Magnetic Stirrer- Principle, Parts, Types, Uses, Examples

A magnetic stirrer is a laboratory device that uses a rotating magnetic field to make a stir bar or rod immersed in a liquid spin very quickly, thus stirring or mixing it . The magnetic field may be created by a rotating magnet or a set of stationary electromagnets under the vessel . A magnetic stirrer may have a heating system to heat the liquid . It is used in chemistry and biology where other forms of stirring may not be viable. It can have different speeds, volume capacities, and positions, and be powered by battery or plug-in.

A magnetic stirrer consists of a magnetic bar placed within the liquid which provides the stirring action. The stir bar`s motion is driven by another rotating magnet or assembly of electromagnets in the stirrer device, beneath the vessel containing the liquid . Stir bars are typically coated with PTFE or glass, so they are chemically inert and do not contaminate or react with the solution they are in . Their shape may vary to increase efficiency during stirring. Their size varies from a few millimeters to a few centimeters. The smaller sizes are often referred to as "fleas".

Magnetic stirrers are simple, silent, and easy to use devices that can mix liquids for various purposes. They can also heat the liquids if needed. They are widely used in laboratories for chemical experiments and synthesis, microbiology, biochemistry, oil analysis, soil suspending, buffer solutions preparation, pH measurement, and more . They have many advantages over other stirring methods, such as preventing intense boiling, boosting reaction rate and evaporation rate, allowing stirring in closed systems without isolation, reducing contamination risk, and occupying less space . However, they also have some limitations, such as being ineffective for dispersing nanoparticles, viscous liquids or thick suspensions, and large volumes .

In this article, we will discuss the principle, parts, types, uses, examples, advantages, limitations, and precautions of magnetic stirrers in detail.

The magnetic stirrer operates on the principles of attraction for opposite charges and repulsion for like charges. The stirring speed is adjustable, and it is frequently used to stir solvents of various viscosities. A micromotor powers a magnet to create a rotating magnetic field that rotates the stirring bar inside the vessel, enabling a thoroughly mixed reaction to take place. The rotating field may be created either by a rotating magnet or a set of stationary electromagnets, placed beneath the vessel with the liquid. It is equipped with a temperature control system that can heat and regulate the sample’s temperature in accordance with experiment requirements, ensuring that the mixed liquid satisfies the experiment’s requirements while keeping the required temperature.

The following diagram illustrates the principle of magnetic stirrer:

 +-----------------+
| |
| Liquid |
| |
| +-----+ | <--- Stirring bar
| | | |
+-----------------+
^ ^
| |
| |
+-----------------+
| |
| Magnet | <--- Rotating or stationary magnet
| |
+-----------------+

A magnetic stirrer consists of two main parts: a stir bar and a hot plate stirrer. The stir bar is a small magnet coated with a non-reactive material, such as polytetrafluoroethylene (PTFE) or glass, that is placed inside the container with the liquid solution. The hot plate stirrer is a device that contains a stationary electromagnet, a hot plate, a temperature control knob, and a speed control knob. The stationary electromagnet generates a rotating magnetic field that causes the stir bar to spin and mix the solution. The hot plate provides heat to the container and can be adjusted by the temperature control knob. The speed control knob regulates the rotation speed of the stir bar according to the viscosity and volume of the solution. Some magnetic stirrers also have a stir bar retriever, which is a magnet attached to a long rod that helps to remove the stir bar from the container after stirring.

The following table summarizes the parts of a magnetic stirrer and their functions:

A magnetic stirrer is a laboratory device that employs a rotating magnetic field to cause a stir bar immersed in a liquid to spin very quickly, thus stirring it. The rotating field may be created either by a rotating magnet or a set of stationary electromagnets, placed beneath the vessel with the liquid.

Some of the characteristics of a magnetic stirrer are:

• Capacity: The capacity of a magnetic stirrer refers to the maximum volume of liquid that can be stirred effectively by the device. The capacity depends on the size and shape of the vessel, the viscosity and density of the liquid, and the power and speed of the stirring motor. Magnetic stirrers can typically stir up to 4 liters of liquid, but some models can handle larger volumes.
• Speed: The speed of a magnetic stirrer is measured in revolutions per minute (rpm) and indicates how fast the stir bar rotates inside the liquid. The speed can be adjusted by a knob or a digital control panel on the device. The speed range varies depending on the model, but it is usually between 100 and 2500 rpm. The optimal speed depends on the type and amount of liquid being stirred, as well as the desired mixing efficiency and homogeneity.
• Temperature: Some magnetic stirrers have a heating function that allows the user to heat up the liquid while stirring it. This is useful for chemical reactions or processes that require a certain temperature range. The temperature can be controlled by a knob or a digital panel on the device, or by an external probe inserted into the liquid. The maximum temperature that can be reached depends on the power and design of the heating element, as well as the thermal properties of the vessel and the liquid. Magnetic stirrers can typically heat up liquids up to 380°C, but some models can go higher or lower.
• Design: The design of a magnetic stirrer includes several components that affect its performance and functionality. Some of these components are:
  • Stir bar: The stir bar is a small magnet coated with PTFE or glass that is placed inside the liquid to be stirred. The shape and size of the stir bar depend on the shape and size of the vessel, as well as the viscosity and density of the liquid. Stir bars are usually bar-shaped and octagonal or circular in cross-section, but they can also have other shapes such as oval, triangular, or star-shaped. Stir bars can range from a few millimeters to several centimeters in length. Some stir bars have a pivot ring around the center that helps them rotate more smoothly and stably.
  • Stirring motor: The stirring motor is the part of the device that generates the rotating magnetic field that drives the stir bar. The motor can be either a permanent magnet that rotates mechanically, or an electromagnet that switches polarity electronically. The motor is usually located beneath the vessel with the liquid, but it can also be placed on the side or above it. The motor determines the speed and power of the stirring action, as well as its noise level and energy consumption.
  • Top plate: The top plate is the part of the device that supports the vessel with the liquid. It can be made of different materials such as stainless steel, ceramic, or aluminum. The material affects the chemical resistance, thermal conductivity, and durability of the top plate. Some top plates have a heating function that allows them to heat up along with

A magnetic stirrer is a simple and easy-to-use device that can mix liquids of different viscosities and temperatures. To use a magnetic stirrer, follow these steps:

• Prepare a container with the liquid solution already inside of it. The container should be made of glass or plastic and have a flat bottom. The volume of the solution should not exceed half of the container`s capacity.
• Place the container on the hotplate of the magnetic stirrer. Make sure the container is centered and stable on the hotplate. If the magnetic stirrer has a temperature sensor, insert it into the solution and secure it with a clamp or a rubber stopper.
• Connect the magnetic stirrer to a power source and turn it on. Adjust the temperature setting knob to set the desired temperature for the solution. Some magnetic stirrers have a digital display that shows the actual and set temperatures. Wait for the hotplate to reach the set temperature before proceeding to the next step.
• Place a magnetic stir bar into the container with the solution. The stir bar should be small enough to fit inside the container and large enough to create a strong stirring action. The shape of the stir bar can vary depending on the type of solution and container. For example, a cylindrical stir bar is suitable for round-bottomed flasks, while an oval or cross-shaped stir bar is better for flat-bottomed containers.
• Turn the speed control knob to adjust the rotation speed of the stir bar. Start with a low speed and gradually increase it until a vortex is formed in the solution. Avoid setting the speed too high, as this may cause splashing, noise, or loss of coupling between the stir bar and the magnet. The optimal speed depends on several factors, such as the viscosity, volume, and temperature of the solution, as well as the size and shape of the stir bar and container.
• Stir the liquid solution until it is uniformly mixed and reaches the desired temperature. You can monitor the temperature by using a thermometer or by reading the display of the magnetic stirrer. You can also add other reagents or samples to the solution while stirring, but be careful not to disturb or dislodge the stir bar.
• Turn off the speed control knob and wait for the stir bar to stop spinning. Then turn off the temperature setting knob and disconnect the power source. Use hot hands or gloves to lift the container from the hotplate. Be careful not to spill or burn yourself with the hot solution.
• Remove the magnetic stir bar from the container using a stir bar retriever or another magnet. Do not use your fingers or metal tools to remove the stir bar, as this may damage it or contaminate it with foreign substances. Rinse and dry the stir bar before storing it in a clean place.
• Clean and dry the container and the hotplate after use. Dispose of or store the solution according to safety regulations and standard procedures.

By following these steps, you can use a magnetic stirrer to mix liquids efficiently and safely in your laboratory experiments. Magnetic stirrers are versatile and reliable devices that can help you perform various tasks such as titrations, extractions, dissolutions, suspensions, and more.

The choice of size, configuration, and applications of magnetic stirrers are used to classify them. Some of the types of magnetic stirrers are as follows:

• Magnetic Mini Stirrer: The laboratory magnetic stirrers for use in laboratories are compact and require less space. They have electronic controls that enable the user to precisely manage the speed. It can withstand destructive and corrosive substances in the lab. The speed regulator manages the rotation per minute (rpm) of the stirrer.

• Magnetic Stirrer with Timer: The motor is automatically turned off after a certain time, thanks to a magnetic stirrer with a timer. A built-in timer will turn the stirrer off when the pre-selected duration of time has elapsed. Additionally, the speed is automatically decreased, and the weight is automatically removed.

• Heavy-Duty Magnetic Stirrer: High mixing capacities are included with the heavy-duty magnetic stirrers. They are also very robust and exhibit good chemical resistance. They may be the ideal instrument for the lab to use for precise findings. The majority of chemical reactions require heated magnetic stirrers.

• Battery Powered Magnetic Stirrer: They can be utilized in places with no electricity or other restricted environments. They are mostly used in incubators, operate on batteries, and have rubber feet to provide stability. They are portable and lightweight to carry.

• Air-operated Turbine Magnetic Stirrer: The air-operated turbine magnetic stirrer is the ideal tool for blending liquids up to 1 liter of solution. A low-pressure air supply drives this magnetic stirrer. The crucial feature of this magnetic stirrer is that it eliminates the risk of sparking from electrical sources.

• Multi-position Magnetic Stirrer: This type of magnetic stirrer can simultaneously stir multiple samples in different containers. It has a large surface area that can accommodate several vessels with different shapes and sizes. It is suitable for parallel experiments or high-throughput applications.

• Magnetic Stirrer with Hot Plate: This type of magnetic stirrer combines the functions of stirring and heating. It has a hot plate that can heat up the liquid solution to a desired temperature while stirring it. It also has a temperature sensor that can monitor and control the temperature of the solution.

• Magnetic Stirrer without Hot Plate: This type of magnetic stirrer does not have a heating function. It is used for stirring solutions that do not require heating or that are sensitive to heat. It is cheaper and simpler than the magnetic stirrer with hot plate.

Magnetic stirrers are widely used in various fields of science and industry to perform mixing, dissolving, heating, or cooling operations. Some of the common applications of magnetic stirrers are:

• Chemistry: Magnetic stirrers are essential tools for chemists to conduct experiments involving chemical reactions, synthesis, analysis, titration, extraction, crystallization, and more. They can mix different types of solvents and reagents at controlled temperatures and speeds, ensuring uniformity and homogeneity of the solutions. They can also prevent bumping and boiling over of volatile liquids by providing gentle and continuous stirring.
• Biology: Magnetic stirrers are useful for biologists to prepare culture media, buffers, solutions, suspensions, and samples for various biological experiments. They can also be used to perform cell culture, DNA extraction, enzyme assays, protein purification, immunological tests, and more. They can maintain optimal conditions for the growth and viability of microorganisms and cells by providing aeration and agitation.
• Pharmacy: Magnetic stirrers are employed by pharmacists to prepare pharmaceutical formulations, such as creams, ointments, gels, lotions, syrups, suspensions, and emulsions. They can also be used to perform quality control tests, such as dissolution, stability, viscosity, and pH measurements. They can ensure the consistency and efficacy of the products by providing thorough mixing and blending.
• Food and beverage: Magnetic stirrers are used by food and beverage industries to process and analyze various food products, such as milk, yogurt, cheese, wine, beer, juice, sauce, jam, honey, and more. They can also be used to perform food safety tests, such as microbial count, acidity, moisture content, and antioxidant activity. They can enhance the quality and flavor of the products by providing proper mixing and heating.
• Environmental: Magnetic stirrers are used by environmental scientists to monitor and evaluate the quality of water, soil, air, and waste samples. They can also be used to perform environmental remediation processes, such as biodegradation, adsorption, precipitation, and oxidation. They can help in the detection and removal of pollutants and contaminants by providing efficient stirring and extraction.

These are some of the examples of how magnetic stirrers are applied in various domains. However, they are not limited to these fields only. Magnetic stirrers can be used for any purpose that requires mixing or heating of liquids in a safe and convenient manner. They are versatile and reliable instruments that can meet the needs of different users and applications.

Magnetic stirrers offer several benefits over other types of stirring devices, such as mechanical stirrers or shakers. Some of the advantages are:

• Temperature control: Magnetic stirrers can heat and regulate the temperature of the sample according to the experiment requirements, ensuring that the mixed liquid meets the desired conditions. Some magnetic stirrers also have a temperature probe that can measure the actual temperature of the sample and adjust the heating accordingly.
• No contamination: Magnetic stirrers use a stir bar that is coated with a non-reactive material, such as PTFE (teflon), that prevents any contact between the stirrer and the sample. This eliminates the risk of contamination or corrosion that may occur with mechanical stirrers that have metal parts or seals.
• High efficiency and precision: Magnetic stirrers can achieve a uniform and thorough mixing of liquids with different viscosities and densities. They can also adjust the stirring speed precisely and maintain a constant speed throughout the experiment. Magnetic stirrers can also handle small volumes of liquid and create a vortex that enhances the mixing action.
• Low noise and maintenance: Magnetic stirrers have no moving external parts, which means they produce less noise and vibration than mechanical stirrers. They also require less maintenance and cleaning, as they do not have any wear and tear or leakage issues. Magnetic stirrers are also compact and easy to use, requiring only a power source and a knob to control the speed and temperature.
• Long-lasting operation: Magnetic stirrers can run for long periods of time without overheating or losing efficiency. They can also perform stirring tasks that require continuous or overnight mixing, such as protein dialysis or extraction. Magnetic stirrers are also durable and resistant to chemical spills and splashes, as they have a protective casing and a spill-resistant design.

Despite the many advantages of magnetic stirrers, they also have some limitations that need to be considered before using them. Some of the limitations are:

• Magnetic stirrers are not effective for stirring viscous liquids or thick suspensions, as they may cause the stir bar to decouple from the magnetic field and stop spinning. For such liquids, mechanical stirrers or overhead stirrers may be more suitable.
• Magnetic stirrers are not suitable for stirring large volumes of liquids (more than 4 liters), as they may not provide enough torque to rotate the stir bar at a sufficient speed. For larger volumes, multiple magnetic stirrers or other types of stirrers may be required.
• Magnetic stirrers may generate heat and noise during operation, which may affect the temperature and quality of the sample. Therefore, it is important to monitor the temperature and speed of the magnetic stirrer and use a water bath or an ice bath to control the temperature if needed.
• Magnetic stirrers may interfere with other magnetic or electronic devices in the vicinity, such as compasses, watches, phones, computers, etc. Therefore, it is advisable to keep such devices away from the magnetic stirrer and use a shielded container for the sample if possible.
• Magnetic stirrers may cause erosion or corrosion of the stir bar or the container due to friction or chemical reactions. Therefore, it is important to choose a stir bar and a container that are compatible with the sample and resistant to wear and tear. It is also recommended to clean and dry the stir bar and the container after each use.
• Magnetic stirrers may pose a risk of fire or electric shock if not used properly or maintained regularly. Therefore, it is essential to follow the manufacturer`s instructions and safety precautions when using a magnetic stirrer. It is also advisable to use a grounded outlet, avoid contact with water or flammable liquids, and turn off the power when not in use.

A magnetic stirrer is a useful device for mixing liquids in a laboratory, but it also requires some precautions to ensure safety and efficiency. Here are some tips to follow when using a magnetic stirrer:

• Choose the appropriate size and shape of the stir bar for the container and the liquid. The stir bar should be about 0.5-1.0 cm smaller than the diameter of the container and should match the shape of the bottom. For example, a round-bottomed flask needs a pivot ring stir bar, while a flat-bottomed beaker needs an oval or octagonal stir bar.
• Place the container on the center of the hot plate and make sure it is stable and balanced. Avoid using containers that are too large or too heavy for the hot plate, as they may cause instability or damage to the device.
• Adjust the speed and temperature gradually and carefully. Start with a low speed and increase it slowly until the desired stirring action is achieved. Do not exceed the maximum speed or temperature recommended by the manufacturer, as this may cause overheating, splashing, or loss of stirring. Monitor the speed and temperature throughout the experiment and adjust them as needed.
• Use a thermometer or a thermocouple to measure the actual temperature of the liquid, not the temperature displayed on the hot plate. The temperature of the hot plate may differ from the temperature of the liquid due to heat transfer and evaporation. Also, use a water bath or an oil bath to heat liquids that are flammable, volatile, or sensitive to high temperatures, as direct heating may cause fire or decomposition.
• Do not leave the magnetic stirrer unattended for long periods of time, especially when heating liquids. Check the device regularly for any signs of malfunction, such as smoke, sparks, noise, or odor. Turn off the device and disconnect it from the power source if any problem occurs. Also, use a timer or an alarm to remind you of the end of the experiment.
• Wear appropriate personal protective equipment (PPE), such as gloves, goggles, lab coat, and shoes, when handling liquids that are corrosive, toxic, or hazardous. Avoid direct contact with hot surfaces or liquids and use tongs or hot hands to move containers. Clean up any spills or splashes immediately and dispose of waste properly.
• Remove the stir bar from the container after stirring by using a magnetic retriever or a magnet. Do not use your fingers or metal tools to fish out the stir bar, as this may cause injury or contamination. Wash and dry the stir bar after each use and store it in a clean and dry place.
• Clean and maintain the magnetic stirrer according to the manufacturer`s instructions. Wipe off any dust or dirt from the device with a damp cloth and disinfect it if necessary. Do not use abrasive or corrosive cleaners that may damage the device. Lubricate any moving parts if needed and replace any worn or broken parts.

Following these precautions will help you use a magnetic stirrer safely and effectively in your laboratory experiments. A magnetic stirrer is a valuable tool for mixing liquids, but it also requires some care and attention to ensure its proper functioning and longevity.

There are many types and models of magnetic stirrers available in the market, each with different features and specifications. Here are some examples of magnetic stirrers that can be used for various purposes in laboratories and industries.

• Hotplate Magnetic Stirrer BS series (Biobase): This is a type of magnetic stirrer that can also heat the liquid solution while stirring it. It has a digital display that shows the temperature and speed settings. It can stir and heat up to 20 liters of liquid at a maximum temperature of 380°C and a maximum speed of 2000 rpm.

• Mini Magnetic Stirrer (Thomas Scientific): This is a portable and lightweight magnetic stirrer that occupies a small space on the benchtop. It has a quiet operation and minimal vibration that reduces the noise level in the lab. It is ideal for stirring small volumes of liquid for general research, life science, industry, quality analysis, and clinical care. It can stir up to 1.5 liters of liquid at a maximum speed of 1500 rpm.

• Magnetic Stirrer JBQ-XJ-MS236 (Biometer): This is a simple and practical magnetic stirrer that has a knob operation for adjusting the stirring speed. It can mix liquids uniformly and support various experimental applications. It has a built-in lithium battery that provides a long battery life of 48 hours, making it suitable for mobile use in different fields. It can stir up to 1 liter of liquid at a maximum speed of 3000 rpm.

• Magnetic laboratory stirrer Cimerac™ Basic (Thermo Scientific): This is a low-cost magnetic stirrer that is suitable for everyday use in the lab. It can stir liquids at speeds ranging from 100 to 2500 rpm. It has control knobs for precisely adjusting the speed and a redesigned top-plate that reduces the risk of breakage. It also has a built-in ring-stand holder that can fit a 1.3 cm diameter support rod.

• Magnetic laboratory stirrer MST (VELP Scientifica): This is a strong, small, and versatile magnetic stirrer that features a brushless motor and a techno-polymer casing that ensures high chemical resistance. It can accept flasks up to 5 liters and stir liquids at speeds ranging from 50 to 1700 rpm. It is designed for long-lasting performance and comes with a three-year warranty. It is ideal for microtitration, BOD, microbiology, and biochemistry applications.

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