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Concept
Magnetic stirrer
Summary
A magnetic stirrer or magnetic mixer is a laboratory device that employs a rotating magnetic field to cause a stir bar (or flea) immersed in a liquid to spin very quickly, thus stirring it. The rotating field may be created either by a rotating magnet or a set of stationary electromagnets, placed beneath the vessel with the liquid. It is used in chemistry and biology as a convenient way to stir small volumes and where other forms of stirring, such as overhead stirrers and stirring rods, may not be viable.
A magnetic stirrer consists of a magnetic bar placed within the liquid which provides the stirring action. The stir bar's motion is driven by another rotating magnet or assembly of electromagnets in the stirrer device, beneath the vessel containing the liquid. Stir bars are typically coated in PTFE, or, less often, in glass; the coatings are intended to be chemically inert, not contaminating or reacting with the reaction mixture they are in. Glass may be viable as an alternative if PTFE is unsuitable due to high temperature or chemical attack. In dissolving metal reductions that use an alkali metal dissolved in a primary amine, PTFE may be attacked to some extent. Birch reductions (a common dissolving metal reduction) are often conducted in a glass vessel, thus indicating that a glass stir bar would likewise be compatible. Glass can be attacked by strong alkali (such as lye) depending on heat, exposure time, and concentration.
Magnetic stirrers are bar-shaped and usually octagonal or circular in cross-section, a pointed oval shape is also common for use in round-bottom flasks. A variety of special shapes exist for more stable or efficient stirring in different conditions or to conform to the shape of small vessels. Many stir bars have a pivot ring around the center on which they rotate. The smallest are only a few millimeters long and the largest several centimeters. The smaller sizes (less than about 10mm) are often referred to as "fleas".
Laboratory hot plates often serve a dual purpose by incorporating both the stirring assembly and a heating element.
Official source
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