Funkerman23
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Stir bar sizes: rules?
As dumb as this sounds: can anybody tell me what size egg/oval stir bars are usually used for 500ml,1& 2 liter RBFs? with luck I'll finally have a
stir plate soon and I am not sure what to look for. For the most part I'll be using these in 24/40 joint sizes but I still have the 19/22 stuff as
well. I thank you for your time as always.
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Arthur Dent
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I don't think there are any rules in particular, you have to evaluate what volume of liquid you want to displace, and then there are factors like the
density of the liquid and will it change during the reaction, will there be a precipitate during the reaction, does it need a vigourous stitting or
simply a basic motion?
Also do you want to mix stuff together, or simply keep a liquid from bumping during a distillation... I say go with your instinct. Bigger in this case
is not necessarily better. You'd be surprised that a small stirbar can conjure up quite a storm in a big beaker!
Robert
--- Art is making something out of nothing and selling it. - Frank Zappa ---
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Dr.Bob
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I think this might be referring to the question of what will fit through a 24/40 joint, perhaps. The largest oval stirbar that fit fit easily through
a 24/40 joint is a 36mm x 16mm one. All of the longer ovals are 19mm wide, which about 1 or 2 mm too wide to fit easily. They can be trimmed to
fit, but that takes some care and caution not to trim your fingers off as well. I don't know what the largest stirbar that will fit a 19/22 joint is,
but I would guess that it is pretty small, maybe a 25 x 12 mm (1 x 1/2"), that would be my best guess.
For 24/40 a 36 mm x 16mm oval will work fine in most reactions from 500 to 2000 ml. For smaller reactions, the 32 x 16 mm (1 1/4 x 5/8") and 25 x 12
mm (1 x 1/2") work pretty well down to about 100ml, then I usually switch to a 19 x 8 mm one (3/4 x 3/8") for 50 - 100 ml. But the largest ovals
(2"+) are really too large to stir most flasks well, even if you shave them down to fit the joint.
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Funkerman23
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Mostly this will be for mixing and to keep the bumping down to a manageable level( anti-climb traps can only do so much). Granted: I don't expect to
stir cement with these or molasses but just general use. I just don't want to get a spinbar stuck in the joints of my flasks. Chinese glass I can
afford to replace, ChemGlass? not so much..
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Rosco Bodine
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Standard taper is diameter/length rate of decrease of 1 per10 and that is to say for each 10mm length of the ground zone the opening diameter
decreases by 1mm ....therefore the clear opening of a 24/40 at the smaller dimension will be 24mm-4mm=20mm Even if a flatted oval is 19mm across the
flats, the diagonal dimension through the centerline axis will be oversize and the minimum of a 29/42 joint would be required for clearance at the
opening. Those flatted ellipitical ovals ( not the "egg" shaped ovals having a single flatted band, but the blade tipped flatted elliptical type ) of
about 3 inch length and 60grams weight from fisher are the most stable alnico stirbars at all speeds which I have found. But no they won't clear the
opening of a 24/40 joint. For all standard ground joint sizes the first number is the largest ground zone opening diameter and the
second number is the length of the ground zone. So you have to do the math to get the clear opening diameter "pass through" clearance dimension. The
curvature of the flask bottom for a round bottom flask will be limiting what size stirbar can be used if the contact point for the spinning stirbar is
on its belly as it should be. Many smaller stirplates do not have sufficiently large drive magnets to couple with the 3 inch bars. So there is also
a sizing matchup to consider there
between the driven element and the driver.
[Edited on 19-4-2012 by Rosco Bodine]
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