Filtration

To carry out this procedure, you will need to assemble a Gooch crucible and funnel on a Buchner flask, clamped for stability using a retort stand (Fig. 20.2). The glass Gooch crucible has a porous disk of sintered ground glass, typically of pore diameter 20-30 µm, which is satisfactory for moderately sized precipitates. Fit the crucible into a Gooch funnel using a rubber cone, put the funnel into a one-holed rubber bung and then into a Buchner flask. The tip of the funnel must project below the side arm of the flask to prevent loss of filtrate down the side arm. Then, connect the Buchner flask to a water pump. Pour your precipitate suspension into the Gooch crucible, using a glass rod to direct the liquid into the centre of the sintered base. The lower end of the glass rod should be close to, but not touching, the sintered-glass base. Never overfill the crucible. The precipitate remaining in the bottom of the beaker should be rinsed out with the filtrate solution: disconnect the pump and pour the filtrate back into the beaker containing the precipitate. The pump should be disconnected by pulling off the vacuum tube from the Buchner flask. On no account turn off the water pump while doing this. You may need to rinse the beaker several times, to collect all of the precipitate in the crucible.

Fig.20.2 Experimental arrangement for filtration of precipitate.

Fig. 20.3 Structure of nickel-dimethylglyoxime complex.