Filter Paper

FILTER PAPER. For separating liquids from solid substances (filtration), unsized paper is used. Depending on the coarseness of the precipitates to be separated and the viscosity of the liquid, paper of varying density is required; the densest paper (so-called hardened filters) is used for retaining precipitates such as freshly precipitated barium sulfate or alumina. The denser the paper, the slower the filtration proceeds. To ensure greater strength of the filter paper in a wet state, up to 25% of flax fiber (fiber length 30--70 mm) is introduced into the paper pulp. For pharmaceutical purposes, filter paper must be (according to State Pharmacopoeia VII) white; when viewed against the light, no pinholes, dark spots, or translucent spots (with reduced density) should be visible in it. It must be free from wood (phloroglucinol test in hydrochloric acid) and starch (iodine solution test), and must not contain water-extractable chlorides; ash is permitted up to 1 mg per each 100 cm² of paper. It would be desirable to require that the aqueous extract from 100 cm² of filter paper also leave no more than a negligible residue (0.5 mg) upon drying at 110°. For filter paper to be suitable for filtering salicylates, the absence of iron salts is necessary. For special purposes, ashless filters are prepared (Mendeleev Plant in Moscow), for which purpose filter paper is washed with hydrochloric acid, then with hydrofluoric acid, and finally with water until all soluble matter is removed; such paper contains less than 0.1 mg of ash in 100 cm². The quality of filter paper from the standpoint of its practical applicability is determined by the rate of filtration, expressed in cubic centimeters of distilled water passing in 1 minute through 1 cm² of the area of a given paper at 20° under a pressure of 50 mm (difference in levels) of a water column; the Herzfeld apparatus serves for this determination, ensuring constant pressure during the test. Solutions that interact with cellulose, such as zinc chloride solutions, acids stronger than 4-normal, alkalis stronger than normal, and the like, cannot be filtered through filter paper. Filter paper is sometimes used as a highly absorbent dressing material. Filter paper impregnated with a solution of saltpeter is used for fumigations in asthma; such saltpeter paper with the addition of fragrant or volatile substances is used for fumigating rooms in order to mask the bad odor of spoiled air and partially destroy (by evolving nitrogen oxides) certain odorous substances in the air.