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How To Choose the Right Laboratory Hot Plates

What Is a Laboratory Hot Plate?

The hot plate is an essential, portable, table-top machine used for certain lab procedures which require heating samples.

Hot plates have a flat surface with an internal heating system. The device can reach a temperature as high as 350℃, hot enough to ignite a spectrum of low-boiling solvents like pentane, hexane, diethyl ether, acetone, and low-boiling petroleum ether.[1] Thus, these substances should never be heated with hot plates, especially in an open vessel.

Digital Hotplate without Stirrer

Figure: An image of a hot plate without a stirrer.

Some hot plates come with a magnetic stirrer, making it easier for the heated liquid to swirl continuously and form a homogenous mixture. For example, adding agar to a medium and gently heating it requires that the mix be stirred continuously until completely dissolved.

Hot plates are preferable to hot baths in student labs because hot baths are dangerous and can overheat, spill, or ignite.[2] Also, it takes more time to cool down, requires expensive mantles, and doesn’t work for all sizes of flasks.

Types of Hot Plates

Based on the design, hot plates are of three types:[3]

  1. Standard hot plate: It’s a machine with only a heating system, this hot plate has no stirrer. However, it requires less maintenance and re-calibration routines.
  2. Magnetic stirrer with hot plates: This hot plate has a heating system and an electromagnet placed under its surface. The electromagnet causes the magnetic bar immersed in the solution to move in a circular motion at a maximum speed of  2,500 rpm.[3] You must note that the heating plate won’t work simultaneously with the magnetic stirrer.
  3. Stirring hot plate: Here, the heating and stirring elements are installed under the machine’s flat top surface. It’s usually the most expensive type, however, it provides uniform heating to high-volume or viscous samples.[3]

Based on the material used in making a hot plate surface, there are:[3]

  • Ceramic hot plates: These plates can withstand a temperature of 350℃. The tops are resistant to corrosion but susceptible to heat shock from metal cylinders.[3] Therefore, always use them to heat glass beakers.
  • Aluminum hot plates: The aluminum tops provide better heat uniformity than any other heating tops, and are more durable and tough to crack. The system is suitable for high-throughput lab operations.[3]
  • Polypropylene hot plates: The plate can’t attain very high temperatures like ceramic and aluminum plates. However, they have strong resistance to chemicals, acids, and solvents. These plates are applied in wet chemistry lab works, such as those that involve acetonitrile, methanol, and ethanol.[3]
  • Stainless steel hot plates: These plates are highly resistant to corrosion, ethers, and most alcohols. They promote aseptic conditions and are suitable for ISO-grade cGMP spaces, clean rooms, and USP-compliant facilities.[3]

Factors to Consider When Purchasing Lab Hot Plates

Here are factors you should consider when choosing hot plates for your lab:[4]

1. Required temperature

The temperature range of different hot plates available on the market is within 250-550℃.[4] However, it’s always recommended to choose an instrument with a temperature range higher than required to heat the sample.

For example, if you need to heat your samples at 250℃, go for a hot plate that provides a maximum temperature of 300℃ or more.

Also, the working temperature of hot plates is influenced by the plate surface’s material:[5]

  • If required to work at a temperature of 300-400℃, use a hot plate with an aluminum surface. They have better conductivity and temperature uniformity.
  • For working temperatures ranging from 400-500℃, use a hot plate made of ceramic glass or iron.
  • If you need a working temperature above 600℃, use a hot plate made of titanium.[5]

2. Hot plate size

Hot plates are available in different sizes and dimensions. It’s recommended to use a plate larger than the size of the vessel you will be working with, though there are several highly efficient mini-hotplate stirrers up for purchase.[5]

However, a larger plate has high power consumption and requires a three-phase power supply.[5] Therefore, purchase the instrument while considering the facilities of your lab.

3. Temperature control

Hot plates come with two types of temperature control:

  • Analog thermostat: Purchase a hot plate with this system when temperature accuracy is not essential.[5]
  • Digital temperature control: This system will provide you precise control over temperature setting and multiple options to set heating rate and timer.[5]

4. Temperature uniformity

If you seek temperature uniformity, purchase a digital hot plate with a stirrer. They are excellent in heat distribution.[5] The temperature uniformity can be checked using a temperature meter and a surface tracker.

5. Capacity

The capacity of the sample subject for heating is an essential factor when purchasing a hot plate. The capacity of the device available on market ranges from 20 liters to 200 liters.[4]

6. Safety features

Safety is a primary concern while working with instruments like hot plates. A range of safety features that come with the device include:[4]

  • Speed ramping to avoid splashing of materials being heated when increasing the speed.
  • Stir protection to shut down the machine automatically in case the stirrer motor fails or stops.
  • Hot top warning indicator to indicate the hot status of the plate.
  • Current limit protection to protect against excessive current.
  • Safety shield to protect against spills, splashes, and aerosols.
  • Probe protection to shut off the heater automatically when the probe disengages from the sample.
  • High-temperature setting to program the maximum temperature the hot plate can reach.[4]

7. Sample viscosity

If working with a viscous sample, then purchase hot plates with high-powered drive magnets. Note that a bigger-sized hot plate can’t handle the high viscosity of a sample. A unit with magnetic coupled strength reduces the chance of the stir bar spinning out.[4]

How To Use Hotplates Safely (Precautions)

Hot plates are not as dangerous as open flame heating methods. However, there are certain precautions that you need to take when using the equipment:[6]

  • Take precautions while heating flammable organic liquid. Do not heat them in an open vessel as their vapor can spill and ignite while coming in contact with any other heating element. Always use a condenser. However, a water bath is recommended to heat such solvents.

Figure: An illustration of the right approach of using a hotplate while heating flammable organic solvents.[6]

  • The temperature and stirrer control of each hot plate varies with the brand, so, the functions of each control should be distinguished well before use.[1]
  • Before using the instrument, ensure that its “ON” and “OFF” switch is working and the heating mantles of the device are quickly cooled.[6]
  • Ensure that the glass you are using to heat your solution is made of heat-resistant material, such as borosilicates. Do not use glasses made of soda-lime glass or “soft glass” because they can’t tolerate frequent temperature changes and can break easily.[6]
  • Avoid using thick-walled glass vessels as they can heat unevenly which causes them to crack during use.
  • Ensure the hot plate is larger than the heating vessel.
  • Before using the container to heat materials, ensure they are not already cracked or broken.
  • Add solvent or reagents to the heating container using a dropping funnel (closed system) when the hot plate is already hot. It prevents the materials from catching flash fire or moisture absorption.[6]
  • Use thermal tongs or gloves to remove the hot containers from the hot plate.
  • It’s essential to learn the position and working of your lab fire extinguisher to control any accidental fire conditions.[6]
  • Do not use hot plates manufactured before 1984. Purchase models that are explosion-proof and have more safety features.[7]
  • Do not heat metal pans or foil on the hot plate at 200℃. It can damage the device.[7]
  • Do not heat items beyond the specific temperature mentioned by the manufacturer.
  • Look out for electrical wires and temperature sensor probes touching the hot plate surface when it’s hot.[7]
  • Do not use an extension cord or power strip to plug in the hot plate, always use GFCI (Ground Fault Current Interrupter) outlets.[7]

Conclusion

Hot plates are an essential lab instrument used for heating lab materials and samples. There are several types available in the market, such as those with and without stirring systems. The hot plate system with a stirrer provides more temperature uniformity while heating samples.

Before purchasing the device consider factors like the size, capacity, safety features, and temperature control to procure the right equipment for your labs.

At Conduct Science, we provide you with a range of highly-advanced hot plates with all the safety features intact.

References:

  1. Nichols Lisa. Hotplates. Retrieved from: Chem Libretexts on Hotplates
  2. Hot plate. Retrieved from: https://en.wikipedia.org/wiki/Hot_plate
  3. Laboratory Hot Plates and Magnetic Stirrer Features Comparison Chart. Retrieved from: https://www.laboratory-equipment.com/blog/hot-plate-comparison-for-digital-analog-magnetic-stirring-plates/
  4. Driscoll Aimee (2020). Retrieved from: https://stirrers.net/blogs/blog/factors-to-consider-when-choosing-a-magnetic-or-hotplate-stirrer
  5. https://www.mrclab.com/types-of-laboratory-hot-plates. Retrieved from: https://www.mrclab.com/a-guide-to-choosing-hot-plates-for-the-laboratory
  6. Hot Plate Use and Safety in Laboratory. Retrieved from: https://chem.wisc.edu/wp-content/uploads/sites/1130/2019/12/HotPlateSafety.pdf
  7. Hot plates. Retrieved from: https://ehs.umass.edu/sites/default/files/Hot%20Plates%20Fact%20Sheet.pdf
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