Filtering the Precipitate
After precipitation and digestion are complete, the precip- itate is separated from solution by filtration using either filter paper or a filtering cru- cible. The most common filtering medium is cellulose-based filter paper, which is classified according to its filtering speed, its size, and its ash content on ignition. Fil- tering speed is a function of the paper’s pore size, which determines the particle sizes retained by the filter. Filter paper is rated as fast (retains particles > 20–25 μm), medium fast (retains particles > 16 μm), medium (retains particles > 8 μm), and slow (retains particles > 2–3 μm). The proper choice of filtering speed is important. If the filtering speed is too fast, the precipitate may pass through the filter paper re- sulting in a loss of precipitate. On the other hand, the filter paper can become clogged when using a filter paper that is too slow.
Filter paper is hygroscopic and is not easily dried to a constant weight. As a re- sult, in a quantitative procedure the filter paper must be removed before weighing the precipitate. This is accomplished by carefully igniting the filter paper. Following ignition, a residue of noncombustible inorganic ash remains that contributes a posi- tive determinate error to the precipitate’s final mass. For quantitative analytical pro- cedures a low-ash filter paper must be used. This grade of filter paper is pretreated by washing with a mixture of HCl and HF to remove inorganic materials. Filter paper classed as quantitative has an ash content of less than 0.010% w/w. Qualita- tive filter paper typically has a maximum ash content of 0.06% w/w.
Filtering is accomplished by folding the filter paper into a cone, which is then placed in a long-stem funnel (Figure 8.6). A seal between the filter cone and the funnel is formed by dampening the paper with water and pressing the paper to the wall of the funnel. When properly prepared, the stem of the funnel will fill with the solution being filtered, increasing the rate of filtration. Filtration is accomplished by the force of gravity.
The precipitate is transferred to the filter in several steps (Figure 8.7). The first step is to decant the majority of the supernatant through the filter paper without transferring the precipitate. This is done to prevent the filter paper from becoming clogged at the beginning of the filtration process. Initial rinsing of the precipitate is done in the beaker in which the precipitation was performed. These rinsings are also decanted through the filter paper. Finally, the precipitate is transferred onto the filter paper using a stream of rinse solution. Any precipitate clinging to the walls of the beaker is transferred using a rubber policeman (which is simply a flexible rubber spatula attached to the end of a glass stirring rod).
An alternative method for filtering the precipitate is a filtering crucible (Fig- ure 8.8). The most common is a fritted glass crucible containing a porous glass disk filter. Fritted glass crucibles are classified by their porosity: coarse (retaining particles > 40–60 μm), medium (retaining particles > 10–15 μm), and fine (re- taining particles > 4–5.5 μm). Another type of filtering crucible is the Gooch cru- cible, a porcelain crucible with a perforated bottom. A glass fiber mat is placed in the crucible to retain the precipitate, which is transferred to the crucible in the same manner described for filter paper. The supernatant is drawn through the crucible with the assistance of suction from a vacuum aspirator or pump.