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blogfast25
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Quote: Originally posted by elementcollector1  | | Might be able to do that... Maybe. The 2-hr reaction will be the tricky part, because the time window is only an hour and 20 minutes long. Could set
up at lunch... |
Much higher up you'll find an entry of mine where I measured hydrogen evolution over time. I found the reaction was basically over after 1 hour, in my
conditions. Most of the experiment time (again, in my conditions) is needed to actually grow the K-balls. And that is a lengthy process and difficult
to speed up...
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elementcollector1
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Hopefully, the magnesium powder will help.
Is there a way to powder the KOH too without it getting too deliquescent?
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blogfast25
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Yes. DON'T bother! Normally dried glass is fine. Your KOH contains at least 10 % water. It's not a problem. My glassware, KOH, Mg, solvent and
T-butanol came straight off the shelf.
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elementcollector1
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Got it.
One thing I've always worried about is the reflux of the t-BuOH. According to Wiki, it boils at just 82 C, and the temperature of the reaction is
around 200 C. Even if it is refluxing, shouldn't the alcohol just boil off instantly as soon as it hits the solvent again?
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blogfast25
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Powdering isn't necessary: flakes will do just fine. It's what everyone else uses. This reaction requires certain phase transfers but it doesn't seem
to be affected by system granulometry much.
Unfortunately we still don't seem to understand what is wrong when it doesn't work.
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elementcollector1
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I thought it was solvent problems. Pretty sure every grade of magnesium imaginable was tested, so it couldn't have been that. Different solvents such
as kerosene and vaseline were tested too, without success if I remember. I'm using pharmacy mineral oil, which claims "100% mineral oil", but I'm
going to boil it just in case.
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blogfast25
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| Quote: Originally posted by elementcollector1 | Got it.
One thing I've always worried about is the reflux of the t-BuOH. According to Wiki, it boils at just 82 C, and the temperature of the reaction is
around 200 C. Even if it is refluxing, shouldn't the alcohol just boil off instantly as soon as it hits the solvent again? |
Nope. That's the definition of TOTAL REFLUX: everything condenses back into the reaction flask, inluding the t-butanol. It's also not necessary to
cool your condenser like crazy: as long as it's has an area that's below the BP of t-butanol it WILL reflux it. Woelen for instance used a vertical
Liebig condenser but with air pumped through the mantle instead of water. Worked perfectly!
[Edited on 25-2-2013 by blogfast25]
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blogfast25
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No, we don't know that. Many solvents were found to work w/o problems: that 'special kerosene' (I forget the name now), normal kerosene, lamp oil,
paraffin, candle wax, Tetralin, paraffinic baby oil, all tested positive. The patent suggest decane (for cesium).
Don't bother boiling your mineral oil, it's a waste of time. Instead spend some time prepping well, and fingers crossed! Don't forget a horse shoe!
[Edited on 25-2-2013 by blogfast25]
[Edited on 25-2-2013 by blogfast25]
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elementcollector1
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Was it Shellsol D70?
'All tested positive' as in confirmed production of K, correct?
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blogfast25
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Yep, Shellsol D70, aka 'a waste of money'. All made K. The few instances (bar sheer incompetence) were it didn't work are poorly understood.
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elementcollector1
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| Quote: Originally posted by blogfast25 | | Much higher up you'll find an entry of mine where I measured hydrogen evolution over time. I found the reaction was basically over after 1 hour, in my
conditions. Most of the experiment time (again, in my conditions) is needed to actually grow the K-balls. And that is a lengthy process and difficult
to speed up... |
I don't understand. Does this mean the reaction is officially over at ~1 hr, then the rest of the time is just coalescing K?
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blogfast25
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In a nutshell: yes. I think you'll agree with me when you've witnessed your first succesful reaction.
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Mailinmypocket
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I'm happy to see somebody else trying this out! I really hope it works for you and then we can maybe get another hint as to why it sometimes doesn't
work... After so many tries my own results never resulted in much more than K dots bobbing around in a fleeting existence 
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elementcollector1
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If I had to guess, you might have added too much t-BuOH, which dissolved right through your potassium. Or not enough magnesium was used. I don't
really know without knowing exactly what you did first.
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Mailinmypocket
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| Quote: Originally posted by elementcollector1 | | If I had to guess, you might have added too much t-BuOH, which dissolved right through your potassium. Or not enough magnesium was used. I don't
really know without knowing exactly what you did first. |
I thought the excess t-BuOH was the culprit also, but given the 10+ runs I did I tried varying the amounts of it +\- and although the balls did grow
bigger on some attempts than others, they never coalesced and always vanished after about 1h30m. I have also tried various types of Mg and KOH, and
solvents... Ill have to go back through my notes and see which attempt was the most fruitful and maybe work up from that. Not until new flasks arrive
though since my last 19/20 erlenmeyer bit the dust..
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elementcollector1
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I'm just using rubber joints instead of ground glass. Are you sure you need ground-glass for this reaction?
Also, what size flask do you typically use? I'm thinking 125mL.
[Edited on 26-2-2013 by elementcollector1]
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Mailinmypocket
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| Quote: Originally posted by elementcollector1 | I'm just using rubber joints instead of ground glass. Are you sure you need ground-glass for this reaction?
Also, what size flask do you typically use? I'm thinking 125mL.
[Edited on 26-2-2013 by elementcollector1] |
Oh no, ground glass isn't necessary, I just prefer it. I made a setup with rubber stoppers and whatnot (see up the thread, not like it worked any
better  ). I was running it in a smaller 50ml flask, but the rubber stopper run
was done in a 125.
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elementcollector1
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Does it make a difference if a 50mL or a 125mL flask is used? For the 50 mL, the reactants would always be completely submerged, but couldn't grow as
large without rendering removal impossible. For the 125mL, the reactants might not always be completely submerged in the oil, but the potassium could
be allowed to grow significantly larger.
Hopefully going to give this experiment a try in a few days, if not today.
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Mailinmypocket
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| Quote: Originally posted by elementcollector1 | Does it make a difference if a 50mL or a 125mL flask is used? For the 50 mL, the reactants would always be completely submerged, but couldn't grow as
large without rendering removal impossible. For the 125mL, the reactants might not always be completely submerged in the oil, but the potassium could
be allowed to grow significantly larger.
Hopefully going to give this experiment a try in a few days, if not today. |
That's one of the reasons I used a small flask (the other being reagent wastage). If there is not enough room for the K to move, coalescence will not
occur as readily. The times I did it in the 125ml I found that there was very little free space in the solvent, especially after the KOH dehydrates
and increases in size.
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elementcollector1
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Got it. The original versuchschemie.net entry used 50mL of liquid in a 125mL flask, which seemed to work well.
Unfortunately, the setup with a huge condenser on a tiny flask can be a bit wobbly... I'll probably need to clamp the condenser to a stand just to
keep the whole thing from falling over.
What do you think about using a sand bath? My electric hotplate isn't all that hot, but a sandbath could be beneficial.
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blogfast25
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| Quote: Originally posted by elementcollector1 | Got it. The original versuchschemie.net entry used 50mL of liquid in a 125mL flask, which seemed to work well.
Unfortunately, the setup with a huge condenser on a tiny flask can be a bit wobbly... I'll probably need to clamp the condenser to a stand just to
keep the whole thing from falling over.
What do you think about using a sand bath? My electric hotplate isn't all that hot, but a sandbath could be beneficial. |
Sand bath is safer and tends to provide more even heat. In the unlikely event of flask breakage the sand provides the first barrier/absorber for the
oil and any potassium.
A minimum of clamping is needed secure the assembly. One retort stand, one bosshead and one retort clamp will do it though. To occasionally swirl the
flask, loosen the retort clamp a bit, swirl, then retighten till secure again.
[Edited on 26-2-2013 by blogfast25]
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elementcollector1
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Well, I gave the experiment 2 tries today. One was a bit messy, and I ended up switching to ground-glass for better stirring and joint integrity.
Here's my general setup for the second run:
Unfortunately, something strange happened: The mineral oil started to froth over, right up into the condenser! At this point I shut everything down.
Is this normal?
One more note: I'm a little confused as to how to add the alcohol. If I inject it through the top of the condenser, it freezes on the sides and
refuses to move. If I inject it into the flask, it instantly boils away. One solution I could see to this is injecting hotter alcohol in so as to give
it a better chance of reaching the flask, but what do you people think?
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Mailinmypocket
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I have found that while the heat is ramping up there is always a sudden frothing that occurs. This is accompanied by dense white fumes. This always
happens between ~170-190c (in my experiments anyways) but has never went into the condenser, that may just be a difference in set ups though? What
kind of mineral oil are you using? I believe this is happening due to the KOH losing its water content and releasing steam, although I don't know for
certain. It does stop frothing after less than a minutes time though as the heat increases!
As for adding the t-butanol, try mixing it up well with a couple milliliters of your mineral oil prior to adding it. Then add the oil mix in
increments or however you were adding the alcohol... It's much easier than dealing with a fraction of a ml at a time.
Next time try and allow the mix to keep heating up even if it froths, provided we aren't talking about ridiculous amounts of it obviously. Once it
calms down wait until the target temperature of about 220c and add the alcohol.
[Edited on 27-2-2013 by Mailinmypocket]
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elementcollector1
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Well, can I re-use the reaction mix I currently have? It's cooled down to RT and is currently lying in my garage.
So, start adding the catalyst *after* the frothing ends. Got it...
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blogfast25
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| Quote: Originally posted by elementcollector1 |
One more note: I'm a little confused as to how to add the alcohol. If I inject it through the top of the condenser, it freezes on the sides and
refuses to move. If I inject it into the flask, it instantly boils away. One solution I could see to this is injecting hotter alcohol in so as to give
it a better chance of reaching the flask, but what do you people think? |
The freezing of the t-butanol shows you're cooling too hard. Try just gentle air cooling instead of brute force cold water.
Also, try my 'one pot method': add the catalyst (ALL of it) at the start, mixed in with the rest of the ingredients. Then assemble and start heating.
Do you have any temperature probe anywhere? Excessive foaming could point to too much heat. Start on a lower setting and gradually crank up the heat
until you've got a gentle rolling boil. The catalyst will NOT instantly boil away, as you claim.
The frothing is almost certaily due to hydrogen evolution, in your case just a bit too fervently. But broadly it's a good sign.
I would keep the failing batch for a later date but start with fresh materials. You can always use the failed batch later on.
[Edited on 27-2-2013 by blogfast25]
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