Laboratory Glassware - Safety When Using Vacuums and Keck Clips

Safety When Using Vacuums and Keck Clips

An absolute vacuum produces a pressure difference of one atmosphere, approximately 14 psi, over the surface of the glass. The energy contained within an implosion is defined by the pressure difference and the volume evacuated. Flask volumes can change by orders of magnitude between experiments. Whenever working with liter sized or larger flasks, chemists should consider using a safety screen or the sash of a flow hood to protect them from shards of glass, should an implosion occur. Glassware can also be wrapped with spirals of tape to catch shards, or wrapped with webbed mesh more commonly seen on scuba cylinders.

Glass under vacuum becomes more sensitive to chips and scratches in its surface, as these form strain accumulation points, so older glass is best avoided if possible. Impacts to the glass and thermally induced stresses are also concerns under vacuum. Round bottom flasks more effectively spread the stress across their surfaces, and are therefore safer when working under vacuum.

When connecting glassware, it is often tempting to use Keck clips on every joint, but this can be dangerous if the system is sealed or the exhaust is in any way restricted; e.g. by wash flasks or drying media. Many reactions and forms of operation can produce sudden, unexpected surges of pressure inside the glass. If the system is sealed or restricted, this can blow the glass apart. It is safer to only clip the joints that need holding together to stop them falling apart and to purposefully leave one or more unclipped; preferably those that are connected to lightweight, small objects like stoppers, thermometers or wash heads, that are pointing vertically upwards and not connected to other items of glassware. By doing so, any significant surge of pressure will cause these specifically chosen tapers to open and vent. This may seem counterintuitive, but it is safer and easier to deal with a controlled escape as opposed to the entire volume being uncontrollably released in an explosion.