How to Process Samples in Round Bottom Flasks

Before providing tips on how to process samples in round bottom flasks it might be useful to describe how these flasks are used in the R&D lab and to set up pilot plant operations.  We’ll then get into describing how you use these flasks – focusing on equipment you’ll want to have to conduct processing under high temperatures.

Typical Uses for Round Bottom Flasks

Heat-resistant glass flasks, beakers and test tubes are common implements in the lab.  Glass is favored because it is easily cleaned, chemically inert and allows researchers to gauge sample quantities being processed.

Round bottom flasks as described in a Google search are more resistant to fracturing under vacuum, as a sphere more evenly distributes stress across its surface. These flasks are often employed to contain chemical reactions run by chemists, especially for reflux set-ups and laboratory-scale synthesis.

Round bottomed flasks can have one or more necks, depending on the application.

The shape of these flasks presents challenges not present with typical flat-bottom beakers and Erlenmeyer flasks simply because they require a supporting system – either clamps or a donut-type base.  A solution to this challenge is found by employing hotplate magnetic stirrers specifically designed for round bottom flasks.

How to Conduct Mixing and Stirring in Round Bottom Flasks

Magnetic stirrers are excellent processing tools for controlled-speed mixing of liquids in the lab.  Volumes handled vary with models but representative examples are from 100 ml to 10 liters.  Stirring speed also depends on models but reach 1600 RPM, as does sample heating capability for hotplate magnetic stirrers. Researchers can set temperatures up to 450⁰C.

How Magnetic Stirrers Work

A magnetic stirrer has a motor-powered rotating magnet in its base to energize a free magnet in the bottom of the flask.  The rotating magnet is the only moving part in the assembly. For that reason magnetic mixers are extremely reliable.

The configuration of the magnet in the flask is based on the application but all flask magnets are coated for cleanability.   The flask magnet (called a flea) in the sample solution is centered over the mixer magnet.  Stirring starts at a low speed to avoid splashing and the stirring magnet being knocked off center.  Stirring is then increased to the selected speed.

As noted above, round-bottom single and multiple-necked flasks have special requirements when used with hotplate magnetic stirrers.  To address this CAT Scientific offers a choice of two designs, the 100 to 500 ml KM 16.4D and the 1,000 to 2,000 ml KM 16.7D, both with heating capacities to 450⁰C and adjustable speeds to 1600 RPM.

Hotplate configuration for these flasks is different than that for flat-bottom flasks and beakers.   It is essential that heat be transferred evenly to the flask surface.  This is achieved by specially designed blocks to provide even distribution over the flask surface vs. the flat heating surface characteristic of standard hotplate magnetic stirrers.

Safety Features for Hotplate Magnetic Stirrers

Both models are equipped to deal with potential failure of the flask by providing a stainless steel container designed to catch and hold samples in the event of breakage.

Both units are equipped with an intuitive control panel that allows setting of operational parameters and the added benefit of the optional Pt100 temperature control probe.  Safety features, enhanced by the probe, guard against overheating, sample loss, software malfunctions and similar conditions that initiate a controlled shutdown of the equipment.

Check our website for additional data on the KM 16.4D and KM 16.7D magnetic stirrers for round-bottom flasks or send us your questions.  We’ll be back to you as quickly as we can with the answers.

Bob Wilcox

Bob Wilcox has represented CAT Scientific’s family of homogenizers, magnetic stirrers, liquid handling and related laboratory equipment since 2002 when Staufen, Germany-based CAT Ingenieurbüro M. Zipperer GMbH established operations in North America. Bob oversees CAT Scientific laboratory apparatus sales and service organization from the company’s headquarters in Paso Robles, CA. He also is in charge of the parent company’s line of JetCat jet turbines, turboprop, and helicopter power plants for hobbyists’ radio controlled fixed wing and helicopter model aircraft. -- Earlier in Bob’s career he was involved in visual and special effects as well as camera and electronics supervisory responsibilities for the motion picture and television industry.

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