Stir bar

Magnetic stir bars, of various sizes.

A stir bar or flea is a small magnetic bar placed within the liquid which provides the stirring action.

General

Stir bars are small items made containing two magnets, coated in PTFE or glass, and come in various forms, like cylindrical, cylindrical with a pivot ring, oval (almond or egg-shaped), triangular, cross, dumbbell (circulus), spherical, saturn, etc. PTFE is more commonly used as it doesn't break when dropped on a hard surface and doesn't crack if exposed to thermal shock, while glass-coated stir bars are used for working with molten alkali metals. Stir bars cannot be used above 225-270 °C (437-518 °F), as PTFE breaks down above that temperature range, while the magnets will begin to lose their strength if heated too long at high temperatures.

Types and use

Each type of stir bar is designed for a specific task:

• Cylindrical stir bar: general purpose stirrer for smooth stirring, used for all flat bottom glassware.
• Plain stir bar: similar in role to cylindrical, but capable of generating more turbulence at low speed.
• Octahedral stir bar: similar to the pivot ring type, but with more turbulence at low speed.
• Oval stir bar: used in round-bottom flasks
• Pivot ring stir bar: used in containers with slightly curved or uneven base, but not very curved like round bottom flasks, the pivot allowing the stir bar to adopt the optimum position for stirring.
• Spherical stir bar: used for stirring in test tubes or for eccentric stirring.
• Triangular stir bar: used for dissolving solids or mixing sediments, the flat base gives a scraping ability for disturbing solids.
• Cross stir bar: used for general purpose stirring but very stable to reduce “jumping”.
• Crosshead stir bar: used in tube shaped containers like test tubes, beakers or flasks.
• Drumbell stir bar: used for flasks with concave bottom, like jars or reagent bottles, capable of generating strong turbulence.
• Saturn stir bar: similar to pivot stir bars in aspect, they are used when stirring powders in liquids without getting stalled.
• Spinring stir bar: allows maximum stabilization of the magnetic stirring bar with the addition of a “hoop” around a standard octagonal bar, best suited for larger open-neck vessels, such as buckets and beakers.

Availability

Magnetic stir bars can be purchased from most lab suppliers. They can also be found on eBay and Amazon

DIY stir bar

A simple stir bar can be made by gluing two magnets on a small cylinder, then coat the item in a chemical resistant coating. This type of stir bar is not however, compatible with many reagents.

See also

• Magnetic stirrer
• Overhead stirrer
• Stir bar retriever

References

Relevant Scienceamdness threads

• PTFE stirring bar dissolved?
• Stir bar sizes: rules?