The Natural Organic Colouring Matters
By
Arthur George Perkin, F.R.S., F.R.S.E., F.I.C., professor of colour chemistry and dyeing in the University of Leeds
and
Arthur Ernest Everest, D.Sc., Ph.D., F.I.C., of the Wilton Research Laboratories; Late head of the Department of Coal-tar Colour Chemistry; Technical College, Huddersfield
Longmans, Green and Co.
39 Paternoster Row, London
Fourth Avenue & 30th Street, New York
Bombay, Calcutta, and Madras
1918
Kaikki kuvat (kemialliset kaavat) puuttuvat // None of the illustrations (of chemical formulas) included.
This colouring matter is well known to the chemist, since white paper impregnated with its solution in a slightly acid or alkaline condition has long been employed, under the name of red and blue litmus-paper, to indicate the presence, in any solution, of alkalis or acids respectively. Alkalis change the colour of red litmus-paper to blue, acids turn blue litmus-paper red. In alkalimetry litmus tincture has, until recently, been the most generally adopted indicator. This use depends upon the fact that the free colouring matter of litmus is red, whereas its alkali salts are blue.
Commercial litmus has the form of small pale blue cubes, composed essentially of gypsum and chalk mixed with but comparatively little colouring matter, which is largely present in the form of a lake.
It is said to be prepared, chiefly in Holland, from various species of lichens, e.g. Lecanora tartarea, Roccella tinctoria, etc., the same, indeed, as are used in the manufacture of orchil (q.v.). Under the combined influence of ammonia and atmospheric oxygen the proximate principles contained in these lichens yield orceïn, the alkali salts of which are purple (orchil); but if potassium or sodium carbonate is present at the same time, the reaction proceeds further, and ultimately azolitmin (the colouring matter of litmus), the alkali salts of which are blue, is produced.
According to Gelis (J. Pharm. Chim., 24, 277; Revue Scient., 6, 50), litmus may be prepared as follows. Orchil-weed is ground and mixed with half its weight of potassium carbonate, and then repeatedly moistened with urine saturated with ammonium carbonate or with an aqueous solution of this salt; the mass soon acquires a brownish-red colour (three days), which gradually becomes purple "(twenty to twenty-five days), and finally blue (thirty days), yielding a litmus of the best quality in forty days. The pulpy mass is mixed with chalk and gypsum, then moulded in the form of cubes, and dried.
By modifying the action of air and ammonia upon orcinol, through the addition of sodium carbonate, De Luynes also succeeded in obtaining the colouring matter of litmus (Comptes rend., 59, 49; Dingl. poly. J., 174, 61 "; Chem. Zentr., 1865, 127; J., 1864, 551). A mixture of i part orcinol, 25 parts crystallised sodium carbonate, 5 parts water, and 5 parts ammonia solution, was heated to 60-80° for four to five days with frequent agitation. On diluting the blue solution thus obtained and acidifying slightly with hydrochloric acid, the colouring matter was precipitated. On washing and drying, it assumed a metallic lustre. It is sparingly soluble in water, but readily soluble in alcohol and in ether.
In making a litmus solution to be employed as indicator, the commercial litmus is extracted with boiling water, the filtered solution is slightly acidified with acetic acid, then carefully neutralised with ammonia, and boiled to expel any excess of the latter. Kept for any lengthened period in stoppered bottles, the solution becomes decolorised in consequence of a reductive fermentation; on exposure to air, however, the original colour is restored. This defect is prevented by saturating the solution with sodium chloride (Reichelt), (compare also Bellamy, J. Pharm. Chim., [v.], 18, 433). A dry litmus-extract may be prepared, according to Vogel, in the following manner (ibid., 45, 64, 70; Chem. News, 1864, 205). Twenty grams powdered commercial litmus are twice digested, each time with 150 c.c. cold distilled water. The second solution, which is alone employed, is divided into two equal portions, one of which is slightly acidified with nitric acid and then mixed with the other. The purplish solution thus obtained is evaporated to dryness on the waterbath, and the granular amorphous mass is kept in a stoppered bottle ready for dissolving in water when required.
For the employment and characteristics of litmus as an indicator, v. R. T. Thomson (J. Soc. Chem. Ind., 6, 198); also art. ACIDIMETRY, vol. i.; Marsh (Chem. News, 61, 2); Berthelot (Ann. Chim, Phys., [vii.], 25, 39); Ronde (Pharm. Zeit, 41, 736); Lescouer (Comptes rend., 123, 811); Lüttke (Zeitsch. anal. Chem., 31, 692); Foerster (ibid., 28, 428); Glaser (ibid., 38, 273).
Litmus exhibits a characteristic absorption spectrum. Ether extracts it from an acid solution, and forms a yellow liquid, whichabsorbs the more refrangible end of the spectrum to a point midway between D and E. If the solution is coloured blue by adding a drop of ammonia, an absorption-band is formed, commencing at D, where it is extremely black, and gradually diminishing to E. A blue aqueous commercial solution shows a well-marked absorption-band at D. Addition of acid changes the colour to red, the band at D disappears, and the spectrum now resembles that of cenolin, the colouring matter of red wine (A. H. Allen, Com. Org. Analysis, 325), (compare also Vogel, Praktische Spectralanalyse, 1877, 269).
Our knowledge of the chemistry of the colouring matters contained in litmus is very meagre. Gelis (J. Pharm. Chim., 27, 477) isolated from it several compounds in the following manner. After extracting commercial litmus with water, the insoluble residue is boiled with dilute caustic alkali and the filtered solution is precipitated with basic lead acetate. The blue precipitate is washed by decantation until it begins to dissolve and colour the wash-water. It is then decomposed with hydrogen sulphide, exposed to air until free from excess of H2S, collected on a filter and digested with dilute ammonia to extract the colouring matter. On adding acid to the filtered solution the main portion of the litmus colouring matters is thrown down as a red flocculent precipitate. The filtrate from this contains a very small quantity of substance (α).
On extracting the dried red precipitate with ether and leaving the orange solution to spontaneous evaporation, it yields a bright red residue (β) containing crystalline needles. This product is insoluble in water, but readily soluble in alcohol, also in alkalis with a violet colour. The portion insoluble in ether is dissolved in alcohol, and on allowing the blood-red solution to evaporate spontaneously it yields a large quantity of a reddish-purple product (y) having a bronze lustre. This represents the colouring matter most abundant in litmus.
The residue, which is insoluble in water, in alcohol, and in ether, contains another product (δ) which is soluble in alkalis, from which it may be precipitated by acids. The three products β, γ, and δ, appear to contain nitrogen.
An examination of litmus was made in 1840 by Kane (Royal Soc. Trans., 1840, 298; Ann. Chim. Phys., [iii.], 2, 129; Annalen, 39> 57 J J- Pharm. Chim., 1841, 569), who isolated from it the chief and characteristic colouring matters azolitmin and erythrolitmin, together with erythroleïn and spaniolitmin.
According to Kane, finely powdered commercial litmus is extracted with boiling water. Most of the colouring matter remains in the form of an insoluble lake in the residue, to which hydrochloric acid is added till effervescence ceases and the mixture is strongly acid. The insoluble matter interspersed with liberated colouring matter is collected on a filter, washed free from acid, dried, and extracted with boiling alcohol. The alcoholic solution is filtered from an insoluble reddish-brown mass (impure azolitmin) and then evaporated to dryness, and the residue is digested with warm ether until it becomes no longer coloured. On distilling the filtered ethereal solution, erythroleïn is left as a purple semi-fluid oily substance. That portion of the alcoholic extract which is insoluble in ether consists of erythrolitmin.
The above-mentioned impure azolitmin is purified, either by dissolving it in a large quantity of boiling water and evaporating the solution to dryness, or by dissolving it in very dilute ammonia, evaporating the solution to dryness, neutralising any residual ammonia by dilute hydrochloric acid, and washing with alcohol until free from ammonium chloride and excess of hydrochloric acid. The residue represents purified azolitmin.
The colouring matter contained in the deeply coloured solution obtained in the first instance by boiling the commercial litmus with water and filtering, is isolated as follows. The solution is precipitated with neutral lead acetate, the purple precipitate thus obtained is well washed, suspended in water, and decomposed with hydrogen sulphide. The mixture of lead sulphide and liberated colouring matter thus obtained is well washed and digested with warm dilute ammonia; the filtered deep -blue solution is evaporated to dryness, the residue is moistened with hydrochloric acid, washed free from ammonium chloride and any excess of hydrochloric acid, with warm alcohol. The residual deep brownish-red powder consists usually of nearly pure azolitmin, more rarely of spaniolitmin, a substance very similar to azolitmin, but which does not contain nitrogen.
Since spaniolitmin occurs so rarely in litmus, and erythrolein is coloured reddish-purple and not blue by alkalis, Kane considers azolitmin and erythrolitmin to be the essential colouring matters of litmus, in which they are combined with ammonia, potash, and lime, and mixed with a considerable quantity of chalk, gypsum, etc.
Azolitmin is a deep brownish-red amorphous powder, insoluble in alcohol and sparingly soluble in water, but readily soluble in alkaline solutions with a pure blue colour. Its ammoniacal solution gives with metallic salt solutions blue or purple precipitates according as they are more or less basic in character. Kane's formula for azolitmin is C9H10NO6, but Gerhardt considers it is best represented by C7H7NO4. It differs from all the other colouring matters isolated from litmus by containing nitrogen. Gerhardt considered it to be derived from orcinol, possibly in accordance with the following equation: C7H6O2 + NH3 + 3O = C7H7NO4 + H2O, or from orcein thus, C7H7NO3 + O = C7H7NO4.
If the percentage composition assigned to this substance is correct, the explanation of the part played by the necessary alkaline carbonate in the manufacture of litmus may be that it facilitates and increases the oxidation of the orcinol, so that the orcein at first formed is changed into azolitmin (Gerhardt, Ch. Org., 3, 816).
Scheitz (Zeitsch. anal. Chem., 1910, 49, 736) has isolated from litmus a blue colouring matter distinct from azolitmin in quantity equivalent to 1,5 per cent, of the weight of the purified material. It consists of a bright brown powder soluble in formic acid, pyridine, and ammonia, forming a bluish-violet solution with the last-named solvent. It absorbs ammonia gas with production of a dark blue ammonia compound, which dissolves in water to a reddish solution. This ammonia compound is a more delicate indicator than the corresponding derivative of azolitmin.
Erythrolitmin, which also constitutes one of the most important ingredients of litmus, is a bright-red powder, sparingly soluble in water and in ether. It is abundantly soluble in alcohol, from which it may be crystallised in the form of dark-red granular crystals. In strong caustic potash it dissolves with a blue colour. With ammonia it forms a blue compound which curiously enough is totally insoluble in water. With metallic salts it forms lakes of a fine purple colour. According to Kane its formula is C13H22O6, and he considers it to be an oxidation product of his erythroleic acid (C13H22O4 ) obtained from orchil.
Erythroleïn forms a crimson semi-fluid mass, almost insoluble in water, soluble in ether and in alcohol with a red colour, and in ammonia with a purple colour. With metallic salts it gives purple lakes. Kane gives its formula as C13H22O2. Its general properties are very similar to those of the above-mentioned erythroleic acid.
Spaniolitmin occurs but rarely in litmus, hence its name. It is a bright-red substance, insoluble in alcohol and in ether, and very sparingly soluble in water. It dissolves in alkalis with a blue colour and gives lakes very similar to those of azolitmin. Kane's formula for it is C9H7O8.
Under the influence of hydrogen sulphide, the colouring matters of litmus are decolorised, Kane's idea being, that a colourless hydrogen sulphide compound is thus formed (v. also Malaguti, Ann. Chim. Phys., (iii.), 37, 206; Vogel, J. pr. Chem., (ii.), 16, 311). Nascent hydrogen, and other reducing agents such as ferrous and stannous oxide, etc., also decolorise them by reduction in the ordinary manner. Azolitmin thus yields colourless leucazolitmin, which, however, rapidly oxidises and becomes coloured on exposure to air. If stannous chloride is added to an ammoniacal solution of azolitmin, purple-coloured stannous -azolitmin is precipitated; if this is boiled with slightly acidulated water there is formed the colourless compound of stannic oxide with leucazolitmin, which, if exposed to air, changes into the bright scarlet stannic-azolitmin.
Deoxidising agents such as sulphurous acid and sulphites do not decolorise the colouring matters of litmus.
Azolitmin and erythrolitmin, suspended in water and submitted to the action of chlorine gas, are decolorised and give yellow chlorine derivatives, chlorazolitmin and chlorerythrolitmin, substances insoluble in water, but soluble in alcohol, ether, and in alkalis.
In his earliest memoir, Kane (Annalen, 36, 324) mentions that on heating the colouring matters of litmus mixed with chalk or gypsum, a red vapour is given off which condenses in the form of crystalline scales (atmérythrin) soluble in alcohol. When heated alone, this substance is not produced. Although Kane makes no subsequent mention of this body it is possible that it was indirubin or even indigotin, since at a later date Wartha (Ber., 9, 217) states that he found some samples of litmus to contain indigotin, recognisable by the violet vapour given off on heating a few cubes of the commercial product in a test tube. Its presence may have been due to the use of urine containing indoxyl in the preparation of the litmus.
Wartha (loc. cit.) gives the foliowingjesults of his examination of litmus. The commercial product is well shaken up with alcohol; the filtered purple solution thus obtained has a green fluorescence, and exhibits in the spectroscope a characteristic absorption band in the green with an almost total absorption of the violet end. The colouring matter (1) itself is obtained on evaporating the solution.
The litmus residue insoluble in alcohol is digested for twenty-four hours with distilled water, and the filtered deep- coloured solution is evaporated to dryness. The extract thus obtained is repeatedly treated with absolute alcohol containing a little glacial acetic acid and again evaporated, so that all traces of water may be removed, and there finally remains a brown powdery mass. On extracting this with absolute alcohol, a large quantity of a scarlet substance (b) is dissolved. It is similar to orcei'n and dissolves in ammonia with a reddish-purple colour. That portion of the brown powder which is insoluble in the acidified alcohol is dissolved in water, the filtered solution is evaporated to dryness, and the residue is repeatedly washed with absolute alcohol and evaporated in order to expel all traces of acetic acid. The residual brown powder, which is very soluble in water, with a reddish-brown colour, but insoluble in alcohol and in ether, is the purified and extremely sensitive colouring matter of litmus (c). Its alkaline solution is blue, its aluminium and tin lakes are violet, and its calcium and barium lakes blue. It appears to be very similar to Kane's azolitmin, but it is said not to contain nitrogen. The yield of these various colouring matters is as follows: (a) 2,3 per cent., (b) 3,4 per cent, (c) 5,7 percent. (Mitchell, Chem. News, 1876, 140).
An examination of the colouring matters of litmus was also made by Rochleder and Skraup (Wien. Anz., 1874, 118; Chem. Zentr., 1874, 424). Other references are Magner, J. Pharm. Chem., 12, 418; Desfosses, ibid., 14, 487; Peretti, ibid., 14, 539.
Of interest also in connection with this subject is the fact that when ethyl-amino-orsellinic acid is oxidised by air in alkaline solution it yields an orange-coloured dye possessing basic properties (Heinrich and Dorschky, Ber., 1904, 37, 1416).
A peculiar blue colouring matter similar to litmus, and called tournesol en drapeaux, has long been manufactured at Grand-Gallargues, Departement du Gard, France, from the Croton tinctorium belonging to the Euphorbiaceæ. Coarse linen cloth is steeped in the deep bluish-green sap expressed from the berries and the tops of the plant, then dried quickly in the open air, and exposed for one to one and a half hours between layers of straw to the ammoniacal vapours of lant or horse-dung (aluminadon) care being taken not to submit them to this influence too long. The cloth thus acquires a deep blue colour. It is then steeped in the sap a second time and dried in air till it acquires a purple or dull green. These blue cloths are or were used by the Dutch farmers for making an infusion with which to impart a red colour to the outside of their cheese, the blue being changed to red by the lactic and butyric acids of the cheese.
According to Joly (Ann. Chim. Phys., [iii.j, 6, in) the colouring matter pervades the entire plant and is readily extracted therefrom by water heated to 50-60°. On being evaporated, an azure-blue resinous mass remains. Acids change the blue colour of its aqueous solution red, and this blue is not restored by alkalis, the colour becoming thereby rather greenish. It is, therefore, probably quite distinct from the colouring matter of litmus, and is, indeed, more similar to the blue colouring matter which can be extracted from another plant belonging to the Euphorbiacea} viz. Mercurialis perennis.
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