Analytical Chem istry - DePauw University

368 Analytical Chemistry 2.0(a)(b)(c)(f)Figure 8.7 Preparing a filter paper cone. The filterpaper circle in (a) is folded in half (b), andfolded in half again (c). The folded filter paper isparted (d) and a small corner is torn off (e). Thefilter paper is opened up into a cone and placedin the funnel (f).(d)(e)glass stirring rodsupernatantprecipitateFigure 8.8 Proper procedure fortransferring the supernatant to thefilter paper cone.while it remains in its beaker, with the rinsings decanted through the filterpaper. Finally, the precipitate is transferred onto the filter paper using astream of rinse solution. Any precipitate clinging to the walls of the beakeris transferred using a rubber policeman (a flexible rubber spatula attachedto the end of a glass stirring rod).An alternative method for filtering a precipitate is a filtering crucible.The most common is a fritted-glass crucible containing a porous glass diskfilter. Fritted-glass crucibles are classified by their porosity: coarse (retainingparticles larger than 40–60 mm), medium (retaining particles greater than10–15 mm), and fine (retaining particles greater than 4–5.5 mm). Anothertype of filtering crucible is the Gooch crucible, which is a porcelain cruciblewith a perforated bottom. A glass fiber mat is placed in the crucible to retainthe precipitate. For both types of crucibles, the precipitate is transferred inthe same manner described earlier for filter paper. Instead of using gravity,the supernatant is drawn through the crucible with the assistance of suctionfrom a vacuum aspirator or pump (Figure 8.9).Ri n s i n g t h e PrecipitateBecause the supernatant is rich with dissolved inert ions, we must removeany residual traces of supernatant to avoid a positive determinate errorwithout incurring solubility losses. In many cases this simply involves theuse of cold solvents or rinse solutions containing organic solvents such asethanol. The pH of the rinse solution is critical if the precipitate containsan acidic or basic ion. When coagulation plays an important role in determiningparticle size, adding a volatile inert electrolyte to the rinse solutionprevents the precipitate from reverting into smaller particles that might passthrough the filter. This process of reverting to smaller particles is called peptization.The volatile electrolyte is removed when drying the precipitate.