Litmus

LITMUS, a coloring substance used as an indicator (see Indicator). It is obtained from lichens belonging to the genera Roccella and Lecanora: R. tinctoria (Canary Islands, Azores), R. fuciformis (Africa, Ceylon, India), Ochrolechia tartarea (Sweden, Norway, Scotland). The crushed plant is mixed in equal parts (by weight) with potash, moistened with a solution of ammonium carbonate, and left to stand for about 40 days. The resulting mass is mixed with chalk and gypsum, formed into cubes, and dried. The commercial preparation consists of dark blue pieces. To give litmus greater sensitivity as an indicator, the commercial litmus must be purified. The purification method by Mohr: uncrushed pieces of litmus are extracted three times with 85% alcohol with heating; the alcohol is decanted, the undissolved residue is leached with hot water, left to stand for 2-3 days. The decanted alkaline, transparent solution is evaporated to approximately ¹/₃, supersaturated with acetic acid, evaporated to a syrupy consistency, and the mass is poured with 90% alcohol. The blue precipitate is filtered, washed with alcohol, and dissolved in water. Litmus is used in the form of an aqueous solution (which should be stoppered not with a cork but with cotton) and in the form of litmus paper. For ordinary purposes, blue and red papers are prepared by impregnating dense, unsized paper with a weakly alkaline or weakly acidic solution of litmus. The main constituent of litmus is azolitmin. With acids, litmus gives a red coloration, with alkalies—a blue one. The zone of color transition from red to blue is between pH = 5.0-8.0.