Laboratory Safety Manual
Chapter 2: General Safety
Section 2.1 - Operational Rules
E. Vacuum Operations
In an evacuated system, the higher pressure is on the outside, rather than the
inside, so that a break causes an implosion rather than an explosion. The resulting
hazards consist of flying glass, spattered chemicals, and possibly fire.
A moderate vacuum, such as 10 mm Hg, which can be achieved by a water aspirator,
often seems safe compared with a high vacuum, such as 10-5 mm Hg. These numbers are
deceptive, however, because the pressure differences between the outside and inside are
comparable. Therefore any evacuated container must be regarded as an implosion hazard.
When working with a vacuum be aware of
implosion hazards. Apply vacuum only to glassware specifically designed for this purpose,
i.e., heavy wall filter flasks, desiccators, etc.
Never evacuate scratched, cracked, or etched glassware. Always check
for stars or cracks before use.
Vacuum glassware which has been cooled to liquid nitrogen
temperature or below should be annealed prior to reuse under vacuum.
Rotary evaporator condensers, receiving
flasks, and traps should be taped or kept behind safety shields when under a vacuum.
All condensers connected to rotary evaporators should at least be
cooled with circulating ice water.
The use of a vacuum for the distillation of the more volatile
solvents, e.g. ether, low boiling petroleum ether and components, methylene chloride,
etc., should be avoided whenever possible. In situations requiring reduced pressure, two
alternatives should be considered; 1) Utilization of Rotovac System, or 2) Solvent
recovery via atmospheric pressure distillation (preferred method).
Water, solvents, or corrosive gases should not be allowed to be
drawn into a building vacuum system.
When a vacuum is supplied by a compressor or vacuum pump to distill
volatile solvents, a cold trap should be used to contain solvent vapors. Cold traps should
be of sufficient size and low enough temperature to collect all condensable vapors present
in a vacuum system. If such a trap is not used, the pump or compression exhaust must be
vented to the outside using explosion proof methods.
After completion of an operation in which a cold trap has been used,
the system should be vented. This venting is important because volatile substances that
have been collected in the trap may vaporize when the coolant has evaporated and cause a
pressure buildup that could blow the apparatus apart.
After vacuum distillations, the pot residue must be cooled to room
temperature before air is admitted to the apparatus.
All desiccators under vacuum should be completely enclosed in a
shield or wrapped with friction tape in a grid pattern that leaves the contents visible
and at the same time guards against flying glass should the vessel collapse. Various
plastic (e.g., polycarbonate) desiccators now on the market reduce the implosion hazard
and may be preferable.
F. Handling Glassware
Glass breakage is a common cause of injuries in laboratories. Only
glass in good condition should be used.
Discard or send for repair all broken, chipped, starred or badly
scratched glassware. Hand protection should be used when picking up broken glass. For
disposal of broken glass see Section 2.5 - "Safety Practices for Disposal of Broken Glassware".
Clean all glassware before sending for repair.
When using glass tubing, all ends
should be fire polished. Lubricate tubing with glycerin or water before inserting into
rubber stoppers or rubber tubing.
Protect hands with leather gloves when inserting glass tubing. Hold
elbows close to the body to limit movement when handling tubing.
Do not store glassware near the edge
of shelves. Store large or heavier glassware on lower shelves.
Use glassware of the proper size. Allow at least 20% free space.
Grasp a three-neck flask by the middle neck, not a side neck.
Do not attempt to catch glassware if it is dropped or knocked over.
Conventional laboratory glassware must never be pressurized.
General Safety Safety Equipment