TN Lab Supply Heavy-Wall Conical Borosilicate 3.3 Glass 2,500ml 2.5L Filter Flask. Also known as vacuum flasks or Büchner flasks, have heavier walls than a standard conical flask to endure effects of vacuum filtration. Side arm with barb allows for flexible hosing to connect to a vacuum pump. During filtration, the filtrate enters and is held in the flask while the sample or residue remains on the filter paper in the funnel. Can also be used as a vacuum trap in a vacuum line. Highly leach resistant to most chemicals.