Basic Working and Usage of Magnetic Stirrer
Stirring processes are common in labs of all shapes and sizes. Regardless of the specific application or process being performed, your lab technicians need to use the best equipment for the job to ensure consistent results. That's why it's so important to know your stirring options and when they're the right choice for your application.
What is a magnetic stirrer?
A magnetic stirrer uses a rotating magnetic field to move a stirring bar through a liquid sample. The motion of this stirring bar thoroughly mixes the sample through rapid motion and agitation. The speed of the magnetic field is controlled by the user, so it can be customized to the specific sample being stirred. These stirrers should be used with glass or other non-metallic beakers to prevent interference with the magnetic field.
Depending on the application, you can configure a magnetic stirrer with a heating plate to heat the sample while it is being stirred. By using the heating plate, you can mix a variety of samples for different applications.
A magnetic stirrer consists of a magnetic bar placed in a liquid to provide stirring action. The movement of the stirring bar is driven by another rotating magnet or electromagnet assembly in the stirrer unit, located below the vessel containing the liquid. The stirring bar is typically coated with PTFE or, less frequently, glass; the coating is intended to be chemically inert and not contaminate or react with the reaction mixture in which it is located.
Laboratory hot plates typically serve a dual purpose by combining a stirring assembly and a heating element. Such heating elements can range in power from a few hundred watts to several kilowatts, and allow simultaneous heating and stirring of the reaction flask. The maximum fluid temperature that can be achieved depends on the size of the flask, the amount of solution to be heated, the power of the heating element, and the amount of insulation provided to the system.
Uses
Magnetic stirrers are commonly used in chemistry and biology, where they can be used to stir sealed vessels or systems without the need for complex rotary seals. They are preferred over gear-driven electric stirrers because they are quieter, more efficient, and have no moving external parts to damage or wear out (other than the simple bar magnet itself). Magnetic stirring bars work well in glass vessels that are typically used for chemical reactions because the glass does not significantly affect the magnetic field. The limited size of the bar means that magnetic stirrers can only be used for relatively small experiments, i.e. 4 liters or less. The stirring bar also has difficulty handling viscous liquids or thick suspensions. For larger volumes or more viscous liquids, some kind of mechanical stirring is usually required. In synthetic chemistry, a combination magnetic stirrer/heater with a built-in temperature control mechanism and temperature probe is often used with a heating bath or cooling bath to keep the reaction vessel placed in the bath at a temperature between approximately -120 and 250 °C.
Overhead stirrers
In addition to a magnetic stirrer, you can also use an electric top-mounted stirrer to mix your samples. While overhead stirrers can handle larger volumes of fluid, they are not suitable for all viscosity levels. Overhead stirrers are equipped with sensors that will shut down the motor if it overheats, which can happen with particularly viscous fluids. Technicians can use overhead stirrers to control many different aspects of operation and cycle settings, but they are limited to the types of samples they can effectively mix.
If you are mixing higher volumes of lower viscosity liquids, an overhead stirrer may be the right choice. However, if your liquids are more viscous and you don't need the extra volume that an overhead stirrer provides, a magnetic stirrer is a better choice because you don't have to worry about overheating.
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