The vacuum effect of a filter flask is used to filter laboratory samples. A filter flask is an Erlenmeyer flask with a specialized arm on the side. This arm connects to a vacuum pump or aspirator, creating a vacuum and lowering the pressure inside the flask.
These flasks are used for filtering samples because the vacuum forces the filtrate into the flask, while residue remains on the filter paper in the funnel. The funnel used for filtration is called a Buchner funnel. This funnel and a stopper can both be used to maintain the vacuum in the flask; a trap is used to prevent water from the aspirator from entering the flask and contaminating the filtrate.
Filter flasks are also called vacuum flasks, Buchner flasks, side-arm flasks and Kitasato flasks. The name Buchner flask is for its inventor, the German chemist Ernst Buchner. Ernst Buchner's patents for the filter flask and its accompanying funnel date back to 1888. The name Kitasato flask references bacteriologist Shibasaburo Kitasato, the co-discoverer of bubonic plague bacteria.
Filter flasks are produced with heavier walls than standard Erlenmeyer flasks to withstand these pressure changes without imploding. This heavier glass makes filter flasks more expensive than comparable pieces of laboratory glassware.