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I did not know there are products such as a magnetic stirrer with a hot plate, enabling one to apply heat and stirring to a solution with relative ease. Did I understand correctly, that device needs a magnetic bar to be added to the solution the device to work? Does that pose any limits when it can be used (if the magnet would react with something..) How much do these devices increase productivity?

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    $\begingroup$ The magnetic stir bars come in all sizes and shapes. They are coated with PTFE (or similar) so that they do not interfere with your reaction. Unless your reaction is very large or very viscous, a magnetic stir bar requires much less time to set up and clean compared to a traditional mechanical stirrer. $\endgroup$ – ron Jan 28 '15 at 17:14
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    $\begingroup$ I have repaired several cheap stirring hotplates. The problem was that they were made with bad glue that eventually wore out, probably wasn't heat tolerant. I used automotive exhaust manifold silicone glue to stick the magnets back on the motor and they worked fine. The silicone had much higher heat tolerance. $\endgroup$ – user137 Jan 28 '15 at 18:30
  • $\begingroup$ @user137 What would be cheap for stirring hot plates? For example, would you have any opinion on this amazon.co.uk/MAPLE-SCIENTIFIC-HOTPLATE-TEMPERATURE-ADJUSTABLE/… $\endgroup$ – character Jan 28 '15 at 19:27
  • $\begingroup$ Stirring hotplates are very expensive for how simple they are. Expect to spend a couple hundred dollars for a decent one, but if you can find a used one on ebay you might do better. I can't offer an opinion on that particular model, since I've never used it. $\endgroup$ – user137 Jan 28 '15 at 19:36
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Most of the things to be told about magnetic stirring are already in Rons comment. A rotating magnetic field is induced by a rotating permanent magnet inside the hot plate magnetic stirrer. This field drives the magnet inside your reaction flask. To ensure chemical inertness in most situations the magnet is coated with PTFE (Teflon) or protected by a thin layer of glass. Magnetic stirring can replace traditional mechanical stirring is most situations and has many benefits over it: it does not require an additional inlet to the reaction flask, which is very handy for reaction under reduced pressure and is also more easily adjustable to different scales due to the possibility to replace the magnet in the reaction flask. Two exceptions are 1) very large/very viscous reactions or reactions in which excessive amounts of solid are present 2) reactions that involve ferromagnetic compounds that interfere with the rotational movement of the magnet inside the flask (as in Urushibara nickle catalyst preparation).

Vogel has a section on reaction heating and stirring in which this and more is discussed extensively. See pages 71-80.

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