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Vacuum Distillation Apparatus Diagram (Cold Trap, Fractions, Labeled)
Vacuum distillation lowers the pressure so that high-boiling or heat-sensitive compounds distill at a lower temperature.
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Equipment
- Two-neck flask
- Distillation head
- Thermocouples or thermometers (vapor and pot temperature)
- Liebig condenser
- Two-way vacuum adapter (switches between forerun and main fraction)
- Receivers (forerun, main fraction)
- Cold trap
- Vacuum controller (or gauge)
- Vacuum pump
- Oil bath, magnetic stirrer and stir bar
- Chiller
Setup tips
- Boiling chips do not work under vacuum (the air in their pores is pumped out). Stir with a stir bar, or bleed in fine bubbles through a capillary, to prevent bumping.
- Put a cold trap in front of the pump so solvent vapor never reaches it.
- Apply the vacuum first, then heat. To finish, stop heating and let it cool before slowly letting the air back in.
- Never use cracked or scratched glassware (it can implode). Use round-bottom or pear-shaped flasks; never evacuate Erlenmeyer or flat-bottom flasks.
- Use a two- or three-way vacuum adapter to switch receivers for the forerun and main fraction without breaking the vacuum.
- Boiling points under vacuum depend strongly on pressure. Record the pressure together with the head temperature (for example, benzaldehyde boils at about 62 °C at 10 mmHg).
Common mistakes
- Relying on boiling chips under vacuum (it bumps).
- Letting air in while the flask is still hot (bumping, suck-back, oxidation).
- Running the pump without a trap, so solvent ruins the pump oil.
- Evacuating an Erlenmeyer or flat-bottom flask.
Always follow your institution's safety rules and your supervisor's instructions.
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