Persio. Cudbear.; Litmus.
Cudbear is a purplish-red powder obtained from Roccella tinctoria, DC., R. Montagnei, Bel., and other species (N.O. Discomycetes), lichens growing chiefly on the Canary Islands, Madagascar, and on the African coasts. The lichens are boiled with water containing chalk in suspension, and the liquor obtained is concentrated in a vacuum to a specific gravity of about 1.06. The residue of lichen is dried, freed from mechanical impurities, and introduced into large vats, with about twice its weight of the concentrated liquor and one-sixth its weight of solution of -ammonia; after being kept at a temperature of 25° to 30° for two or three months, with occasional stirring, the mass is dried and powdered (see also under Litmus). Cudbear occurs as a purplish-red powder, which imparts a rich red colour to acid and neutral liquids; the colour is again changed to purplish-red on the addition of alkalies.
Uses.—Cudbear is employed as a colouring agent, especially in the preparation of syrups having an acid reaction. Solution of cudbear (Liquor Persionis) is prepared by boiling 3.75 of cudbear, in fine powder, with 100 of distilled water for ten minutes, then straining and making up to 100, if necessary, with distilled water.
- Tinctura Persionis, B.P.C.—TINCTURE OF CUDBEAR. 1 in 8.
- Used to impart a bright red colour to acid liquids. The addition of about 10 per cent. of burnt sugar to the tincture will render it suitable for imparting a brownish-red tint.
Litmus is a blue pigment, obtained from various lichens, chiefly Roccella tinctoria, DC. (Cape Verde), R. Montagnei, Bel. (Madagascar and Mozambique), and Dendrographa leucophaea, Darbish (California) (N.O. Discomycetes). The coarsely powdered lichen is mixed with pearl ash and solution of ammonium carbonate and submitted for several weeks to a slow process of fermentation, during which a red colouring matter is produced, which gradually changes to blue. Chalk and gypsum are then added, the mixture is passed through a sieve, then formed into small rectangular cakes and dried. Litmus occurs in dark blue or bluish-violet, finely granular, friable, and slightly aromatic, rectangular cakes. To prepare a sensitive indicator, commercial litmus is treated as described under Solutio Litmi.
Partially soluble in water or alcohol, forming solutions with a deep blue colour, which are changed to red by acids.
Constituents.—Litmus contains several colouring matters, viz., erythrolitmin, azolitmin, erythrolein, and spaniolitmin, of which azolitmin and erythrolitmin appear to be the chief, but they are probably not homogenous substances. The colouring matter upon which the use of litmus as an indicator depends is a feebly acid, red body, the salts of which have an intense blue colour. The lichens from which litmus is prepared contain lecanoric acid (R. tinctoria), erythrin (R. Montagnei), and orcin. Lecanoric acid is diorsellinic acid, and is converted by alkalies into orsellinic acid. Erythrin is erythrite orsellinate, and is converted into erythrite and orsellinic acid. All these substances are colourless. Orsellinic acid yields by further change orcin, from which, by the action of air in the presence of ammonium carbonate, the colouring matters are produced. These appear to be oxidation products of amino-orcinol. Litmus also contains large quantities of chalk and gypsum.
Uses.—Litmus is much employed as an indicator; its colour is changed to red by acids, and the blue colour is restored by alkalies. As its colour is affected by carbonic acid, titrations in which carbon dioxide gas is liberated are better conducted with methyl orange as the indicator (helianthine). Methyl-orange solution is prepared by dissolving 0.2 of methyl orange in 2.5 of alcohol and sufficient distilled water to produce 100. It is comparatively unaffected by carbon dioxide, hydrogen sulphide, or hydrocyanic acid, but should not be used for titrating organic acids.
- Solutio Litmi.—B.P. TEST SOLUTION OF LITMUS.
- Litmus, in powder, 10; alcohol, 100; distilled water, 100. The litmus is boiled with 40 of the alcohol for one hour in a vessel provided with a reflux condenser, the operation is twice repeated with 30 of the alcohol, and the washed litmus is then digested in the distilled water and filtered. Blue litmus paper is prepared by dipping bibulous paper in this solution and drying. Red solution of litmus is prepared by adding very dilute hydrochloric acid to solution of litmus until the colour just changes to red. Red litmus paper is prepared by dipping bibulous paper in this solution and drying.