Bubblers provide a pressure relief system within the Schlenk line as well as allowing the inert gas flow rate to be easily monitored. Bubblers act as a one-way valve to allow gas to escape the inert gas manifold, whilst preventing atmospheric air and moisture contaminating reactions on the Schlenk line. Several types of bubbler are commonly used in the laboratory, all of which have their own advantages and disadvantages.
Oil bubblers are the most common bubbler employed in research and teaching laboratories due to their simplicity and low cost. They are filled with silicone or mineral oil, which have high chemical resistance. The major drawback of oil bubblers is their susceptibility to ‘suck back’ when backfilling evacuated flasks with inert gas; this can cause oil to leak into the inert gas manifold unless a secondary vessel is placed between the inert gas manifold and oil bubbler. Alternatively, a one-way check valve that is rated to an appropriate pressure can be installed to prevent oil suck back. Another disadvantage of oil bubblers is the low inert gas pressure due to the low density of the oil; this means that the inert gas exiting the Schlenk line has to be restricted to increase the gas pressure to facilitate cannula transfers and cannula filtrations.
A typical oil bubbler.
Over-pressure or pressure-relief bubblers are oil-based bubblers that contain an adjustable spring-loaded check valve which allows for fine control of the inert gas pressure within the Schlenk line. This design also prevents oil suck back. When used in tandem with a regulator, the over-pressure bubbler can be adjusted to relieve pressure only when exceeding a set limit, meaning that high inert gas pressures can be safely reached, enabling faster cannula transfers and cannula filtrations. The major disadvantage is the high cost when compared to simpler oil bubbler designs.
An over-pressure bubbler.
Mercury bubblers work on a similar principle to oil bubblers but provide much higher inert gas pressures within the Schlenk line due to the high density of the liquid metal. Mercury bubblers are often over 76 cm in height (1 atmosphere can support 760 mmHg) which allows the inert gas manifold to be fully evacuated under vacuum, rather than being flushed or purged with inert gas. The major disadvantage with mercury bubblers is the health and safety issues concerned with handling mercury and disposing of waste. Mercury is also incompatible with many chemicals that may be used on the Schlenk line. Mercury bubblers should only be used in well ventilated fume hoods or when attached to an appropriate exhaust system; furthermore, an additional flask equipped with a gas inlet/outlet adapter can be employed to collect small beads of mercury that may exit the bubbler.
A typical mercury bubbler.