chem101st
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Overhead stirrer vs stir bar?
Is there ever a time when an overhead stirrer is absolutely necessary? If so, why, Viscosity? Or can they always be used interchangeably?
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Diachrynic
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Overhead stirrers are able to provide a lot more torque than a magnetic stir bar, so for highly viscous solutions or solutions with a lot of
precipitate, where a magnetic stirrer just doesn't work, you may need overhead stirring. Furthermore overhead stirring can be a lot more violent even
in non-viscous solutions, may be able to provide agitation closer to the walls of the flask (look at the Hershberg stirrer for example). This also
means that professional labs like to use overhead stirrers for things like the preparation of Grignard reagents or other organometallic reagents,
because the violent agitation, that magnetic stirrers have trouble providing, can scrape and activate the e.g. magnesium surface (making the reaction
start more reliably).
I would assume you can always replace magnetic stirring with overhead stirring, but note that it takes one neck of your flask and that it is a lot
more effort to set up, so it's usually only done when necessary.
we apologize for the inconvenience
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chem101st
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Thanks!
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Texium
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One other niche situation when mechanical stirring is necessary is when you’re running a reaction using iron filings. Although it’s rather
entertaining to see all the metal get stuck to the magnetic stir bar, it doesn’t allow for very efficient mixing!
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