Litmus

Litmus

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.

Other Species:
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.