Filtration

FILTRATION (from the Greek philtron—love potion; particularly clear wine was considered as such), one of the methods for separating a liquid phase from a solid one (precipitates, turbidity) by passing the system through porous materials that retain dense particles. Filtration differs from straining by the use of denser porous materials: in the filtered liquid (filtrate), no visible particles should be detectable with the naked eye. In laboratory and pharmaceutical practice, filtration is usually performed through filter paper using glass or porcelain funnels. When using funnels with a 60° angle, one prepares Berzelius filters for them (Fig. 1), which are especially convenient in cases where it is desirable to collect the precipitate.

Figure 1. Funnel with a 60° angle and folding of paper for a Berzelius filter.

in the filtered liquid (filtrate) no visible particles should be detectable with the naked eye. In laboratory and pharmaceutical practice, filtration is usually performed through filter paper using glass or porcelain funnels. When using funnels with a 60° angle, one prepares

Fig. 2.

Figure 3.

Figure 2. Funnel with glass rods to accelerate filtration. Figure 3. Nutsche: a—to vacuum pump; b—for draining the filtrate. Berzelius filters for them (Fig. 1), which are especially convenient in cases where it is desirable to collect the precipitate. To accelerate filtration, fluted funnels are used (see Funnel, Fig. 8) or glass rods are inserted between the walls of the funnel and the filter (Fig. 2). On

Figure 4.

Figure 5. Figure 4. Perforated porcelain funnels. Figure 5. Pleated (folded) Gay-Lussac filter. Buchner funnels (see Funnel, Fig. 3) and Nutsche filters (Fig. 3), which operate under reduced air pressure in the receiver, flat filters in the form of circles of the appropriate size are used. To accelerate filtration in cases where it is not required to save the precipitate, funnels of other shapes are used (see Funnel, Figs. 1, 2) and perforated funnels (Fig. 4); for all these funnels, folded or pleated Gay-Lussac filters are prepared by folding the paper in a fan-like manner (Fig. 5). Funnels with filters should be used in stands to avoid spilling the liquid (Figure 6); when filtering directly into a flask, a thread, a strip of paper, or a glass rod should be placed between the neck of the flask and the funnel, thus creating a gap for air to escape. Recently, glass filters with a porous glass bottom have been produced in our country, through which filtration is performed without paper (Fig. 7). The Schott factory in Jena was the first to release such filters. The pore size of these filters is indicated by numbers: No. 1 allows particles with a diameter of about 100 μ to pass, No. 2—50 μ, No. 3—20 μ, and No. 4—5 μ. The most frequently used forms of these glass filters (Goslaborreaktivsbyt) are shown in Fig. 7; forms "d" and "e" are used for suctioning liquid from easily stirred-up precipitates. To facilitate filtration, methods of enlarging precipitates are used (e.g., preliminary centrifugation), lowering the viscosity of the liquid phase (filtration while heating, see Funnel, Figs. 10 and 11), or the liquid is clarified beforehand by causing the formation of coarse-grained precipitates in it, which entrain (adsorb) fine turbidity. For this purpose, substances such as whipped egg white, gelatin, serum (in the presence of tannins),

Figure 6. Setups for filtration.

f=™*3j

Figure 7. Porous glass filters. milk (for acidic liquids, e.g., berry juices), or scraps of filter paper boiled with water, clarifying clays (floridin, gumbrin), etc., are added to the liquid being clarified; sometimes proteins are added and then coagulated by boiling. Filtration through porous candles (Berkefeld, Chamberland, etc.)—see Berkefeld candle, Filters in laboratory practice. - On an industrial scale, filtration methods that allow for high productivity are used; mainly filter presses are used—apparatuses invented by the engineer Nydge in 1828 (Figure 8). For laboratory and pharmaceutical work, small filter presses exist. Continuous-action filters, drum, disk, and flat (plan-filters) filters possess higher productivity. Regarding the filtration of gases—see Gas masks. - In drop analysis (qualitative microanalysis according to Feigl and Tananaev), filtration refers to the passage of liquid through the capillaries of filter paper; the "filtrate" in this sense is considered to be the spot of liquid at some distance from the drop applied to any point on the paper. I. Oberhard