hot plate

Magnetic stirrer

A stir bar mixing a solution on a combined hot-plate magnetic-stirrer device. The left knob controls the stirring rate and the right knob controls heating.
Other names	Magnetic mixer
Uses	Liquid mixing
Inventor	Arthur Rosinger
Related items	Stir bar Vortex mixer Static mixer

Different sizes magnetic stir bars

A magnetic stirrer is a laboratory device consisting of either a rotating magnet or stationary electromagnets creating a rotating magnetic field. This device is used to cause a stir bar (aka a flea) immersed in a liquid to spin very quickly, agitating or mixing the liquid. A magnetic stirrer often includes a provision for heating the liquid.

Stirrers are often used in laboratories, especially in the fields of chemistry and biology. They are preferred over gear-driven motorized stirrers because they are quieter, more efficient, and have no moving external parts to break or wear out (other than the simple bar magnet itself). Due to its small size, a stirring bar is more easily cleaned and sterilized than other stirring devices.

Magnetic stirrers avoid two major problems with motorized stirrers. Firstly, motorized stirrers use lubricants, which can contaminate the reaction vessel and the product. Secondly, in motorized stirrers, the sealing of the connection between the rotating shaft of the stirrer and the vessel can be problematic, especially if a closed system is needed (e.g., due to OSHA, or environmental regulations, or because a process works only if oxygen, water or dust are excluded).

Magnetic stirrers also have drawbacks. For example, the limited size of the stirring bar means it can only be used for relatively small (under 4 liters) experiments. In addition, viscous liquids or thick suspensions are extremely difficult to mix using this method, although there are some stirrers with special magnets to overcome this problem.

History

Arthur Rosinger of Newark, New Jersey, U.S.A. obtained US Patent 2,350,534, titled Magnetic Stirrer on 6 June 1944, having filed an application therefor on 5 October 1942.^[1] Mr. Rosinger's patent includes a description of a coated bar magnet placed in a vessel, which is driven by a rotating magnet in a base below the vessel. Mr. Rosinger also explains in his patent that coating the magnet in plastic or covering it with glass or porcelain makes it chemically inert.

The plastic-coated bar magnet was independently invented in the late 1940s by Edward McLaughlin, of the Torpedo Experimental Establishment (TEE), Greenock, Scotland, who named it the 'flea' because of the way it jumps about if the rotating magnet is driven too fast.

An even earlier patent for a magnetic mixer is US 1,242,493, issued 9 October 1917 to Richard H. Stringham of Bountiful, Utah, U.S.A. Mr. Stringman's mixer used stationary electromagnets in the base, rather than a rotating permanent magnet, to rotate the stirrer.

The first multipoint magnetic stirrer was developed and patented by Salvador Bonet of SBS Company in 1977. He also introduced the practice of noting the denomination of stirring power in "liters of water", which is a market standard today.

Heating elements may range frm 120 W or lower to 500 W or more. The maximum reachable fluid temperature depends on the size of the flask, the quantity of solution to be heated, and the power of the heating element.

See also

• Stir bar
• Shaker
• Stirring rod
• Static mixer

References

External links

• DIY Stir plate Make your own stir plate from an old computer fan.
• Short video of a home made stir plate.
• [1] Arthur Rosinger's patent on "Magnetic Stirrer"