Litmus. Lacmus. Turnsole. Tournesol. Lacca Coerulea. Lacca Musica. Laquebleu, Fr. Lackmus, G.—Three purple or blue coloring substances are known in commerce, obtained from lichenous plants. They are called severally litmus, orchil, and cudbear. Litmus is yielded by numerous species of lichens which, under the French name of orseille, are brought into commerce from the coasts of Sweden and Norway; from the Alps, the Pyrenees, and other European mountains; from the European and African coasts of the Mediterranean; from the Canaries, Madeira, and various other rocky islands, especially from Mozambique, Madagascar, and Angola, and from California. The species of lichens concerned represent various genera; at present the two most important of the lichens are said to be the Roccella Montagnei of Mozambique and the Dendrographa leucophoea of California. In the north of Europe Lecanora tartarea Ach., or Tartarean moss, and on the Mediterranean coasts the Roccella tinctoria Ach., or orchilla weed, at one time furnished the great bulk of the orseille of commerce. The chromogenous ethers which constitute the coloring matter of the lichens may be readily detected in situ by reagents. (P. J., Nov. 19, 1904.) In some of the lichens these coloring principles are chiefly situated in the cortex; in others they are especially abundant in the gonidial layer.
The principles in these plants upon which their valuable properties depend are themselves colorless, and yield coloring substances by the reaction of water, air, and ammonia. They are generally acids or acid anhydrides, and are named lecanoric, orsellic, erythric, etc., according to their origin. Lecanoric (diorsellinic) acid, C16H14O7, the original constituent of most of these plants, when boiled with water or alkaline solutions, is changed into orsellinic acid, as follows: C16H14O7 + H2O = (C8H8O4)2
|Orsellinic acid, C6H2||/ CH3|
fuses at 176° C. (348.8° F.), and decomposes into orcin, C6H3(CH3)(OH)2, and CO2. The same decomposition is readily effected by distillation with milk of lime. Orcin combines with ammonia gas to form C8H8O2.NH3, the solution of which exposed to the air becomes colored reddish by the formation of orcein, C7H7NO3. This latter compound forms the basis of the commercial orseille extract (orchil or archil).
Kane described a deep red crystalline substance, erythrolitmin, and a brownish-red coloring principle, azolitmin, C7H7NO4. This latter is considered as the distinctive coloring matter of the commercial litmus, and is now found in commerce under that name and is used as an indicator. It is nearly insoluble in cold water or benzene, but dissolves in alcohol with red, and in ether with yellow color. It appears to have the characters of a weak acid, the salts of which are blue and the potassium and calcium compounds of which exist in litmus.
To test the value of the plants as dye-stuffs, they may be macerated in a weak solution of ammonia, or a solution of calcium hypochlorite may be added to their alcoholic tincture. In the former case a rich violet-red color is produced; in the latter, a deep blood-red color appears, but soon fades.
Lacmus or litmus is prepared chiefly if not exclusively in Holland. The process consists in macerating the coarsely powdered lichens, in wooden vessels under shelter, for several weeks, with occasional agitation, in a mixture of urine, lime and potash or soda. A fermentation ensues, and the mass, becoming first red and ultimately blue, is, after the last change, removed, mixed with calcareous or silicious matter to give it consistence, and with indigo to deepen the color, and then introduced into small molds, where it hardens. It is said that ammonia may be substituted for urine in the manufacture of litmus, but that the reactions necessary for the production of the coloring matter of the litmus are due to the action of the enzyme which is found in the lichens. Litmus occurs in rectangular cakes, from a quarter of an inch to an inch in length, light, friable, finely granular, of an indigo-blue or deep violet color. It has the combined odor of indigo and violets, tinges the saliva a deep blue, and is somewhat pungent and saline to the taste. From most vegetable blues it differs in not being rendered green by alkalies. It is reddened by acids, and restored to its original blue color by alkalies. For a method of purifying commercial litmus by Foerster, see Nat. Drug., 1904, 460.
Its chief use in pharmacy is as an indicator. For this purpose it is employed either in infusion or in the form of litmus paper. The infusion, formerly called tincture of litmus, may be made in the proportion of one part of litmus to twenty of distilled water, and two parts of alcohol may be added to preserve it. Litmus paper is prepared by first forming a strong clear infusion with one part of litmus to four of water, and dipping slips of white unsized paper into it, or applying it by a brush to one surface only of the paper. The paper should then be carefully dried, and kept in well-stoppered vessels, from which the light is excluded. It should have a uniform blue or slightly purple color, neither very light nor very dark. As a test for alkalies the paper may be stained with an infusion of litmus previously reddened by an acid, care being taken to avoid all excess. By gas light it is said that the change of color cannot be determined by the eye exactly, as the blue of litmus becomes mauve; but this may be obviated by watching the process through a green glass, by which the faintest trace of blue becomes discernible. (P. J., 2d ser., vi, 479.) For the official method of preparing litmus paper, see Tests, Teat Solutions, etc., PART III.
Orchil, or archil, as prepared in England, is in the form of a thickish liquid, of a deep reddish-purple color, but varying in the tint, being in one variety redder than in another. The odor is ammoniacal. It is made by macerating lichens in a covered wooden vessel, with an ammoniacal liquor, either consisting of stale urine and lime, or prepared by distilling an impure salt of ammonia with lime and water. (Pereira.) For details as to the method of preparation, see Chem. News, 1874, 143. It is occasionally adulterated with the extracts of colored woods, as logwood, sappan-wood, etc. A mode of detecting these adulterations is given by F. Leeshing in the Chem. Gaz. of June 1, 1855, 219. A sulphonated derivative of orchil is much used in coloring food-stuffs under the name of "vegetable red."
Cudbear is a purplish-red powder procured in the same manner as orchil; but the mixture, after the development of the color, is dried and pulverized.
The point in which the preparation of these coloring substances differs from that of litmus appears to be, that potassium or sodium hydroxide is added, in the latter, to the ammoniacal liquid used. Orchil and cudbear are employed as dye-stuffs and f6r coloring purposes, and sometimes, in like manner with litmus, as indicators (see Cudbear). For chemical constituents of lichens, see A. J. P., 1898, 455.