Using a magnetic stirrer requires little setup and is a convenient option, especially when it comes to low viscosity fluid mixing.
Although the setup is simple, you do have some decisions to make, one being which stir bar also known as a stirring flea to use. These differ in size, shape, and material, and there are actually a lot of options to consider. There are many shapes of stir bar available, each suitable for different applications, as described in the table below.
This is a common type of stir bar and is suitable for various applications, usually with flat-bottomed vessels. These ones have a pivot ring around the center. They are designed to maintain the optimum position in vessels with curved or uneven bottoms. The ring helps to reduce vibrations and wear, and reduce the effects of friction from contact with the vessel. Oval or ellipse-shaped stirring bars are ideal for ensuring homogeneous mixing in round-bottomed beakers. This type is good for stable stirring at high speeds.
The cross shape results in additional turbulence for better mixing. The bar sits at the bottom of the vessel, so can also be good for stirring up sedimentation. Crown-shaped stir bars can be ideal for use in a cuvette or test tube. They can be used in other containers as well, so might be a good option if you need to switch between two types.
The unique shape of these bars enables them to create a large amount of turbulence at low speeds. They are ideal for use in vessels with arched bottoms and provide high stability. These bars can provide lots of turbulence even at slow speeds. Images used in this table are sourced from IKA. Stir bars are sometimes available in sets of different shapes, like the one below. This will result in more motion and increase the efficiency of mixing.
The shape and opening size of the vessel could have an impact of the size of the stir bar you choose. For example, for a round-bottomed flask, even if the widest diameter of the vessel is relatively large, a smaller bar will be required to ensure there is no catching on the sides of the flask near the bottom. Some vessels may have very small openings resulting in a desired bar not being able to fit.
While this is rare considering the small size of most bars, if it does happen, you may need to go smaller. The size of the magnet inside the stirrer, which may be referred to as the drive magnet, is of great importance when choosing a stir bar size. The length of the bar should be about equal to the magnet size to avoid spinout.
Spinout occurs when the bar loses its coupling with the internal magnet of the stirrer and the bar stops spinning altogether. Stir bars significantly longer than the internal magnet might reduce the max speed the bar may be spun. This increases drag without providing proportionately stronger magnetic coupling.
Some manufacturer specifications will tell you the maximum recommended length for a stir bar used with the magnetic stirrer. Manufacturer specifications for an IKA Topolino magnetic stirrer.
If exposed to a strong magnetic field, they can become weaker. They are often sold in pairs to avoid this, though they can lose strength through mishandling. Neodymium magnets are stronger than ceramic and Alnico magnets, and have the highest coercivity of all permanent magnet types.
A stir bar magnet is placed inside a container, immersed in the fluid. A separate magnet or magnets is placed underneath the container, so that it attracts to the stir bar magnet. The lower magnet is usually attached to a motor that spins it. If the magnets are close enough, the stir bar magnet spins inside the container. You can purchase a stir-plate mechanism that has a motor and the magnet underneath from laboratory equipment suppliers.
In the video below, we make our own version using an inexpensive fan from a computer and a few magnets. Should you make your own stir-plate or purchase one? That depends both on your budget and how you will use it. Lab quality stir-plates come with built in speed controls that usually work quite well. Lab stir-plates often incorporate a heating element, handy if you need to keep your solution warm.
A homemade stir-plate made with recovered materials like a fan from a PC can be quite inexpensive. This project is popular among home brewers who prepare their own yeast for beer. If you use a lab-quality stir-plate, be sure to check what type of magnet is used inside. A neodymium magnet is the strongest type of permanent magnet, and typically works best with these strong Stir Bars. A ceramic or Alnico magnet inside the stir-plate is much weaker than a neodymium magnet.
While it will still work with the neodymium stir bars shown here, it is possible to demagnetize the ceramic or Alnico magnet inside the stir-plate. Stronger torque translates to greater stirring ability before the magnet-to-magnet pair "slips. The actual strength you'll see depends on a number of interrelated factors, which is hard to quantify in a single number.
Important factors include:. Below is a quick demonstration of how to use a stir bar magnet. The stir-plate mechanism uses a fan from a computer and an old PC power supply as a 12V source. You might use a simple DC power plug for input power rather than a big power supply. Also consider using a potentiometer to adjust the speed. How about a bigger volume of water? What can a little stir bar magnet do inside a 5-gallon bucket?
A lot! While we can't tell you exactly which magnets will work for your application, choose sizes like those used in our videos. A bit of experimentation should yield an inexpensive and effective stirrer. The Stir Bar Magnets come in four sizes:.
In our videos, we used a strong D6C cylinder magnet to retrieve the stir bar from the fluid. Our TIN1 sample package of magnets has a useful collection of magnet sizes and shapes, which we used for the stir plate mechanism. The B block magnets we taped to the fan came from the TIN1 sample pack. For help identifying the poles of magnets, see: Which Pole is North?
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