A Primer on Magnetic Stirrers

Author: Bob Wilcox

Laboratory magnetic stirrers are clever mixing tools that, most simply defined, use a rotating magnet in the base of the unit to cause a bar magnet in solution flasks or beakers to likewise rotate. A U.S. patent was filed on this device by one Arthur Rosinger of Newark NJ on October 5, 1942 and was issued on June 6, 1944.

Magnetic Stirrer development

Over the years substantial improvements have been made on the design and performance of magnetic stirrers.

Today, they find wide application in labs, pilot plants and small production facilities to process a variety of substances such as liquids, solvents, lacquers, oils, polymers and specialty chemicals. Chemists appreciate magnetic mixers because they are efficient, quiet and can operate for hours without supervision. Moreover they take no lunch hours or coffee breaks.

Added features include ease of cleaning and disinfecting. Aside from the rotating magnet in the base there are no moving parts to wear out.

Magnetic mixer capabilities

Most units are fitted with a hotplate allowing samples contained in glass flasks to be heated as a further option in process development. Heating capability and the amount of material that can be processed depend on the size of the unit but temperatures to 500?C and capacities to 10 liters are typical. Mixing speed can be steplessly adjusted from, for example 50 to 1600 RPM; a soft-start/stop feature avoids samples splashing from the container.

In fact the primary limit to the utility of these mixers is viscosity. Unlike overhead mixers and stirrers there is no direct connection between the drive motor magnet and the free magnet in the beaker. This is what limits their use in most cosmetic formulations.

As to the stirring magnets a variety of configurations are available designed to the size and configuration of the sample container. All should have inert coatings to facilitate cleaning and disinfecting, thereby avoiding cross-contamination between samples.

Magnetic Stirrer Control Panels

Researchers have full control over the operation of their magnetic mixer with control options based on the particular model. On the CAT MCS67 programmable stirrer for example options include on/off for power, heating plate and the magnetic motor; plate temperature, motor RPM and operating time. When coupled to the optional but highly recommended platinum resistance temperature probe the set and actual sample temperature will be displayed. Researchers can also program heating and cooling rates and set the operating timer in days, hours and minutes.

Safety Features

The above mentioned temperature probe plays a dual role in the operation of a magnetic mixer. When attached to the mixer it effectively takes over the operation according to the programmed parameters. It along with features built into the mixer also play a role in the safe operation of magnetic stirrers. All
safety features are designed to shut the system down in the event of a malfunction.

Events that trigger a shut down include:
– The hotplate temperature exceeding the maximum allowable temperature
– A breakage or disconnect of the platinum temperature probe
– Breakage or cracks in the flask or beaker
– Electronic failure
– A dramatic drop in the sample temperature

Record Keeping Capabilities

What would lab life be like without record keeping? We’ll never know, but what we can do is facilitate the process, which is a critical part of experimentation in the manufacture of cosmetics. An RS232 interface on magnetic mixers provides connectivity to PCs and printers to archive operating data that include temperature, time, and motor speed.

Admittedly this is a very brief introduction to magnetic stirrers. In a future blog post Later we may look at some of these and other features in detail with the hope of giving practicing chemists ideas on how the versatile tool can be put to work in their operations.

Bob Wilcox has represented CAT Scientific’s family of homogenizers, magnetic stirrers, liquid metering and related laboratory equipment since 2002. In addition to heading the sales function in the US he is chief technician for the CAT equipment service organization.

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