Magnetic Stirrers versus Overhead Stirrers
Comparing the pros and cons of different stirrer models
Often has an investigator ventured to formulate their favorite buffer, or perhaps even an unfamiliar one, without considering the depth of knowledge required beyond simply following the recipe for assembly, ensuring compounds are added in the correct proportions. Is heat required to get the components into solution, or alternately, is heat emitted? What size, shape, and empty volume in the flask is appropriate? How viscous is the solution, and does it gain viscosity as it mixes or cools? The answers to these questions and others can determine whether it is better to stir from above or below.
Stirring from below
Magnetic stirrers mix by coupling a stir bar to a magnetic drive, creating a vortex whose radius and centripetal force are dependent on the speed of rotation, the length of the stir bar, and the viscosity of the solution. If these properties are mutually incompatible, stirring will cease, often with the stir bar uncoupled from the magnetic drive, caroming around the flask.
Hotplate magnetic stirrers are ubiquitous on common-use benchtops. They can be operated intuitively using everyday laboratory supplies without accessorizing overmuch—Erlenmeyer flasks, flat-bottomed beakers, and stir bars. Additionally, an all-in-one solution satisfies both parsimony and convenience, as integrating a hotplate into a stir plate obviates the need for external flame, clamp stands, and esoteric tools of maintenance and adaptation.
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The ubiquity of magnetic stirrers mirrors the straightforward nature of their available features and options. IKA Corporation offers various hotplate stirrers from the base model RH series to the higher-end RCT series, among which one can choose myriad digital displays, differing maximum temperatures, volumes (always rated for water), and ranges of revolution. The highest priced magnetic stirrer usually runs close to the lowest-priced overhead setup. With any of these magnetic units, one can expect safe and simple mixing of most buffers for the long life of the instrument.
However, the presumably inert Teflon coating of stir bars can acquire pocks and abrasions over time, attracting and incorporating trace metals including gold, palladium, nickel, and iron. This can be a significant concern for analytical chemists performing reactions driven by metal ion catalysis, which can be pushed unexpectedly to the right of meticulously prepared equations. Therefore, it can be advantageous to stir solutions with stainless steel impellers driven by motors. Moreover, the variety of impellers, torques, and speeds available in overhead stirrer assemblies can enhance the precision and reproducibility inherent in mixing, homogenizing, or emulsifying an array of solutions.
Stirring from above
Impellers impart flow topographies intrinsically suited to their milieu. A propeller-shaped impeller blade creates motion parallel to its axis of rotation, resulting in a top-to-bottom, low-shear motion ideal for stratification or solid suspension. Cruciform, pivoting, or collapsible blades generate a perpendicular, radial flow, moving liquid sideways then vertically, as in high-shear emulsification, requiring higher RPMs than can be achieved with magnetic stirrers. Square shapes produce a tangential flow, with low-shear horizontal movement, most useful for high-viscosity liquids. In fact, the overhead stirrer reigns supreme in its ability to handle viscosity, especially with solutions constituted under heat that gain viscosity as they cool, such as polymers and lotions. The ability in all cases to place the impeller high or low, centrally or on the margins of its vessel, allows for much finer control of mixing dynamics and desired outcome.
Caframo Lab Solutions offers Ultra Torque, Universal, and Ultra Speed model overhead stirrers, in addition to nearly a score of impellers in stainless steel or PTFE, and assorted stands, collets, and chucks. The low-speed Ultra Torque BDC1850 is optimized for high-viscosity mixing, with a maximum torque of 568 Newton-centimeters, while the Ultra Speed BDC6015 is eponymous to its maximum rate of rotation, ideal for emulsion and homogenization. The Universal BDC3030 treads the middle ground for speed, torque, and capacity, and thus the high road for adaptability.