Magnetic Stirring Bar Selection & Use
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, 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 effective, efficient 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 fluid, 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 efficiency. Therefore, the containing vessel should be as thin as practical.
The selection of the shape of the magnetic stirring bar also influences the resulting vortex in that multi-sided shapes may be more efficient in moving solutions and certain shapes have been designed to provide a tailored fit in specialty vessels such as test tubes, cuvettes and spectrophotometer cells.
Magnetic stirring bars are generally made of Alnico (an alloy of aluminum, nickel, iron and cobalt) magnets encapsulated in an FDA approved PTFE coating. 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.
Newer to the market, the Bel-Art Products Spinfinity magnetic stirring bars use Alnico magnets encapsulated in plastic for increased durability in granular slurries.