Making a bleed for fractionation.
Updated: Dec 20, 2022
To make a bleed for a vacuum distillation you will require a 'Quickfit' or similar air-inlet tube fitted with a B19 cone with a 90° bend and screwthread connector.
Firstly remove the plastic screw thread connector and wearing heat resistant gloves, grasp the air-inlet tube at the top. Hold the tube section of the bleed horizontally in a Bunsen burner flame that has been set on maximum heat. The position on the glass tube where the heat is applied should be approximately 1”- 2” from the lower end and the tube should be moved back and forth and rotated during this heating process: this motion spreads the heat and prevent thermal shock to the glassware. This glass tube should be positioned just above the top of the inner flame cone within the Bunsen flame, i.e. the hottest point, to maximise the heating.
When the glass is red hot and the end of the tube is becoming molten and just starting to sag, grip the far end of the bleed tube with your other gloved hand and rotate to prevent the glass drooping. Immediately step back and quickly pull the glass apart between both gloved hands. With practice, both ends of the glass will still be attached but the central section that was molten should now be hair thin. Allow to cool and then break the capillary section of the glass at a point near the far end where it first becomes extremely thin, by pinching it with your finger and thumb.
The air inlet has now become a bleed and is ready to be inserted into the socket in the flanged lid of the reaction flask. The bleed should then be shortened to the point that it just reaches the base of the flask when inserted into the socket. Refit the plastic screw thread safety adaptor and couple to a flexible pipe that is connected to the side arm on a Buchner flask containing a low boiling solvent - if the fractionation requires this.