Introduction: Magnetic Stirrer Hotplate

A magnetic stirrer hotplate is a very desirable piece of equipment for working with chemicals and
solutions. Commercially manufactured versions can cost $ 200 or more and are beyond the budget of most
hobbyists. A cheaper alternative is to construct one yourself from readily available components, and this
should not cost more than $ 25 to $ 30, even if you need to buy some of the parts new. In this example,
I used a second hand coffee percolator bought at a garage sale for $ 8, together with a 12 V dimmer control
( $ 3 on eBay ), 2 magnets ( $ 3 on eBay ), a computer cooling fan ( mine was salvaged from an old
computer, but you could buy a new one for $ 2.50 on eBay ), a 12 V plug pack ( $ 4 on eBay) and some
small nuts and bolts ( about $ 2 worth ).
The first step was to remove the unwanted parts of the percolator, leaving only the hotplate base. If
this has an iron hotplate ( test it with a magnet ), it would need to be replaced with a piece of aluminium,
copper or brass sheet. Old aluminium saucepans are a good source of material for this purpose. Take care to
reassemble the heating element in its original state and ensure that the wiring is safe ! ( If you are not sure
about this, get someone who knows what they are doing to check it )

Next, two strong magnets were attached to the fan blade, connected by a ferrous metal strip to form a
long bar magnet configuration as in the diagram below. I used construction adhesive together with fine
copper wire ties to secure the magnets in position. If you have magnets with a hole through the middle, these
can be attached with small machine screws and nuts. Ensure that the magnets are equally balanced so that
there is minimal vibration when the fan is running, otherwise the fan bearings will not last very long.

Step 1: Magnetic Stirrer Hotplate

The fan was then mounted on the underside of the percolator base, using some short ( 8mm ) spacers
between the fan housing and the base of the percolator to allow the magnets to spin freely. Suitable spacers
can be cut from an old ball point pen casing with a fine hacksaw. The percolator I used had an ABS plastic
base which does not interact with the rotating magnetic field. If the base plate had been steel, it would have
been necessary to cut out a circle the same size as the fan and cover the hole with a non magnetic material
( plastic, thin plywood, aluminium, etc ) otherwise the stirrer's magnetic field would be weakened to the
extent that the stirrer bar would not rotate properly. After mounting the fan, some extended legs were needed
to provide clearance for the fan, as seen below.

Step 2: Magnetic Stirrer Hotplate

Finally, the dimmer control was attached to a metal plate alongside the hotplate and the wiring
completed, with the 12 V power supplied from the plug pack. I also added a bracket at the back of the
hotplate for a retort rod.

This piece of apparatus is ideal for preparing solutions of chemicals, recrystallising chemicals from
solvents, or other laboratory operations requiring gentle heating and stirring. It is limited to temperatures up
to 100 C, but this is adequate for the majority of uses. As with any stirrer hotplate, it should not be used to
heat low flash point solvents like diethyl ether in open containers, due to the danger of ignition from the
motor or the hot element.
Magnetic stirring is most suitable for low viscosity mixtures without appreciable solid particles. The
size of the magnetic stirring bar should be increased when working in larger vessels. It is possible to make
your own stirrer bars by enclosing small cylindrical magnets inside vinyl, polypropylene or polyurethane
plastic tubes and heat sealing the ends. The plastic needs to be resistant to attack by any solvents used;
Teflon is best but is harder to seal. If Teflon is needed, it is probably better to buy a commercial stirrer bar.

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