cnidocyte
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Anti bumping measures
Firstly I've had the idea that stirring prevents bumping for a while now but I can't remember where I got this idea and don't know how valid it is.
Does stirring prevent bumping? Secondly I plan on getting some boiling chips but in the meantime what measures can I take to prevent bumping. I hear
most ceramics are fairly porous and provide sufficient nucleation sites. Would ceramic chips from a broken plate/cup provide sufficient nucleation
sites to prevent bumping?
[Edited on 31-1-2011 by cnidocyte]
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Lambda-Eyde
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To your last question the answer is yes. Many times I have found that a stirring bar is sufficient.
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Panache
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a bleed though a capillary tube of some inert gas is very good too, esp under vacuum, you can heat this also using a length of heating tape around the
feed tube.
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Contrabasso
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The little glass "cubes" from a broken car window are free and if you walk a mile in an urban area you should find enough to fill a matchbox -which
will satisfy your lifelong need. Alternatively get a small piece of (any) glass say an inch square and break it up using a hammer or rough pliers
(caution sharp edges!).
Stirring does work BUT it can be hard to stir evenly inside a reaction vessel
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entropy51
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Magnetic Stirring Promotes Smooth Boiling
| Quote: | Use of a seamless Teflon-covered magnetic stirring bar in conjunction with a magnetic stirrer
effectively prevents violent bumping and has a tendency to decrease the foaming prevalent in so
many reactions. A particular advantage of the Teflon bar is that vacuum distillations can be
conducted without having to introduce extraneous gases. Distillations can be conducted at much
lower pressures with consequent decrease in decomposition of thermally sensitive compounds.
Another (though minor) advantage is that ordinary still heads and one-neck flasks can be used.
Stirring bars are effective when either oil baths or glass fabric heating mantles are used. They
stand rough usage and are inert to almost all reagents and extremes of temperature.
Magnetic stirring is also recommended in oontinuous liquid-liquid extractions-both in the
extractor, to insure better contact between the two liquids, and in the pot, to prevent bumping
and discourage foaming. The bumping or foaming that may occur during a Soxhlet extraction,
where the effectiveness of ordinary ebullators decreases as the extractina solution becomes concentrated,
can he prevented by magnetic stirring.
Another advantage of the magnetic stirrer is that its effectiveness is not marred by a temporary
increase in internal pregsure which forces liquid into capillaries and sometimes clogs them.
In consequence, the temperature can fall without the danger that subsequent reheating will
cause bumping. |
JORDAN J BLOOMFIELD
University of Arizona, Tucson
J Chem Ed, Volume 39, Number 7, July 1962 / 355
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Saerynide
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My current and previous labs don't even *have* boiling chips. I realized that when you move away from college organic chem into actual research, no
one uses boiling chips anymore. The first time, I asked for boiling chips and a post doc looked at me like I was stupid.
I think no one uses em because they contaminate and they also dont work under vacuum. We just use stir bars for all our anti-bumping needs.
If you are in a real pinch, you can always use broken ceramic or a piece of brick.
[Edited on 2/2/2011 by Saerynide]
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spirocycle
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if you use PTFE chips, it wont contaminate
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Wizzard
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@Copntrabasso- Greta idea! But only tempered glass will form those lovely little pieces- Regular glass will break into very irregular little pieces
with sharp conchoidal fractures.
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cnidocyte
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Quote: Originally posted by Contrabasso  | | The little glass "cubes" from a broken car window are free and if you walk a mile in an urban area you should find enough to fill a matchbox -which
will satisfy your lifelong need. Alternatively get a small piece of (any) glass say an inch square and break it up using a hammer or rough pliers
(caution sharp edges!). |
Interesting. Tempered glass is pretty easy to find, I didn't know it was a good source of nucleation sites. I'd definitely opt for tempered glass over
ordinary glass though cuz sooner or later I'd be bound to cut myself with normal glass shards. Lately I've just been stirring and every time I check
on it I see a whirlpool of little bubbles so it seems to be doing a good job of keeping the solution from superheating.
[Edited on 2-2-2011 by cnidocyte]
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ScienceSquirrel
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I have seen boiling chips used in big solvent stills in heating mantles.
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1281371269
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Found these on eBay(.co.uk) which might be of interest:
http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=350...
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peach
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You may need to be careful using safety or toughened glass, as things like car windscreens are now laminated with layers of things that aren't glass
to prevent the thing caving in on the passengers as shards if it gets broken.
I believe the cube glass you find on the street from telephone boxes and such is going to be laminated to produce that effect.
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Contrabasso
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I've been using cubes of broken toughened glass as found on the street literally! If they are loose cubes they work fine. Usually they come from
broken side windows. Broken windscreens are usually laminated so the broken glass stays together and the centre film of plastic obviously spoils the
performance as anti bumping beads. But remember that the right stuff is free, pick it up, wash it use it, repeatedly!
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Arthur Dent
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| Quote: Originally posted by peach | | I believe the cube glass you find on the street from telephone boxes and such is going to be laminated to produce that effect. |
as the police officer is arresting the vandal thug who broke the telephone booth windows: "But officer, I only did this because I need boiling stones
for my 2000 ml 3-neck boiling flask!"  
I found an excellent source of boiling stones... there are two great alternatives to the expensive lab stuff available at dollar stores: In the crafts
section, you can find small transparent glass beads with a through hole for necklaces, bracelets and stuff, and the other thing? A one-pound bag of
good ol' colorful glass marbles! I used the latter for a long time before discovering the small glass beads.
Robert
Robert
--- Art is making something out of nothing and selling it. - Frank Zappa ---
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slinky
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Magnetic stirring works great for distillations, especially vacuum distillations. The only time magnetic stirring isn't good is when you're distilling
a liquid that has solids inside. For example when distilling impure solvents the small metallic salts at the bottom of the flask combined with the
moving stir bar will scratch your flask.
Broken ceramic works, small chips with a lot of rough surface area exposed perform well. I've used ceramic chips many times.
Barbecue grill stones have a tremendous amount of surface area and work great. Example: BBQ Grill Cleaning Stone
Lately though I've been using glass fabric. 3m Bondo Fiberglass Cloth is a tight weave of glass fibres made into a cloth. It works really well in lieu of boiling stones. Just place a a small
cutting in the flask. I replaced my glass bead column packing with it. It's inert and has much more surface area than glass beads. It works great to
insulate the top of flasks and will last much longer than aluminium foil. It's cheap too 
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peach
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Glass blowing suppliers, for making coloured and decorative glass, sell glass frit; grit. The glass blower will add this to produce swirls and
patterns in the glass. There are quite a few online places who sell the more decorative things.
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cnidocyte
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I was distilling some methanol out of a mixture containing nitromethane the other day and when I checked on it, it was bumping because the stir rod
got stuck at the bottom of the flask. I'll never make the mistake of aggitating a superheated solution again but I bet it could have flash boiled by
itself if the stirrer suddenly got unstuck and started turning again. This kinda thing happening when theres nitromethane in the flask is an explosion
waiting to happen. This has happened me a few times while stirring solutions, I don't know why a small stir rod gets stuck in a large RB flask like
that. Only thing I can think of is maybe it wasn't placed directly over the centre of the hotplate therefore the rotating magnetic field wasn't
exerted on it uniformly.
[Edited on 22-2-2011 by cnidocyte]
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unionised
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For a lot of the things I distil, I find a matchstick wrapped with copper wire (so it sinks) works just fine.
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slinky
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| Quote: Originally posted by cnidocyte | I don't know why a small stir rod gets stuck in a large RB flask like that. Only thing I can think of is maybe it wasn't placed directly over the
centre of the hotplate therefore the rotating magnetic field wasn't exerted on it uniformly.
[Edited on 22-2-2011 by cnidocyte] |
Proper placement over the stirrer important. I've found that when I turn the speed on too high the stir bar can't keep up. This causes the stir bar to
come out of sync with the magnet within the stirrer. The stir bar just sits @ the bottom of the flask and rattles around. On my stirrer if I exceed
60-70% of max power at some point in time the stir bar will lose sync and rattle around.
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