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Efficient and Inexpensive Apparatus for Hot Filtration

Carlos C. Romão & Herminio P. Diogo
J. Chem. Educ. 78(1), 65 (2001)

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Most experiments described in organic chemistry manuals contain at least one filtration step. Hot filtration, which allows removing insoluble impurities, is particularly important in recrystallization processes. When hot filtration is done through filter paper in a funnel, crystallization of the product inside the cold stem of the funnel often causes severe product losses and blocking.

Figure 1. Apparatus for hot filtration.

To avoid this inconvenience and keep the funnel hot, many textbooks suggest using a short-stemmed funnel at the top of an Erlenmeyer flask that contains a small amount of the solvent used in the recrystallization. If the Erlenmeyer is placed on a hot-plate or steam bath, the solvent vapor keeps the funnel hot, preventing early crystallization. With the exception of water, this technique generates toxic or flammable solvent vapors that require working under a hood and may represent a fire hazard. The alternative jacketed funnels with sintered glass disks are expensive and require pumping to circulate heating fluid or water. In our first-year laboratory course an inexpensive and efficient hot filter was built from inexpensive Erlenmeyer flasks, which can be modified by simple glass-blowing techniques.

Screw-thread glass adapters (Schott, no. 28625) were welded to the mouth and side of the neck of a 250-mL Erlenmeyer flask and a large hole (ca. 5 cm diameter) was made in its bottom (Fig. 1). Screw caps (Schott, GL32 and GL18) with ca. 5 mm bores with rubber, silicone, or Teflon O-rings that prevent escape of hot liquid are screwed to each adapter. A long-stemmed funnel is inserted through the bottom hole and through the cap in the neck. It is tightened in position, leaving a tip ca. 2 cm long outside the cap, adjusting the funnel wall to the rim of the hole in the bottom of the flask. A smaller funnel, with a bent tip, is inserted through the bore on the side arm and tightened. The assembled apparatus is clamped upside down and hot water is then poured into the flask through this side funnel. After the folded filter paper is placed inside the hot central funnel, filtration may start and the concentrated solution will flow along the central stem without crystallization taking place. If the hot water needs replacement, the side arm is rotated downwards and the water drips out. The side arm is then replaced in the upright position, a refill of hot water is added, and filtration is continued.