Tips for choosing the right stirring bar for your experiment

Author: Sophie Bolton
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Date: Aug 02, 2018

Choosing a stirrer bar

When it comes to magnetic stirring, the stirring bar (also known as a flea) is an integral part of the process.

The right type of bar can make a considerable difference to the effectiveness of an experiment. Mixing can be made more efficient, and the quality and uniformity of reactions can be improved. The right choice can also reduce wear and abrasion of the coating of the bar itself.

What to consider in magnetic stirring bars

There are a number of factors to consider when choosing a stirring bar for your experiment:

Size

Be sure to check that the bar is the appropriate size for the vessel being used – ensure it’s small enough to fit through the opening.  

Shape

There are a range of shape options to consider, each of which has a slightly different effect.

  • Cylindrical bars are one of the most commonly used shapes. They’re designed for stirring small volumes in vials and tubes.
  • Pivot ring bars are similar to cylindrical bars, but the additional pivot ring can help to position the bar in the optimum place and enable smoother stirring, while helping to reduce friction.
  • Cross shaped bars are typically good for stable stirring with a deep vortex, and are a good general purpose option.
  • Elliptical or curved stirring bars are designed to work best with round bottom flasks.
  • Octagonal stirring bars provide a greater surface area and added turbulence due to their interrupted profile.


Composition

Magnetic stirring bars need to be coated in a material that won’t react with the sample. The most commonly used coating is PTFE (Teflon) due to its heat resistant and chemically inert nature. Glass-coated options are ideal for very high temperature applications, as well as for use with abrasive materials.

Rare earth stirring bars

These can usually be identified by a carbon black spot on the bar, or by ‘RE’ in the product name. Rare earth magnets have a much greater magnetic strength, making them useful for stirring viscous samples. In contrast to standard stirring bars, they’re almost completely resistant to demagnetisation, meaning they don’t require regular replacement.

When to consider mechanical (overhead) stirring

In some instances, magnetic stirring may not be sufficient for an experiment. For example, if your sample is particularly viscous, or the viscosity is expected to increase during the experiment, magnetic stirring will likely not be powerful enough for the mixing process. Similarly, if there are solids or crystals in your sample then the magnetic stirring bar may grind them between the stirrer and the glass.

In such cases, mechanical (overhead) stirring is advised.

Click here to read more on which overhead stirrer would be suitable for your application.

Which stirring bars are recommended with Radleys equipment?

We have tested a range of stirring bars and have the following recommendations for use with Radleys equipment:

Carousel 12 Plus – We recommend the 16.5 mm Rare Earth Cross Shape PTFE Stirring Bar for the standard Carousel 12 tubes and the 10 mm Cross Shaped PTFE Stirring Bar for the reduced volume Carousel 12 tubes.

Carousel 6 Plus – For 250 ml, 170 ml and 100 ml flasks, choose the 25 mm Rare Earth Elliptical PTFE Stirring Bar. We also recommend a Rare Earth Elliptical PTFE Stirring Bar with the 25 ml and 50 ml flasks, but the smaller 15 mm size cross and octagonal bars would also work.

GreenHouse Plus – We recommend the 15 mm Pivot Ring PTFE Stirring Bar, but a cylindrical bar would also be suitable.

StarFish – For flasks between 100 ml and 500 ml, the 25 mm Elliptical PTFE Stirring Bar is recommended, but the 30 mm cross shape bar is also suitable. The smaller 15 mm Rare Earth Elliptical Starring Bar is recommended for the 25 ml and 50 ml flasks, with the added option of using the smaller cross shaped and octagonal bars.

Click to read our full Recommended Stirring Bar Guide

Even with this information in mind, selecting a stirring bar can often be a case of trial and error. We recommend trying a few different types of stirring bar to determine the best one for your specific application.

We offer evaluation kits, each with a range of stirring bars in different shapes and sizes. You can order these using catalogue number RR98095 (PTFE Magnetic Stir Bar Evaluation Kit, Pack 30) or RR71200 (Stirrer bar evaluation kit - for flasks, Pack 10).

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