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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, 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.
|Use for General Stirring Applications||Use for High Temperature Stirring||Use for Viscous Solutions High Speed Stirring||Use for Abrasive Slurries Extended Stirring Applications|
|Brand / Type||Spinbar Teflon PTFE||Spinbar Pyrex Glass||Spinbar Rare Earth Samarium Cobalt||Spinfinity|
|Magnetic Type||Alnico||Alnico||Samarium Cobalt||Alnico|
|Covering||Teflon PTFE||Clear Pyrex Glass||Teflon PTFE||Plastic|
|Features / Benefits||Wide selection of shapes and sizes to fit vessels of all types; Individual shapes create different vortexes for efficient and effective stirring||Heated stirring applications up to 274°C (525°F)||Reliable Magnetic Coupling – Superior magnetic energy reduces frequency of spinout in the most vigorous applications||Durable – Last 2.5 to 3 times longer under conditions known to traditionally cause material breakdown|
|FDA grade Teflon PTFE low friction coating is durable and inert||Glass casing has zero absorption and porosity||FDA Grade Teflon PTFE low friction coating is durable and inert||Outer shell made from FDA approved materials|
|Color selection for color-coding work processes||Distinct Color – Green color lets you quickly identify Spinbar Rare Earth Samarium Cobalt||Distinct Color – Quickly identify the Spinfinity difference by its purple color|