Hot Plate Usage and Heating Reactions

Burns: The hot plate emits heat while operational and for a period after shutdown. With temperatures reaching up to 500°C, severe burns can occur. 

Electric Shock: The power cord contacting the surface of the hot plate can lead to insulation melting and subsequent electric shock. 

Fire Hazard: 

• Older hotplate models may pose a risk of electrical sparks from switches or thermostats, potentially igniting combustible materials. 

• Caution is advised when heating flammable substances. 

• Hot/stirrer plates pose an additional risk if the wrong feature is activated accidentally. 

• Hot plates lack explosion-proof or intrinsically safe design. 

Equipment Selection and Maintenance: 

• Utilize only hot plates approved by a Nationally Recognized Testing Laboratory (NRTL) (e.g., UL certified) 

• Review manufacturer instructions and consider device registration for timely notifications regarding warnings or recalls. 

• Inspect equipment before each use, remove damaged equipment from service 

Glassware and Setup: 

• Use heat-resistant glassware and check for cracks before use 

• Ensure hot plate surface exceeds vessel size to prevent spills 

• Add boiling stones when heating liquids 

• Exercise caution when condensing materials to prevent vessel cracking 

Operation: 

• Keep electrical cords away from hot plate surface 

• Turn off hot plate when not in use 

• Minimize unattended or overnight operation; consider timers or auto-shutoff 

• Use medium to medium-high settings for heating liquids 

• Verify heating element is off when only stirring 

Safety Measures: 

• Use fume hoods for heating flammable liquids 

• Provide secondary containment for heated flammable liquids to prevent spills 

• Avoid heating strong oxidizing materials in oil baths to prevent reactions 

Several research institutions have reported incidents involving hot plates, with some leading to significant damage. The following hot plates are known models to have issues with spontaneous and/or runaway heating or other safety concerns, like “OFF” not disconnecting power to the heating element.  

• Corning: PC-35, PC-351, PC-200, PC-220, PC-320, PC-400D, PC-420, PC-420D 
• VWR: 7x7 Aluminum Top 
• Troemner: 97042-714 Professional Ceramic 7x7, 97042-642 Advanced Ceramic 7x7 
• Fisher: Isotemp 11- 600-49H, Isotemp 11-700-49H 
• Chemglass: Optimag-St CG1994-10, Optimag-St CG1994-50, Optimag-St CG1993-T-50 
• Cimarec*: SP46925, H-4954.xx 

* Sold under Thermolyne, Barnstead/Thermolyne, and Fisher names depending on age. 

Notes: 

• This is not an exhaustive list and other models may also have safety issues. 

• Exposure to elements such as cold, moisture, corrosive gas, and vapor are known to increase the chance of processor failures, although problems have also been observed in nearly new equipment. 

• Hot plates manufactured before 1984 do not have temperature feedback controls and can spontaneously and rapidly heat beyond the set temperature while in the ‘ON’ position. 

• Stir-only plates may generate heat and even overheat and cause fires while stirring viscous liquids or mixtures with solids due to under-powered motors.   

Conducting experiments at elevated temperatures significantly increases the risk of fire, explosion, over-pressurization, and chemical exposure. Assess all hazards before beginning experiments at elevated temperatures. Consider the chemicals’ boiling points, flashpoints, and potential for thermal decomposition temperature, as these will determine the temperature to which you can safely heat the materials. Evaluate not just the starting materials, but also the products and byproducts of side reactions for their thermal stability. Also, take into account the reactivity between materials and determine if there is a potential for runaway reaction, production of gas, or explosion. If you need assistance assessing the properties of your materials or reactivity hazards, consult your Principal Investigator or contact the Chemical Hygiene Officer. There are certain chemicals that should not be heated, for example pyrophoric compounds, strong oxidizers, and peroxides.  Never heat any mixture or solution that evolves vapors in a closed system unless you have taken into account the pressure that will be produced as a result. When scaling up a reaction, reevaluate the hazards, especially if it is more than triple the original scale.

Set-Up Considerations

• Heating Methods:
  • Hot plates, heating mantles, heating blocks
  • Never use an open flame (e.g. Bunsen burner) to heat a reaction
• Equipment Setup:  
  • Ensure heat source can be removed easily without disturbing the reaction mixture
  • Use lab jacks and clamps for quick heat source removal
• Stirring:  
  • Incorporate a stirring mechanism for even heating
  • Consider mixture viscosity and the importance of even mixing
  • Choose appropriate magnetic stir bar size
  • Consider whether mechanical overhead stirring is needed
• High-Temperature Reactions Involving Solvents:  
  • For temperatures near or above solvent boiling point, extra controls should be utilized to prevent vaporization and pressure build-up
  • Use a condensing apparatus and continuously vent through a bubbler
• Reactions Requiring Inert Atmospheres: 
  • Use a Schlenk line with exit bubbler (vs. balloon) for venting
• Gas-Evolving Reactions and Distillations: 
  • • Only use hose/adapter attachments
    • Avoid constrictions (e.g., needles, stopcock connectors)
    • Use non-combustible materials for insulating distillation pots

Engineering Controls 

Always use the following engineering controls to reduce risk: 

• Temperature Regulation:  
  • Opt for thermocouple/thermostat-controlled heating systems when available 
  • For critical temperature-dependent reactions, use a multi-neck flask with a thermometer adapter to directly monitor the mixture's temperature 
• Secure Setup:  
  • Firmly clamp the thermocouple probe within the heating bath medium 
  • Fasten condenser tubing with wire to prevent leaks or flooding 
  • Caution: Avoid mixing water and hot oil to prevent dangerous splattering 
• Accessibility:  
  • Utilize a lab jack for swift removal of the heating bath from the reaction vessel 
• Additional Precautions:  
  • Evaluate the need for a blast shield based on potential hazards 

Administrative Controls

Always use the following work practices to reduce risk: 

• Pre-Heating Preparations: 
  • Clear the fume hood of unnecessary flammable and combustible materials and keep them away from the heating apparatus 
• During Heating: 
  • Temperature Control:  
    • Allow the reaction to reach and stabilize at the target temperature before leaving the area 
    • Utilize built-in hotplate timers (if available) or separate audible timers to manage heating duration and prompt regular checks 
  • Labeling:  
    • Clearly mark stir and heat control knobs for easy identification 
  • Supervision:  
    • Never leave heated reactions unattended 
    • If the hot plate must be used while unattended, make a plan for reducing the risks 
• Post-Heating Safety: 
  • Allow complete cooling to room temperature before handling the reaction 
  • Unplug heating equipment when not in use or use an appliance timer to ensure that the heat source is disconnected from power when the lab is unoccupied  

Wear this personal protective equipment: 

• Lab coat with sleeves fully extended to the wrists 

• Safety glasses 

• Gloves, compatible with the hazardous material being processed 

• Heat-resistant gloves are recommended for handling hot glassware or baths 

• Full-length pants 

• Closed-toe shoes 

Related links: 

• Personal Protective Equipment 

• PPE Support for Researchers 

• University of Pennsylvania – Fact Sheet: Heating Reactions 
• University of Pennsylvania – Hot Plate Malfunctions and Misuse 
• NDSU - Safety Flash: Hot Plate Fire 
• Catching Up With Runaway Hot Plates