For the first decade of my career as a synthetic organic chemist, the only stirring mechanism I ever used was the magnetic type, with or without a separate heating mantle, sometimes as a dual hot-plate stirrer. Then one day, while following a literature prep that called for an overhead stirrer, I decided that I knew more than the author and turned again to my trusty magnetic stirrer. Big mistake. When I returned from a coffee break, the stir bar had stopped doing its thing and the reaction vessel was nearly frozen solid. After carefully disposing of the reaction flask and stirrer, I followed the prep to the letter and got my product, but not before wasting nearly half a day.
Overhead stirrers are specified often when either reaction or mixing volumes are very large, typically above two liters, but even more frequently when the operation is initially viscous or expected to develop significant viscosity.
As illustrated by the above example, viscosity can creep up without notice. Some reactions turn to thick slurries or form heavy precipitates as they progress. Similarly, syrup preparations begin as pure water and end as sludge-like products, particularly on cooling.
CAT Scientific (Paso Robles, CA) provides further examples for putting aside your magnetic stirrer and setting up an overhead mixing system: mixtures of water and organic solvents or of soluble and insoluble materials, suspensions, slurries, pastes, creams, and polymer processes that show a wide range of viscosities.
Magnetic stirrers, whether built into a hot plate design or not, will set a lab back anywhere from $200 to about $900. More features, stronger magnetics, more sophisticated controls, and greater robustness translate to higher prices.
The high end of the price range for magnetic stirrers is the price entry point for overhead stirrers, with the “sweet spot” for laboratories in the $1,000–$5,000 range. Overhead designs provide significantly higher mixing/ stirring capability, greater volume capacities, and stronger agitation for processes involving highly viscous liquids.
What to look for
One consistent requirement for laboratory stirrers, be they magnetic or overhead, is a robust motor and protective motor housing. “Users need to be assured that stirrers will work reliably, accurately, and safely at all times,” notes Nicole Kvasnicka, product marketing manager at Heidolph North America (Elk Grove Village, IL). “Regardless of the stirrer type, the motor must always be protected from the lab environment.”
Magnetic stirrers have the advantage, she says, for rapid dissolution of small samples or “any simple application on the benchtop of a low viscosity.”
For Refika Bilgic, managing director at IKA Works (Wilmington, NC), process consistency, reproducibility, and operational safety are the three primary things to look for in an overhead stirrer.
She explains the relationship between stirring or agitation forces for magnetic versus overhead stirrers, and how they relate to application: “The choice comes down to volume and viscosity, as well as the materials you’re stirring. If a high level of sedimentation is expected, a magnetic stir bar will most likely get stuck and not run efficiently. In this situation an overhead stirrer is definitely the better alternative. In general, any sample processing smaller than 50mPa-s [milli Pascal-seconds] and one liter is suited for a magnetic stirrer; anything greater than both values should be processed by an overhead stirrer.”
Selection of the stirring element—there are dozens of designs and sizes—also depends on the application, the viscosity, the desired speed range, and the samples. “Depending on which stirring element is used, a variety of effects can occur,” Bilgic observes. “For example, a stirring element that is three-bladed or has an anchor shape creates different results in flow efficiency and direction. Stirrer designs can also result in different levels of shearing forces and homogeneity.”
For additional resources on stirrers, including useful articles and a list of manufacturers, visit www.labmanager.com/stirrers