Magnetic stirrer

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A magnetic stirrer or magnetic mixer is a laboratory device which uses a rotating magnetic field to cause a stir bar to spin very quickly. The rotating field may be created either by a rotating magnet or a set of stationary electromagnets, placed beneath the vessel with the liquid. The magnetic stirrer is a very useful and convenient method of stirring liquids and suspensions and it's crucial in air-free processes.

Design

A magnetic stirrer is composed of two main components: the hot-plate stirrer device and the stir bar.

The hot-plate stirrer is a piece of equipment which, as its name suggests, has the form of a hot plate and houses the magnet/electromagnet used for engaging the stir bar. Most are actual hot plates, being capable of both producing heat as well as a rotating magnetic field. They are easy to tell in that they have two buttons (or set of controls for advanced models), each for setting the temperature of the hotplate and the stirring speed, the main plate is metal and tend to be larger. Stir plates lack the heating elements and are used only for stirring. They are smaller than hot-plate stirrers and the outer framework is made of plastic.

The stir bar consists of a small magnet or ferromagnetic material, which is coated in Teflon, though glass-coated stir bars also exist and are suitable for working with molten alkali metals. PTFE is more commonly used as it doesn't break when dropped on a hard surface and doesn't crack if exposed to thermal shock. While the most common stir bar form is that of gelatin capsule, other types, such as almond/egg/olive, cross, cross-head, dumbbell, star, triangular prism exist.

A less common component, but still important, is the stir retriever. The device consists of a magnet embedded in a PTFE rod and has the appearance of an antenna rod. It is used to retrieve the stir bar from liquids, without having to pour out the liquid.

Performance

Advantages

The main advantage of magnetic stirrers is their simple construction, the lack of moving parts and low noise during operation. Since the stir bars are made of PTFE or glass, which are inert, they can be used to stir corrosive solutions. Since glass and most non-ferrous materials have poor magnetic permeability, this allows for stirring in sealed systems, making this method vital in air-free processes. Magnetic stirrers are also easier to clean than other stir devices and since do not use rotating parts there is no risk of leaking lubricant in the stirred liquid.

Disadvantages

Magnetic stirring cannot be used at very high temperatures, as the protective coating of the stir bars will degrade. Very large volumes of liquids cannot be stirred properly, nor do viscous liquids or thick suspensions.

Availability

Magnetic stirrers, along with stir bars and stir retrievers can be purchased from most lab suppliers. They can also be found on eBay and Amazon.

DIY magnetic stirrer

A simple magnetic stirrer can be made by simply gluing two or more magnets (depending on the choice and size) on the rotating part of an electric motor. A computer fan can be used as electric motor. To control the speed of the fan, a voltage regulator should be added to the circuit. A good tutorial can be found here.

If you want to make a hot-plate magnetic stirrer, you will first need to properly separate the stirring part from the heating element, otherwise the heat will slowly degrade the magnet over time. For the hotplate, a heating element, either home made from Nichrome or an already existing hot-plate heating element is added inside a metal disk, which will act as the hotplate. To insulate it from the stirring section, an insulator such as mineral wool is added on the bottom of the heating plate. Make a rotating stirrer as explained below, and place it below the heating element, but make sure the insulation does not interfere with the rotating motor. The temperature and rotation speed are controlled through a thermostat and a voltage regulator respectively. Keep in mind that this type of stirrer is not easy to make and if you don't know what you're doing it's best to appeal to the help of an electrician, though it would be simpler to just buy a hotplate magnetic stirrer.

A simple way to make a stir bar is to take a piece of ferromagnetic material, such as a piece of iron, steel or a small magnet and coat it with a protective layer, such as Plasti Dip or Teflon paint. This however, does not offer total inertness and may not work at high temperatures. Unfortunately, PTFE cannot be properly melted, so coating a stir bar in Teflon at home is not possible or may not hold. Sealing the bar in glass is another option, but you will need to leave a bit of space inside, as metals tend to dilate more than glass during heating and there's a risk it may crack the glass coating if not properly done.

Stir retrievers can be done by simply adding a small magnet inside a test tube and dip the tube in the stirring solution to retrieve the stir bar. There's no need to seal the tube. However, a more simple way of retrieving the stir bar is to use a magnet from outside the flask to draw the stir bar through the walls of the flask and drag it to the vessel opening. Do not use very strong magnets, as if they slip from your hand, they might crack the glass.

Projects

  • Stir liquids, duh!

Safety

Since it generates a magnetic field, magnetic stirrers may attract ferrous materials, which may contaminate the stirrer plate. The magnetic field however it's too weak to significantly affect most electronic devices, which means it's safe to handle most electronic devices near it.

See also

References

Relevant Sciencemadness threads