Litmus Botanical: Roccella tinctoria (D. C.)
Family: N.O. Lichenes
---Synonyms---Lacmus. Orchella Weed. Dyer's Weed. Lacca caerulea. Lacca musica. Orseille. Persio. Rock Moss. Lichen Roccella. Roccella phycopsis. Roccella Pygmaea. Turnsole. Touresol. Laquebleu.
---Part Used---The whole plant, for its pigment.
---Habitat---Seashore rocks on all warm coasts and some mountain rocks.
---Description---Various origins are ascribed to the name Roccella. It may be derived from rocca (a rock), or from the red colour produced by the plants. It occurs in an Italian Natural History of 1599.
Roccella tinctoria is a small, dry, perennial lichen, in appearance a bunch of wavy, tapering branched, drab-coloured stems from 2 to 6 inches high, springing from a narrow base. These bear nearly black warts at intervals, the apothecia or means of fructification peculiar to lichens. It is found principally on the Mediterranean coasts but other species from other localities are also sources of commercial Litmus.
Blue and Red Orchil or Archil are used for dyeing, colouring and staining. The red is prepared by steeping the lichen in earthen jars and heating them by steam. The blue is similarly treated in a covered wooden vessel. They are used as a thickish liquid for testing purposes.
Cudbear, prepared in a similar way, is also used as a dye. It is dried and pulverized, and becomes a purplish-red in colour.
The preparation of Litmus is almost exclusively carried on in Holland, the details being kept a secret. About nineteen kinds seem to be there, varying very much in value.
The lichens are coarsely ground with pearlashes, and macerated for weeks in wooden vessels in a mixture of urine, lime and potash or soda, with occasional stirring. In fermentation the mass becomes red and then blue, and is then moulded into earthy, crumbling cakes of a purplish-blue colour. The scent is like violets and indigo and the taste is slightly saline and pungent. Indigo is mixed with inferior kinds to deepen the colour.
Blue Litmus Paper is prepared by steeping unsized white paper in an infusion or Test Solution of Litmus, or by brushing the infusion over the paper, which must be carefully dried in the open air.
Red Litmus Paper is similarly prepared with an infusion faintly reddened by the addition of a small percentage of sulphuric or hydrochloric acid.
Vegetable red, much used in colouring foods, is a sulphonated derivative of orchil.
---Constituents---The lichen contains a brown resin, wax, insoluble and lichen starches, yellow extractive, gummy and glutinous matters, tartrate and oxalate of lime and chloride of sodium. The colouring principles are acids or acid anhydrides, themselvescolourless but yielding colour when acted upon by ammonia, air and moisture.
The chief of these are Azolitmin and Erythro-litmin, sometimes called leconoric, orsellic and erythric acids.
The dye is tested by adding a solution of calcium hypochlorite to the alcoholic tincture, when a deep blood-red colour, quickly fading, should appear, or the plants can be macerated in a weak solution of ammonia, which should produce a rich violet-red.
---Medicinal Action and Uses---Demulcent and emollient. A decoction is useful in coughs and catarrhs.
Litmus is used officially as a test for acids and alkalis. Acids impart a red colour to blue Litmus and alkaloids cause reddened Litmus to return to its original blue. It may be used in solid or liquid forms as well as on the papers.
---Adulterations---Orchil is often adulterated with extracts of coloured woods, especially logwood and sappan wood.
Two of the chief sources of Litmus are now R. Montagnei of Mozambique and Dendrographa leucophoea of California.
Lecanora Tartare, or Tartarean Moss, was formerly much used in Northern Europe.
R. pygmaea is found in Algeria\.
R. fuciformis is larger, with flatter, paler branches.
R. phycopsis is smaller and more branched.
Inferior kinds of Litmus are prepared from species of Variolaria, Lecanora and Parmelia.
British Pharmaceutical Codex, An Imperial Dispensatory Medical Practitioners And Pharmacists 1911
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.