Tech Team · Kintek Solution

The temperature at which a rotary evaporator evaporates methanol depends on the operating pressure and the desired vapor temperature. Methanol has a boiling point of 64.7°C at atmospheric pressure (760 mmHg), but under vacuum conditions, its boiling point drops significantly. For example, at a pressure of 95 mbar, methanol boils at approximately 25°C, and at 123 mbar, it boils at around 30°C. The rotary evaporator's vacuum system allows for precise control of pressure, enabling evaporation at lower temperatures to protect heat-sensitive materials. The bath temperature is typically set about 20°C higher than the desired vapor temperature to ensure efficient evaporation while minimizing the risk of bumping. For methanol, this means a bath temperature of around 45–50°C for a vapor temperature of 25–30°C. The condenser is usually set to around 0°C to ensure effective condensation of the methanol vapor.

Key Points Explained:

1. Boiling Point of Methanol Under Vacuum:

   • At atmospheric pressure (760 mmHg), methanol boils at 64.7°C.
   • Under vacuum conditions, the boiling point decreases significantly. For example:
     • At 95 mbar, methanol boils at approximately 25°C.
     • At 123 mbar, methanol boils at around 30°C.
   • This reduction in boiling point is crucial for protecting heat-sensitive materials and speeding up the evaporation process.

2. Role of Vacuum in Rotary Evaporators:

   • Rotary evaporators operate under high vacuum conditions (typically around 1 torr or 1.33 mbar).
   • The vacuum pump reduces the pressure inside the system, lowering the boiling points of solvents like methanol.
   • This allows evaporation to occur at much lower temperatures than at atmospheric pressure.

3. Temperature Settings for Methanol Evaporation:

   • Vapor Temperature: The desired vapor temperature for methanol evaporation is typically 25–30°C, depending on the pressure.
   • Bath Temperature: The water bath temperature is usually set about 20°C higher than the vapor temperature to ensure efficient heat transfer. For methanol, this means a bath temperature of around 45–50°C.
   • Condenser Temperature: The condenser is typically set to around 0°C to ensure effective condensation of methanol vapor.

4. 20/40/60 Rule:

   • This rule suggests setting the bath temperature about 20°C higher than the desired vapor temperature and the condenser temperature about 20°C lower.
   • For methanol, this translates to:
     • Vapor temperature: 25–30°C.
     • Bath temperature: 45–50°C.
     • Condenser temperature: 0°C.

5. Importance of Rotation and Thin Film Formation:

   • The rotary flask rotates at a constant speed, creating a thin film of the process medium on the inner wall.
   • This thin film ensures uniform heating and increases the surface area for evaporation, speeding up the process.
   • Rotation also reduces the risk of bumping, which can occur if the liquid boils too vigorously.

6. Practical Operating Procedure:

   • Lightly grease all joints to ensure a proper seal.
   • Connect the chiller and vacuum pump.
   • Set the heat bath to the desired temperature (45–50°C for methanol).
   • Turn on the rotovap motor at approximately 100 RPM.
   • Start the vacuum pump and inject 20% of the total liquid volume into the rotovap.
   • Adjust the injection valve to match the input speed to the output speed, maintaining a stable chiller temperature.
   • Once stable, the rotovap can complete evaporation without supervision.

7. Safety Considerations:

   • Lower temperatures reduce the risk of bumping, which can cause sudden boiling and splashing of the solvent.
   • Always ensure the condenser is properly cooled to prevent solvent loss and ensure efficient condensation.
   • Monitor the vacuum pressure to maintain consistent evaporation conditions.

By following these guidelines, methanol can be efficiently evaporated using a rotary evaporator at temperatures significantly lower than its atmospheric boiling point, ensuring the safety and integrity of heat-sensitive materials.

Summary Table:

Parameter	Value
Boiling Point (760 mmHg)	64.7°C
Boiling Point (95 mbar)	~25°C
Boiling Point (123 mbar)	~30°C
Vapor Temperature	25–30°C
Bath Temperature	45–50°C
Condenser Temperature	0°C
20/40/60 Rule	Bath: 20°C > Vapor, Condenser: 20°C < Vapor

Need help optimizing your rotary evaporator setup? Contact our experts today for tailored solutions!