DocX
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Stir bar get stuck in RBF
Really annoying to actually ask have this noobish question, but I just want to get it right from the beginning.
I have a heating mantle w stirrer sporting a 1000 ml RBF. I'm doing a synth right now where success is really about stirring. I haven't finished my
DIY overhead yet, and the magnetic ones also keep failing me since they get stuck in the RBF all the time!
I don't know why, I'm using small ones. Does anyone have a hunch of the right size of a stirr fish in a 1000 ml RBF?
Preferably with link?
Doc
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CharlieA
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Quote: Originally posted by DocX  | Really annoying to actually ask have this noobish question, but I just want to get it right from the beginning.
I have a heating mantle w stirrer sporting a 1000 ml RBF. I'm doing a synth right now where success is really about stirring. I haven't finished my
DIY overhead yet, and the magnetic ones also keep failing me since they get stuck in the RBF all the time!
I don't know why, I'm using small ones. Does anyone have a hunch of the right size of a stir fish in a 1000 ml RBF?
Preferably with link?
Doc |
Just some idle thoughts:
...how did you get the stir bar into the flask? I'm guessing it is stuck in a viscous reaction mix???
...lab supply houses sell "stir bar retriever's, which are just a magnet sealed into the end of a section of plastic tubing or rod, which you use to
"fish out" the stir bar. I am not sure how effective they are with a viscous mixture. If the mix is viscous, can you thin it out by adding more
solvent?
I suppose you could always break the flask!
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BromicAcid
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I have had stir bars that are just too large for a 24/40 joint that can be pushed through but at that point there's no coming back since you couldn't
grab it to pull it back through, is that what you're talking about? Sometimes also the bars are too long and you kinda have to do a shake dance with
the flask to try to get it to drop straight down the neck. Sometimes the smallest bar that can fit and be easily removed is, well, small. So you
need to get a flask that has a larger center neck if only to fit a larger stir bar in the flask. Not sure which of these issues you're facing. What
is the size of the neck that's causing you issues?
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Jenks
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I bought a few football-shaped stir bars that would have provided a powerful stir - only to have them be too large to fit through a 24/40 joint. I
even started buying flasks with 24/29 joints to get around this. The alternative is to file the wide part of the Teflon, which seems like an ugly
solution, so I haven't tried.
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Fery
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Use a magnet. Cheap neodymium magnet disk from outside of the flask attaches to one pole of the stir bar inside the flask so you can move the stir bar
and drive it through the neck of the flask. Or use advanced tool for that - PTFE coated remover which you put inside of the flask
https://www.laboratorni-potreby.cz/magmichadelka-vcelky/vyta...
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DocX
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Oh no, I see now how badly I phrased myself. Sorry folks, not really stuck. It won't spin because it rubs against the curved
glass, even though I'm using a stir fish. It's naturally PTFE-coated, has a good curve and isn't too long, something like 1-1,2 cm. Still it will only
spin at the lowest speed, and not all the time.
So my real question is: what kind of stir fish do you guys use for magnetic stirring in a 1000 ml RBF? How big?
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charley1957
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Kinda sounds as if you may have an intermittent stir motor. Are you sure it’s running all the time it’s supposed to be running?
You can’t claim you drank all day if you didn’t start early in the morning.
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DocX
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| Quote: Originally posted by charley1957 | | Kinda sounds as if you may have an intermittent stir motor. Are you sure it’s running all the time it’s supposed to be running?
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Yeah, it works fine when using a straight stir bar in a beaker or a flat-bottom.
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Texium
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Perhaps just a weak magnet then. Try a football shaped stir bar if you haven’t yet. It should spin with less friction in the RBF.
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DocX
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| Quote: Originally posted by Texium | | Perhaps just a weak magnet then. Try a football shaped stir bar if you haven’t yet. It should spin with less friction in the RBF.
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Yeah, that's what I'm using. I call it a "stir fish". But the problem persists, both with my stir plate and my heating mantle.
That why I was wondering about the best size of a football shaped stir bar in a 1000 ml RBF.
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Cathoderay
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Is that "fish" one that you bought or is it a used one?
Some magnetic materials handle heat better than others, heat can temporarily or permanently reduce the strength of the magnetic field.
Neodymium magnets are common these days and cheap but they generally handle heat poorly. Some formulas lose strength at 80 C.
Ceramic (Ferrite) magnets are fine with heat and inexpensive.
Alnico magnets are the best with temperature but more expensive.
Samarium Cobalt are good with temperature but the most expensive.
Note that the relative cost I'm stating is for the raw material not the finished
stir bar.
Unfortunately I don't know of an easy way of determining what the material is under the plastic coating. Try another brand.
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DocX
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I bought it. Amazon stuff. I wasn't aware of the different qualities available, but of course there are. There always is. Thank you for the new
knowledge!
Now, the reaction I was using it for weren't done above 50 C, so the temperature issue wasn't the cause.
I'll just try a few other ones and try and find the optimal size. It's just a pain in the old donkey to order a lot of different sizes just to try
them out. So I hoped someone would offer me their experience.
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Cathoderay
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I'm supposing that there is.
You never know what a used stir bar has been exposed to.
Of course the Teflon coating should not be used above 260 C anyway, you would not want to use one when distilling sulfuric acid for instance.
The only other thing I can think of is that you would want the curvature of the stir bar to be such that the ends do not rub on the bottom of a RBF.
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DraconicAcid
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I know that some brands of stirring plates simply go to fast for the magnet to keep up with (especially when the stir plates are old, for some
reason), so it just sits there and wiggles.
Please remember: "Filtrate" is not a verb.
Write up your lab reports the way your instructor wants them, not the way your ex-instructor wants them.
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DocX
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| Quote: Originally posted by Cathoderay |
The only other thing I can think of is that you would want the curvature of the stir bar to be such that the ends do not rub on the bottom of a RBF.
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Yeah, and that's where I'm thinking this goes wrong. I don't use a large stir bar, but when I try the smaller one I do have, it's spinning just fine.
But that's too small to create enough of a vortex. And since the curvature of 1000 ml RBF:s are more or less the same for everyone, I wondered what
size of a curved stir bar you guys use.
Sorry it took me so long to formulate the real question 
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Osmiridium
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| Quote: Originally posted by DraconicAcid | | I know that some brands of stirring plates simply go to fast for the magnet to keep up with (especially when the stir plates are old, for some
reason), so it just sits there and wiggles. |
Sounds kinda strange to me. Isn't the speed usually adjustable? At least I have never encountered magnetic stirrers without regulation.
| Quote: Originally posted by DocX | Oh no, I see now how badly I phrased myself. Sorry folks, not really stuck. It won't spin because it rubs against the curved
glass, even though I'm using a stir fish. It's naturally PTFE-coated, has a good curve and isn't too long, something like 1-1,2 cm. Still it will only
spin at the lowest speed, and not all the time.
So my real question is: what kind of stir fish do you guys use for magnetic stirring in a 1000 ml RBF? How big?
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Well, this is way too small for a 1l flask! Like that it will never work properly, especially if the contents gets viscous.
I actually use bigger ones even in smaller flasks if vigorous stirring is necessary. Here I'd use one with 4cm may be even bigger.
PTFE has little friction so a standard one should be fine. If you have some, the football shaped are better suited for round bottomed flasks. If
stirring is still to weak the plate might be to far away or the magnets are weak.
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DocX
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| Quote: Originally posted by Osmiridium |
Well, this is way too small for a 1l flask! Like that it will never work properly, especially if the contents gets viscous.
I actually use bigger ones even in smaller flasks if vigorous stirring is necessary. Here I'd use one with 4cm may be even bigger.
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Thank you! But my problem is that the larger ones won't spin, and the ones that do are too small to create enough stirring.
I guess I'll just have to get started on that overhead stirrer project that has been dormant for a while. Cordless drill motor with the fitting still
attached, Arduino control. Speed regulation (naturally), timer, intervals and every function I see necessary to jam in there over time. Now all I need
is free time.
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Jenks
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I suspect that the magnet in my hot plate/stirrer degrades over time due to being close to the heated surface. So if you can find a new stirrer to
test on, maybe the stir bars are fine. If you do buy new, large football shaped stir bars, watch out that they are not too wide for the flask joint. I
bought a name brand (VWR), 41 mm stir bar to try to make sure the magnet would be good, but even though the width was supposed to be only 19mm, it
still wouldn't fit through a 24/40 joint. I should have known better - the minimum width inside the 24/40 is 18 mm - I just thought they wouldn't make
a stir bar that wouldn't fit through a standard joint.
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Osmiridium
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Ok, it's a problem with the magnets then. Have you checked that the stir bar is magnetic itself? Who knows if some cheap Chinese ones could possibly
be only a Teflon coated iron rod.
It's a cool idea with the overhead stirrer, though magnetic stirring far simpler to use, installed way faster and should actually pose no problem with
that flask size, granted you don't attempt to stir some gunky sticky mass. Overhead stirrers are only necessary with really big flasks or certain
setups.
If you don't need the heating plate cos you are using a heating mantle, you could easily build a quick and dirty magnetic stirrer yourself. Take a
computer fan and glue some neodymium magnets to it. Speed regulation is possible with some of these. This way you could use as strong magnets that you
could possibly use iron bars as stirrers. The temperature under such a heating mantle shouldn't rise much above 50°C (without ventilation).
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DraconicAcid
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| Quote: Originally posted by Osmiridium | | Quote: Originally posted by DraconicAcid | | I know that some brands of stirring plates simply go to fast for the magnet to keep up with (especially when the stir plates are old, for some
reason), so it just sits there and wiggles. |
Sounds kinda strange to me. Isn't the speed usually adjustable? At least I have never encountered magnetic stirrers without regulation.
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Yes, and even on the lowest setting, the stir bars can't keep up.
Please remember: "Filtrate" is not a verb.
Write up your lab reports the way your instructor wants them, not the way your ex-instructor wants them.
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DocX
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Yup, no problem with them. I don't know if my heating plate is poorly constructed, but I do sometimes have problems even when stirring in flat beakers
or crysdishes. The bar tends to jam, jump out of circulation and jump around uncontrollably.
Naturally, my heating plate and mantle were cheap. Vevor and Faithful (a truly Chinese brand name). The Lidl versions of lab equipment.
Also, the goddamn mantle has broken and won't heat up anymore. But that's another story.
| Quote: Originally posted by Osmiridium |
It's a cool idea with the overhead stirrer, though magnetic stirring far simpler to use, installed way faster and should actually pose no problem with
that flask size, granted you don't attempt to stir some gunky sticky mass. Overhead stirrers are only necessary with really big flasks or certain
setups. |
Yes, or whenever the magnetic stirring fails  .
| Quote: Originally posted by Osmiridium |
If you don't need the heating plate cos you are using a heating mantle, you could easily build a quick and dirty magnetic stirrer yourself.
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I DO love DIY alternatives, but any practically useful stirrer needs to be incorporated into a heating source. Fully possible to build one that holds
up to temperatures though.
But my next project will probably be a DIY rotovap. It's fully possible if one can just CNC some PTFE!
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