History.—The manufacture of litmus was probably discovered by the Dutch about the latter end of the seventeenth century [Litmus is not mentioned by, and therefore, was probably unknown to, Caspar Bauhin (Pinax, 1671), and to Dale (Pharmacologia, 3tia ed. 1737). The earliest authors in whose works I have found it mentioned are, Pomet (History of Drugs, Eng. ed. 1712), and Valentine (Hist. Simpl. 1716).].
Preparation.—Litmus is obtained by the united influence of water, air, ammonia, and either potash or soda, on any of the tinctorial lichens capable of yielding orchil. If the potash or soda be omitted, the product is not litmus, but orchil.
The manufacture of litmus has been described by Ferber [Neue Beytrage zur Mineralgeschichte verschiedener Länder, Bd. i. S. 378, Meitun, 1778. Ferber describes the process as practiced at Amsterdam. He says that the pulp is ground in a mill, and forced through a hair cloth, before it is placed in the molds.], by an anonymous writer [Nicholson's Journal of Nat. Phil. Chem. and the Arts, vol. ii. p. 311, 1799. The notice of litmus is a translation from an article in the Journal du Commerce. The lichen is said to be ground in a mill, and sifted through a brass wire sieve before maceration. The moulds are described as being 1 1/4 inches by 8-10ths of an inch.], by Morelot [Mémoire sur le Lichen François, vulgo tournesol en pain, in the Mém. de la Soc. Méd. d'Emulation, t.v.p. 281.], and by Amédéé Gélis [Journ. de Pharm. t. xxvii. p. 476, 1840.]. From their accounts it appears that the lichen is macerated for several weeks, with occasional agitation in a mixture of urine, lime, and potashes, in a wooden trough under shelter. A kind of fermentation takes place, and the lichen becomes first reddish, and subsequently blue. When the pulp has acquired a proper blue colour, it is placed in proper moulds, and the cakes thus procured are subsequently dried.
The moulds are either of steel or brass, and consist of two parts: the lower one divided into rectangular cells, and the upper one supporting a series of metallic rods bearing small metallic disks, so arranged as to accurately fit the cells of the lower piece.
The lower piece is immersed in the pulp with which they are filled, and the excess of pulp is then scraped off by means of a wooden spatula. The upper piece being then applied, the disks enter the cells, and force out the moulded cakes of litmus.
It appears from Gélis' experiments, that any of the lichens which serve for the production of orchil may be used in the preparation of litmus [Ferber saw the Roccella lichen use at Amsterdam. Morelot stated that Variolaria orcina yielded it. Nees and Ebermaier (Handb. d. Med. Pharm. Bot. Bd. i. s. 49); and Thomson (Org. Chemistry, p. 284), on the other hand, say that Lecanora tartarea is employed.—An orchil-maker, under my care in the London Hospital, told me that he had been accustomed to make litmus of pipe clay, starch, soda, and orchil liquor, and he gave me some specimens of it thus prepared. Gélis prepared it with Roccella tinctoria (in the scutelliferous state), Roccella fuciformis, and the mixture of Lecanora parella β pallescens, and Isidium corallinum, sold under the name of Orseille d'Auvergne; but the last mentioned plnats yielded a less fine product than the others.].
The urine serves for the production of carbonate of ammonia, and the lime employed abstracts the carbonic acid.
The Dutch manufacturers add chalk or sulphate of lime, and some siliceous or argillaceous substance, to give body and weight to the litmus. Indigo is also introduced, doubtless for the purpose of increasing the blue colour of the cakes.
Description.—Litmus is imported from Holland, in the form of small, rectangular, light, and friable cakes of an indigo-blue colour. Examined by the microscope we find sporules, and portions of the epidermis and mesothallus of some species of lichen, moss leaves, sand, &c. The odour of the cakes is that of indigo and violets. The violet odour is acquired while the mixture is undergoing fermentation, and is common to all the tinctorial lichens. It has led some writers into the error of supposing that the litmus-makers use Florentine orris in the manufacture of litmus. The indigo odour depends on the presence of indigo in the litmus cakes.
Composition.—An accurate and complete analysis of litmus is yet a desideratum. In 1840, Dr. Kane [Phil. Trans. for 1840, p. 298.] submitted it to examination, and obtained from it four colouring principles, to which he gave the names of erythrolein, erythrolitmine, azolitmine, and spaniolitmine. These, in their natural condition, are red, and the blue of litmus, he says, is produced by combination with a base. "There are, properly speaking," he adds, "only two characteristic colouring matters in litmus—the erythrolitmine and the azolitmine; for the erythrolein is coloured crimson purple only by alkalies, and the spaniolitmine occurs but very seldom. In the litmus of commerce these colouring substances are combined with lime, potash, and ammonia, and there is mixed up in the mass a considerable quantity of chalk and sand."
Gélis [Journ. de Pharmacie, t. xxvii. p. 483, 1840.] has published some interesting observations on litmus. He says that litmus owes its colour to four different coloured products, which he designates by the letters A, B, C, and D. The ash of litmus he found to contain carbonate of potash, carbonate or sulphate of lime, alumina, silica, traces of oxide of iron, chlorine, sulphuric acid, and phosphoric acid.
I shall provisionally call the proper colouring matter of litmus derived from the lichen, lichen-blue.
Lichen-Blue; Litmus Blue. By these terms I understand the peculiar blue colouring matter of litmus, which is soluble in water, and is reddened by acids. It is probably either some modification of orceine, or some allied principle. It may perhaps be a mixture or compound of several colouring principles.
It is soluble in both water and spirit, yielding a coloured solution, which, in the concentrated state, has a purple colour when viewed by transmitted light; but in the dilute state it is pure blue. Viewed by transmitted candle-light it has a reddish colour. An aqueous infusion of litmus neither reddens turmeric paper nor occasions a precipitate with a solution of chloride of calcium. It contains, therefore, no free alkali or alkaline carbonate.
It is reddened by acids; and also by many of the metallic salts—as corrosive sublimate, sulphate of copper, sulphate of iron, &c. The infusion of litmus, which has been reddened by acids, has its blue colour restored by alkalies, alkaline earths, the alkaline and earthy sulphurets, the alkaline carbonates, the soluble borates, the tribasic phosphate of soda, the alkaline cyanides, &c. An infusion of litmus is decolorized by chlorine and by the alkaline hypochlorites. Certain deoxidizing agents also deprive it of colour; as sulphuretted hydrogen, hydrosulphuret of ammonia, sulphurous acid, the hyposulphites, nascent hydrogen (obtained by adding hydrochloric acid and zinc to an aqueous infusion of litmus), and the protosalts of iron. If an infusion of litmus be left in contact with sulphuretted hydrogen, in a well-stopped bottle, for a few days, the liquid is decolorized, but reacquires its colour by exposure to the air or oxygen gas.
Characteristics. The lichen-blue is an aqueous infusion of litmus, is distinguished from other vegetable blues by the action of acids and alkalies on it (see supra); for most vegetable blues and purples (as red cabbage juice, syrup of violets, &c.) are changed to green by alkalies, whereas, lichen-blue does not undergo this change.
If a lump of moistened litmus be laid on turmeric paper, the latter is reddened by it; but by the application of heat the redness disappears.
When litmus cakes are thrown into diluted hydrochloric acid, a copious effervescence ensues, and a solution of chloride of calcium is obtained.
If a cake of litmus be ignited in the outer cone of the flame of a candle, a whitish violet tint is communicated to the flame, indicative of the presence of potash.
If the ashes of litmus be thrown into diluted hydrochloric acid, violent effervescence takes place; a solution of chloride of calcium is obtained, and a quantity of siliceous sand remains undissolved.
Impurities.—Inferior samples of litmus (including all those usually found in English commerce) contain indigo [See two papers by the author in the Pharmaceutical Journal, vol. ix. p. 12, 1849; and vol. x. p. 325, 1850.], the presence of which in litmus cakes is proved by their odour; by the coppery lustre which they acquire when rubbed with the nail; by digesting them in oil of vitriol, by which a blue solution of sulphate of indigo is obtained; and by heating them in a watch glass or platinum capsule, by which indigo-vapour (characterized by its well-known odour and reddish violet colour) and crystals of indigo are obtained.
Uses.—Litmus is employed as a test for acids and alkalies. The former communicate a red colour to blue litmus: the latter restore the blue colour of reddened litmus. (The action of various salts on litmus has been before stated.) If the litmus present be reddened by an unboiled, but not by a boiled, water, we may infer that the acid present is a volatile one; probably carbonic acid, or perhaps sulphuretted hydrogen. Reddened litmus may have its blue colour restored not only by alkalies, &c., as before mentioned, but also by carbonate of lime dissolved in water by a considerable excess of carbonic acid.
1. TINCTURA LACMI; Tincture of Litmus. (Litmus one part; Distilled Water twenty-five parts. M.)—Though called tincture, it is in reality an infusion of litmus. In order to preserve it, a portion (about 1/10th part) of spirit may be added to it. If required to be more concentrated, the proportion of litmus should be augmented. Some persons first bruise the litmus in a mortar, and then tie it up in a linen bag before steeping it in the water. By keeping in a closely-stopped bottle, its blue colour disappears, but is shortly restored on the admission of atmospheric air.
2. CHARTA EXPLORATORIA COERULEA; Blue Test Paper; Blue Litmus Paper.—This is prepared by dipping slips of paper in a strong and clear infusion of litmus; or by brushing the infusion over the paper.
Bibulous or unsized paper is usually preferred, on account of the facility with which it imbibes the liquid to be tested; and also because the alum which frequently enters into the composition of the size affects the colour of litmus. Professor Graham, however, recommends good letter paper; or, if the infusion is applied to one side only, thin and sized drawing paper. Faraday [Chemical Manipulation] recommends the infusion to be prepared from an ounce of litmus and half a pint of hot water. The Prussian Pharmacopceia of 1827 orders one part of litmus and four parts of water. Others employ one part of litmus and six parts of water.
In order to obtain extremely delicate test paper, the alkali in the litmus is to be almost neutralized by a minute portion of acid. To effect this, divide the filtered infusion of litmus into two parts; stir one portion with a glass rod which has been previously dipped into very dilute sulphuric acid, and repeat this until the liquid begins to look reddish: then add the other portion of liquid, and immerse the paper in the mixture.
Good litmus paper should be uniform in its colour, and neither very light nor very dark. When it has a purplish tint, it is a more delicate test for acids than when its colour is pure blue. When carefully dried, it may be preserved by wrapping it in stiff paper, and keeping it in well-stopped vessels in a dark cupboard or drawer.
Books of test papers, bound up like bankers' cheque-books, are sold in the shops, and are very convenient. They are about 1 3/4 inches long and 3/8ths of an inch wide. To preserve them they are kept in leathern cases.
Blue litmus paper is used to detect the presence of acids and of certain salts which react as acids.
3. CHARTA EXPLORATORIA RUBEFACTA; Reddened Test Paper; Red Litmus Paper. This is prepared with an infusion of litmus which has been slightly reddened by an acid. Blue litmus paper may be extemporaneously reddened by exposing it for a few seconds to the vapour of acetic acid; but for preserving, it is better to prepare the paper with litmus which has been reddened by a minute portion of dilute sulphuric acid: the acetic acid being objectionable, on account of its volatility.
Red litmus paper is employed as a test for alkalies and certain salts (see supra), which react as bases.