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kilomentor | 17 January, 2008 20:16
Today’s Kilomentor blog pertains to a laboratory technique, which is particularly appropriate at a scale of 5-10 litres where it becomes problematic to pick them up and pour from them. It is sometimes necessary either to filter a solution without sucking the solvent dry and exposing the slurried solid or to draw away the liquid phase from a reactor flask to leave behind the solid phase. This can be particularly useful if the solid is sensitive to the air or moisture in the atmosphere. it can also be used when the slurry material would block the filter, make the flow impractical, and prevent a filtration from being completed. Removing a liquid phase from a vessel to leave the solid inside is called inverted filtration.
An apparatus for inverted filtration is described in Organic Synthesis Coll. Vol. 2 1966. A figure is printed on pg. 610. The solution is drawn up from the bottom of the reactor through a tube using gentle suction and carried over through the tube to a second vessel. A filter entirely encloses the end of the tube that is inserted into the reaction mixture. The filter is prepared from an extraction thimble and a rubber stopper, which has a single hole drilled through the top to accommodate the glass tube. The thimble is stuffed with glass wool to hold the thimble centered around the glass tube.
During the reaction the filtering assemblage is held above the reaction solution but inside the reactor. When the time for the inverted filtration arrives, the assemblage is pressed down by pressing the glass tube into the solution sliding it further down on the stopper which blocks a neck of the reactor flask. When the filter assembly is at least partially dipping into the solution, clear liquid will pass through the extraction thimble By the application of pressure to the reactor or gentle suction on the tube (by way of the filtrate receiving flask) the liquid which has been filtered through the extraction thimble will be forced up the tube and over into the receiving flask. Thus only liquid, which passes through the filter is transferred.
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