Magnetic Stirrer: Parts, Principle, and Types

The magnetic stirrer is an essential piece of laboratory equipment used for stirring or mixing in laboratory experiments by creating a rotating magnetic field. The rotating magnetic field is created with the help of a rotating magnet (i.e., stir bar) and a stationary magnet. It is also known as a magnetic mixer. Richard Stringham patented the first magnetic stirrer in 1917.

Table of Contents

Major parts of magnetic stirrer

The magnetic stirrer is user-friendly for preparing mixtures and consists of two major components which include; a stir bar and a hot plate stirrer device. A hot plate stirrer device also contains different parts within it such as; a stationary electromagnet, a hot plate, a temperature controlling knob, and a speed controlling knob. Similarly, the stir bar retriever is also a less commonly used part of a magnetic stirrer.

Stir bar

It is a magnetic bar placed in the container containing a solution used for the agitation of liquid mixture or solution. It is coated with nonreactive material (such as polytetrafluoroethylene (PTFE) or glass) that protects the magnet and prevents contamination of the liquid medium in which they are immersed for agitation. The shape of the stir bar varies (i.e., octagonal, circular, etc.) and depends on two factors; vessel shape and viscosity of the stirring medium. Similarly, the stir bar’s size also varies from a few millimeters to a few centimeters.

Hot plate

It helps to provide a plane surface for the vessel to hold and heat. It is generally made up of ceramic or stainless steel that provides good chemical-resistant performance.

Stationary electromagnet:

It is a simple square bar magnet, a U magnet, or a composite SSMCmagnet placed beneath the vessel with the liquid. Its rotation induces the rotation of a stir bar immersed in a liquid.

Temperature controlling knob:

It helps to control the temperature based on the viscosity of liquid type, liquid volume, and the size of the vessel.

Speed controlling knob:

It helps to control the rotating speed (i.e., RPM) of the stir bar as per requirement.

Stir bar retriever

It is a separate magnet at the end of the long stick coated with inert material (i.e., PTFE). It helps in the removal of the stir bar from the vessel.

Principle

The magnetic stirrer is an important laboratory device that can be used over a wide temperature range and with any chemical agents, in an open or in closed system.

The main purpose of a magnetic stirrer is to generate a magnetic field that helps to mix solutions. When the main switch of the magnetic stirrer is turned on, the heating mechanism and electromagnetic effects are activated. The heating mechanism in it is regulated by a temperature-controlling knob that heats the solution. Whereas, the electromagnetic effect induced by the device interacts or interferes with the motionless magnetic bar. As a result, the stir bar rotates and results in the proper mixing of the solution. The solution also rotates along with the magnetic bar during the mixing process. For proper mixing, the vessel containing the bar magnet must be placed at the center of the plate. Similarly, the size of the bar magnet should be selected based on the vessel’s size and the solution’s viscosity.

Types of magnetic stirrer

Based on the size, configuration, and applications; magnetic stirrers are of the following types;

Mini magnetic stirrer

They are closed-packed and occupy less space in the laboratory.
They are noiseless in operation with low vibration making them convenient to use.
They also consist of a speed regulator that helps control the bar magnet’s speed (in RPM).
They are resistant to corrosive and hazardous chemical agents in the lab.

Magnetic stirrer with timer

A magnetic stirrer with a timer helps to close down the motor automatically after the set period runs out.

Heavy-duty magnetic stirrer

They have a high mixing range and are long-lasting.
Internally, it consists of an electronic management device that automatically controls the speed concerning the load.
They are also highly unreactive to chemicals.

Battery-powered magnetic stirrer

These portable magnetic stirrer work with batteries and are ideal to be used in fields, incubators, or anywhere without electricity.

Air-operated turbine magnetic stirrer

It uses a low-pressure air supply to operate it.
It helps in mixing large volumes of solution (Upto 1L).
It abolishes the risk of sparkling from electrical sources.
Ideal to stir solutions that produce hazardous vapors.

Uses of magnetic stirrer

Compared to other mechanical stirrers, magnetic stirrers are simple in architecture and also are easy to operate.
It helps to mix solutions for several hours or even overnight.
Stirring bars are smaller in size so are more easily cleaned and sterilized than other stirring devices.
It allows stirring in a closed system (i.e., less contamination).
Manual stirring is labor intense and time confusing process. Magnetic stirrer helps to save time for researchers.
They are used in place of mechanical stirrers as they produce less noise and are comparatively more efficient.
They occupy less space in the laboratory and are portable.

Limitations of magnetic stirrer

The size of the stir bar needs to be changed from time to time, depending on the nature of the solution.
Because of the limitation in the size of the stir bar, the device can only be used for small-scale experiments.
It cannot be used to stir viscous or dense suspension or a high volume of liquid.
It cannot be operated at extremely high temperatures because the outer covering of the stir bar might deteriorate

References

Stir bars. K&J Magnetics, Inc. Retrieved on 26th September 2022. Retrieved from https://www.kjmagnetics.com/blog.asp?p=stir-bars
The complete guide to magnetic stirrers and their components.Laboratory/ Microcentrifuge. Retrieved on 27th September 2022. Retrieved from https://neuationcentrifuges.wordpress.com/tag/magnetic-stirrer-components/