Comparison Chart

MAGNETIC STIRRING BAR SELECTION & USE
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Magnetic stirring is used in a number of common laboratory procedures,
yet the importance of selecting the best stirring bar for each particular application
is often overlooked. The drive magnet, vessel shape (including the opening of the vessel
that the stirbar has to t through), viscosity and abrasiveness of the materials should all
be considered when choosing the size, shape and materials (coating and magnet) of the
magnetic stirring bar in order to achieve eective, ecient stirring results.
First, let’s discuss magnetism. On the most basic level, we know that opposite poles attract.
A magnetic stirrer has a drive magnet, generally a bar or U-shape made of a metallic alloy or a ceramic
which rotates powered by a drive motor. The magnetic poles of the drive magnet are typically about two
inches apart in bench top models, and can be up to six inches apart in larger units used to mix 50 gallons
of liquid solution. For optimum magnetic coupling, the distance between the magnetic poles of the drive
magnet and the length of the stirring bar should be equal. Bars too long or too short in proportion to the
drive magnet will not have optimum coupling which is important to reduce spinout.
Once a magnetic stirring bar has been placed in a container with solution, it should be positioned directly
over the center of the drive magnet. The stirring speed should be increased slowly, until the desired vortex
pattern is achieved. Should the magnetic stirring bar lose its coupling with the drive magnet because of
the speed of the drive magnet, viscosity of the uid, or an improperly selected stirring bar length, it is said
to have “spun-out.”
Vertical distance between the drive magnet and the stirring bar should be kept to a minimum for the best
coupling and stirring eciency. Therefore, the containing vessel should be as thin as practical.
The selection of the shape of the magnetic stirring bar also inuences the resulting vortex in that
multi-sided shapes may be more ecient in moving solutions and certain shapes have been designed
to provide a tailored t in specialty vessels such as test tubes, cuvettes, spectrophotometer cells and
round bottomed boiling asks.
Magnetic stirring bars are generally made of ALNICO (an alloy of aluminum, nickel, iron and cobalt)
magnets encapsulated in FDA approved Teon
®
, known for its inert chemical compatibility and heat
resistance. In addition, magnetic stirring bars also use high energy magnetic materials, such as Samarium
Cobalt which increases strength of coupling with a drive magnet and helps to reduce spinout when
mixing viscous solutions or mixing at high speeds.