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 perhaps the most common bubblers employed in research and teaching laboratories, primarily 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, or when cooling flasks without first increasing the gas flow. This can easily contaminate the reaction vessel with atmospheric air and moisture, or even suck oil into the inert gas manifold of the Schlenk line. Another disadvantage of oil bubblers is the reduced inert gas pressures in the Schlenk line due to the low density of the oil. This means that cannula transfers and cannula filtrations are typically quite slow.
Over-pressure or pressure-relief bubblers are oil-based bubblers that contain a spring-loaded valve mechanism 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 compared to simpler oil bubbler designs.
Mercury bubblers work on a similar principle to oil bubblers but provide much higher inert gas pressures within the Schlenk line. The pressure can be manually controlled to increase the gas flow during certain manipulations by partially blocking the exhaust tubing with a finger or thumb. 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. Note: Appropriate safety and containment measures must be in place when using a mercury bubbler.