a long shot but have ya tried a brief wash of your magnesium with hydrochloric acid just before popping it into the solvent? It might help to
activate the surface. I personally haven't tried such an approach yet.
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@blogfast
double posting means posting more than one response before anyone else has responded. On this page alone you've made two pairs of posts. If you feel
the need to post more then simply edit your last post and use dividers.
I admit i made a some double posts before but i've stopped now.
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@MagicJigPipe
Maybe you can "clean" your oil by heating it with just magnesium turnings to destroy whatever "goodies" the companies put in there. Then let it settle
overnight and decant off the liquid. In the lab i normally destroy stabilizers and other goodies in solvents with sodium.
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