§ 311. Scheibler’s Process.—A method very different from those just described is one practised by Scheibler. This is to precipitate the phosphotungstate of the alkaloid, and then to liberate the latter by digesting the precipitate with either hydrate of barium or hydrate of calcium, dissolving it out by chloroform, or, if volatile, by simple distillation. The convenience of Scheibler’s process is great, and it admits of very general application. In complex mixtures, it will usually be found best to precede the addition of phosphotungstic acid[337] by that of acetate of lead, in order to remove colouring matter, &c.; the excess of lead must in its turn be thrown out by SH2, and the excess of SH2 be got rid of by evaporation. Phosphotungstic acid is a very delicate test for the alkaloids, giving a distinct precipitate with the most minute quantities (1⁄200000 of strychnine and 1⁄100000 of quinine). A very similar method is practised by Sonnenschein and others with the aid of phospho-molybdic acid. The details of Scheibler’s process are as follows:—
[337] The method of preparing this reagent is as follows:—Ordinary commercial sodium tungstate is treated with half its weight of phosphoric acid, specific gravity, 1·13, and then allowed to stand for some days. Phosphotungstic acid separates in crystals.
The organic mixture is repeatedly extracted by water strongly acidified with sulphuric acid; the extract is evaporated at 30° to the consistence of a thin syrup; then diluted with water, and, after several hours’ standing, filtered in a cool place. To the filtered fluid phosphotungstic acid is added in excess, the precipitate filtered, washed with water to which some phosphotungstic acid solution has been added, and, whilst still moist, rinsed into a flask. Caustic baryta or carbonate of potash is added to alkaline reaction, and after the flask has been connected with bulbs containing HCl, it is heated at first slowly, then more strongly. Ammonia and any volatile alkaloids are driven over into the acid, and are there fixed, and can be examined later by suitable methods. The residue in the flask is carefully evaporated to dryness (the excess of baryta having been precipitated by CO2), and then extracted by strong alcohol. On evaporation of the alcohol, the alkaloid is generally sufficiently pure to be examined, or, if not so, it may be obtained pure by re-solution, &c.
The author has had considerable experience of Scheibler’s process, and has used it in precipitating various animal fluids, but has generally found the precipitate bulky and difficult to manage.
§ 312. Grandval and Lajoux’s Method.[338]—The alkaloids are precipitated from a solution slightly acidified by hydrochloric or sulphuric acid by a solution of hydrarg-potassium iodide. The precipitate is collected on a filter, washed and then transferred to a flask; drop by drop, a solution of sodium sulphide is added; after each addition the suspended precipitate is shaken and allowed to stand for a few minutes, and a drop of the liquid taken out and tested with lead acetate; directly a slight brown colour appears, sufficient sodic sulphide has been added. The liquid is now left for half-an-hour, with occasional shaking. Then sulphuric acid is added until it is just acid, and the liquid is filtered and the mercury sulphide well washed. In the filtrate will be the sulphate of any alkaloid in solution; this liquid is now made alkaline with soda carbonate and shaken up, as in Dragendorff’s process, with appropriate solvents; such, for example, as ether, or chloroform, or acetone, or amylic alcohol, according to the particular alkaloid the analyst is searching for, and the solvent finally separated and allowed to evaporate, when the alkaloid is found in the residue.
[338] “Dosage des alcaloides à l’aide de l’iodure double de mercure et de potassium,” par MM. A. Grandval et Henri Lajoux, Journ. de Pharmacie, 5 sér. t. xxviii. 152-156.
§ 313. Identification of the Alkaloids.—Having obtained, in one way or other, a crystalline or amorphous substance, supposed to be an alkaloid, or, at all events, an active vegetable principle, the next step is to identify it. If the tests given in Dragendorff’s process have been applied, the observer will have already gone a good way towards the identification of the substance; but it is, of course, dangerous to trust to one reaction.
In medico-legal researches there is seldom any considerable quantity of the material to work upon. Hence the greatest care must be taken from the commencement not to waste the substance in useless tests, but to study well at the outset what—by the method of extraction used, the microscopic appearance, the reaction to litmus paper, and the solubility in different menstrua—it is likely to be. However minute the quantity may be, it is essential to divide it into different parts, in order to apply a variety of tests; but as any attempt to do this on the solid substance will probably entail loss, the best way is to dissolve it in a watch-glass in half a c.c. of alcohol, ether, or other suitable solvent. Droplets of this solution are then placed on watch-glasses or slips of microscopic glass, and to these drops, by the aid of a glass rod, different reagents can be applied, and the changes watched under the microscope as the drops slowly evaporate.
§ 314. Sublimation of the Alkaloids.—A very beautiful and elegant aid to the identification of alkaloids, and vegetable principles generally, is their behaviour towards heat.
Alkaloids, glucosides, the organic acids, &c., when carefully heated, either—(1) sublime wholly without decomposition (like theine, cytisin, and others); or (2) partially sublime with decomposition; or (3) are changed into new bodies (as, for example, gallic acid); or (4) melt and then char; or (5) simply char and burn away.
Many of these phenomena are striking and characteristic, taking place at different temperatures, subliming in characteristic forms, or leaving characteristic residues.
One of the first to employ sublimation systematically, as a means of recognition of the alkaloids, &c., was Helwig.[339] His method was to place a small quantity (from 1⁄2 to 1⁄4000 of a mgrm.) in a depression on platinum foil, cover it with a slip of glass, and then carefully heat by a small flame. After Helwig, Dr. Guy[340] greatly improved the process by using porcelain discs, and more especially by the adoption of a convenient apparatus, which may be termed “the subliming cell.” It is essentially composed of a ring of glass from 1⁄8 to 2⁄3 of an inch in thickness, such as may be obtained by sections of tubing, the cut surfaces being ground perfectly smooth. This circle is converted into a closed cell by resting it on one of the ordinary thin discs of glass used as a covering for microscopic purposes, and supporting a similar disc. The cell was placed on a brass plate, provided with a nipple, which carried a thermometer, and was heated by a small flame applied midway between the thermometer and the cell; the heat was raised very gradually, and the temperature at which any change took place was noted. In this way Dr. Guy made determinations of the subliming points of a large number of substances, and the microscopic appearances of the sublimates were described with the greatest fidelity and accuracy. On repeating with care Dr. Guy’s determinations, however, I could in no single instance agree with his subliming points, nor with the apparatus he figures and describes could two consecutive observations exactly coincide. Further, on examining the various subliming temperatures of substances, as stated by different authors, the widest discrepancies were found—differences of 2 or even 3 degrees might be referred to errors of observation, a want of exact coincidence in the thermometers employed, and the like; but to what, for example, can we ascribe the irreconcilable statements which have been made with regard to theine? According to Strauch, this substance sublimes at 177°; according to Mulder, at 184·7°. But that both of these observations deviate more than 70° from the truth may be proved by any one who cares to place a few mgrms. of theine, enclosed between two watch-glasses, over the water-bath; in a few minutes a distinct sublimate will condense on the upper glass, and, in point of fact, theine will be found to sublime several degrees below 100°.
[339] Das Mikroskop in der Toxicologie.
[340] Pharm. Journ. Trans. (2), viij. 719; ix. 10, 58. Forensic Medicine, London, 1875.
Since this great divergency of opinion is not found either in the specific gravity, or the boiling-points, or any of the like determinations of the physical properties of a substance, it is self-evident that the processes hitherto used for the determination of subliming points are faulty. The sources of error are chiefly—
(1.) Defects in the apparatus employed—the temperature read being rather that of the metallic surface in the immediate vicinity of the thermometer than of the substance itself.
(2.) The want of agreement among observers as to what should be called a sublimate—one considering a sublimate only that which is evident to the naked eye, another taking cognisance of the earliest microscopic film.
(3.) No two persons employing the same process.
With regard to the apparatus employed, I adopt Dr. Guy’s subliming cell; but the cell, instead of resting on a metallic solid, floats on a metallic fluid. For any temperature a little above 100° this fluid is mercury, but for higher temperatures fusible metal is preferable.
The exact procedure is as follows:—A porcelain crucible (a in fig.), about 3 inches in diameter, is nearly filled with mercury or fusible metal, as the case may be; a minute speck (or two or three crystals of the substance to be examined) is placed on a thin disc of microscopic covering glass, floated on the liquid, and the cell is completed by the glass ring and upper disc. The porcelain crucible is supported on a brass plate (b), fixed to a retort-stand in the usual way, and protected from the unequal cooling effects of currents of air by being covered by a flask (c), from which the bottom has been removed. The neck of the flask conveniently supports a thermometer, which passes through a cork, and the bulb of the thermometer is immersed in the bath of liquid metal. In the first examination of a substance the temperature is raised somewhat rapidly, taking off the upper disc with a forceps at every 10° and exchanging it for a fresh disc, until the substance is destroyed. The second examination is conducted much more slowly, and the discs exchanged at every 4° or 5°, whilst the final determination is effected by raising the temperature with great caution, and exchanging the discs at about the points of change (already partially determined) at every half degree. All the discs are examined microscopically. The most convenient definition of a sublimate is this—the most minute films, dots, or crystals, which can be observed by 1⁄4-inch power, and which are obtained by keeping the subliming cell at a definite temperature for 60 seconds. The commencement of many sublimates assumes the shape of dots of extraordinary minuteness, quite invisible to the unaided eye; and, on the other hand, since the practical value of sublimation is mainly as an aid to other methods for the recognition of substances, if we go beyond short intervals of time, the operation, otherwise simple and speedy, becomes cumbersome, and loses its general applicability.
There is also considerable discrepancy of statement with regard to the melting-point of alkaloidal bodies; in many instances a viscous state intervenes before the final complete resolution into fluid, and one observer will consider the viscous state, the other complete fluidity, as the melting-point.
In the melting-points given below, the same apparatus was used, but the substance was simply placed on a thin disc of glass floating on the metallic bath before described (the cell not being completed), and examined from time to time microscopically, for by this means alone can the first drops formed by the most minute and closely-adherent crystals to the glass be discovered.
Cocaine melts at 93°, and gives a faint sublimate at 98°; if put between two watch-glasses on the water-bath, in fifteen minutes there is a good cloud on the upper glass.
Aconitine turns brown, and melts at 179° C.; it gives no characteristic sublimate up to 190°.
Morphine, at 150°, clouds the upper disc with nebulæ; the nebulæ are resolved by high magnifying powers into minute dots; these dots gradually become coarser, and are generally converted into crystals at 188°; the alkaloid browns at or about 200°.
Thebaine sublimes in theine-like crystals at 135°; at higher temperatures (160° to 200°), needles, cubes, and prisms are observed. The residue on the lower disc, if examined before carbonisation, is fawn-coloured with non-characteristic spots.
Narcotine gives no sublimate; it melts at 155° into a yellow liquid, which, on raising the temperature, ever becomes browner to final blackness. On examining the residue before carbonisation, it is a rich brown amorphous substance; but if narcotine be heated two or three degrees above its melting-point, and then cooled slowly, the residue is crystalline—long, fine needles radiating from centres being common.
Narceine gives no sublimate; it melts at 134° into a colourless liquid, which undergoes at higher temperatures the usual transition of brown colours. The substance, heated a few degrees above its melting-point, and then allowed to cool slowly, shows a straw-coloured residue, divided into lobes or drops containing feathery crystals.
Papaverine gives no sublimate; it melts at 130°. The residue, heated a little above its melting-point, and then slowly cooled, is amorphous, of a light-brown colour, and in no way characteristic.
Hyoscyamine gives no crystalline sublimate; it melts at 89°, and appears to volatilise in great part without decomposition. It melts into an almost colourless fluid, which, when solid, may exhibit a network not unlike vegetable parenchyma; on moistening the network with water, interlacing crystals immediately appear. If, however, hyoscyamine be kept at 94° to 95° for a few minutes, and then slowly cooled, the edges of the spots are arborescent, and the spots themselves crystalline.
Atropine (daturine) melts at 97°; at 123° a faint mist appears on the upper disc. Crystals cannot be obtained; the residue is not characteristic.
Solanine.—The upper disc is dimmed with nebulæ at 190°, which are coarser and more distinct at higher temperatures; at 200° it begins to brown, and then melts; the residue consists of amber-brown, non-characteristic drops.
Strychnine gives a minute sublimate of fine needles, often disposed in lines, at 169°; about 221° it melts, the residue (at that temperature) is resinous.
Brucine melts at 151° into a pale yellow liquid, at higher temperatures becoming deep-brown. If the lower disc, after melting, be examined, no crystals are observed, the residue being quite transparent, with branching lines like the twigs of a leafless tree; light mists, produced rather by decomposition than by true sublimation, condense on the upper disc at 185°, and above.
Saponin neither melts nor sublimes; it begins to brown about 145°, is almost black at 185°, and quite so at 190°.
Delphinine begins to brown about 102°; it becomes amber at 119°, and melts, and bubbles appear. There is no crystalline sublimate; residue not characteristic.
Pilocarpine gives a distinct crystalline sublimate at 153°; but thin mists, consisting of fine dots, may be observed as low as 140°. Pilocarpine melts at 159°; the sublimates at 160° to 170° are in light yellow drops. If these drops are treated with water, and the water evaporated, feathery crystals are obtained; the residue is resinous.
Theine wholly sublimes; the first sublimate is minute dots, at 79°; at half a degree above that very small crystals may be obtained; and at such a temperature as 120°, the crystals are often long and silky.
Theobromine likewise wholly sublimes; nebulæ at 134°, crystals at 170°, and above.
Salicin melts at 170°; it gives no crystalline sublimate. The melted mass remains up to 180° almost perfectly colourless; above that temperature browning is evident. The residue is not characteristic.
Picrotoxin gives no crystalline sublimate. The lowest temperature at which it sublimes is 128°; the usual nebulæ then make their appearance; between 165° and 170° there is slight browning; at 170° it melts. The residue, slowly cooled, is not characteristic.
Cantharidin sublimes very scantily between 82° and 83°; at 85° the sublimate is copious.
The active principles of plants may, in regard to their behaviour to heat, be classed for practical purposes into—
§ 315. Melting-point.—The method of sublimation just given also determines the melting-point; such a determination will, however, seldom compare with the melting-points of the various alkaloids as given in text-books, because the latter melting-points are not determined in the same way. The usual method of determining melting-points is to place a very small quantity in a glass tube closed at one end; the tube should be almost capillary. The tube is fastened to a thermometer by means of platinum wire, and then the bulb of the thermometer, with its attached tube, is immersed in strong sulphuric acid or paraffin, contained in a flask. The thermometer should be suspended midway in the liquid and heat carefully applied, so as to raise the temperature gradually and equably. It will be found that rapidly raising the heat gives a different melting-point to that which is obtained by slowly raising the heat. During the process careful watching is necessary: most substances change in hue before they actually melt. A constant melting-point, however often a substance is purified by recrystallisation, is a sign of purity.
§ 316. Identification by Organic Analysis.—In a few cases (and in a few only) the analyst may have sufficient material at hand to make an organic analysis, either as a means of identification or to confirm other tests. By the vacuum process described in “Foods,” in which carbon and nitrogen are determined by measuring the gases evolved by burning the organic substance in as complete a vacuum as can be obtained, very minute quantities of a substance can be dealt with, and the carbon and nitrogen determined with fair accuracy. It is found in practice that the carbon determinations appear more reliable than those of the nitrogen, and there are obvious reasons why this should be so.
Theoretically, with the improved gas-measuring appliances, it is possible to measure a c.c. of gas; but few chemists would care to create a formula on less than 10 c.c. of CO2. Now, since 10 c.c. of CO2 is equal to 6·33 mgrms. of carbon, and alkaloids average at least half their weight of carbon, it follows that 12 mgrms. of alkaloid represent about the smallest quantity with which a reliable single combustion can be made.
The following table gives a considerable number of the alkaloids and alkaloidal bodies, arranged according to their content in carbon:—
TABLE SHOWING THE CONTENT OF CARBON AND NITROGEN IN VARIOUS ALKALOIDAL BODIES.
| Carbon. | Nitrogen. | |||
|---|---|---|---|---|
| Asparagin, | 36 | ·36 | 21 | ·21 |
| Methylamine, | 38 | ·71 | 45 | ·17 |
| Betaine, | 44 | ·44 | 10 | ·37 |
| Theobromine, | 46 | ·67 | 31 | ·11 |
| Theine, | 49 | ·48 | 28 | ·86 |
| Indican, | 49 | ·60 | 2 | ·22 |
| Muscarine, | 50 | ·42 | 11 | ·77 |
| Lauro-cerasin, | 52 | ·47 | 1 | ·53 |
| Amanitine, | 57 | ·69 | 13 | ·46 |
| Narceine, | 59 | ·63 | 3 | ·02 |
| Colchicine, | 60 | ·53 | 4 | ·15 |
| Oxyacanthine, | 60 | ·57 | 4 | ·42 |
| Solanine, | 60 | ·66 | 1 | ·68 |
| Trimethylamine, | 61 | ·02 | 23 | ·73 |
| Jervine, | 61 | ·03 | 5 | ·14 |
| Sabadilline, | 61 | ·29 | 3 | ·46 |
| Aconitine, | 61 | ·21 | 2 | ·16 |
| Nepaline, | 63 | ·09 | 2 | ·12 |
| Colchicein, | 63 | ·44 | 4 | ·38 |
| Veratroidine, | 63 | ·8 | 3 | ·1 |
| Narcotine, | 63 | ·92 | 3 | ·39 |
| Veratrine, | 64 | ·42 | 2 | ·91 |
| Delphinine, | 64 | ·55 | 3 | ·42 |
| Physostigmine, | 65 | ·49 | 15 | ·27 |
| Rhœadine, | 65 | ·79 | 3 | ·65 |
| Cocaine, | 66 | ·44 | 4 | ·84 |
| Gelsemine, | 67 | ·00 | 7 | ·10 |
| Conhydrine, | 67 | ·12 | 9 | ·79 |
| Staphisagrine, | 67 | ·5 | 3 | ·6 |
| Chelidonine, | 68 | ·06 | 12 | ·34 |
| Atropine, Hyoscyamine, | 70 | ·58 | 4 | ·84 |
| Sanguinarine, | 70 | ·59 | 4 | ·33 |
| Papaverine, | 70 | ·79 | 4 | ·13 |
| Delphinoidine, | 70 | ·9 | 3 | ·9 |
| Morphine and Piperine, | 71 | ·58 | 4 | ·91 |
| Berberine, | 71 | ·64 | 4 | ·18 |
| Codeine, | 72 | ·24 | 4 | ·68 |
| Thebaine, | 73 | ·31 | 4 | ·50 |
| Cytisine, | 73 | ·85 | 12 | ·92 |
| Nicotine, | 74 | ·08 | 17 | ·28 |
| Quinine, | 75 | ·02 | 8 | ·64 |
| Coniine, | 76 | ·81 | 11 | ·20 |
| Strychnine, | 77 | ·24 | 8 | ·92 |
| Curarine, | 81 | ·51 | 5 | ·28 |
§ 317. Quantitative Estimation of the Alkaloids.—For medico-legal purposes the alkaloid obtained is usually weighed directly, but for technical purposes other processes are used. One of the most convenient of these is titration with normal or decinormal sulphuric acid, a method applicable to a few alkaloids of marked basic powers—e.g., quinine is readily and with accuracy estimated in this way, the alkaloid being dissolved in a known volume of the acid, and then titrated back with soda. If a large number of observations are to be made, an acid may be prepared so that each c.c. equals 1 mgrm. of quinine. A reagent of general application is found in the so-called Mayer’s reagent, which consists of 13·546 grms. of mercuric chloride, and 49·8 grms. of iodide of potassium in a litre of water. Each c.c. of such solution precipitates—
| Of | Strychnine, | ·0167 | grm. |
| „ | Brucine, | ·0233 | „ |
| „ | Quinine, | ·0108 | „ |
| „ | Cinchonine, | ·0102 | „ |
| „ | Quinidine, | ·0120 | „ |
| „ | Atropine, | ·0145 | „ |
| „ | Aconitine, | ·0268 | „ |
| „ | Veratrine, | ·0269 | „ |
| „ | Morphine, | ·0200 | „ |
| „ | Narcotine, | ·0213 | „ |
| „ | Nicotine, | ·00405 | „ |
| „ | Coniine, | ·00416 | „ |
The final reaction is found by filtering, from time to time, a drop on to a glass plate, resting on a blackened surface, and adding the test until no precipitate appears. The results are only accurate when the strength of the solution of the alkaloid is about 1 : 200; so that it is absolutely necessary first to ascertain approximatively the amount present, and then to dilute or concentrate, as the case may be, until the proportion mentioned is obtained.
A convenient method of obtaining the sulphate of an alkaloid for quantitative purposes, and especially from organic fluids, is that recommended by Wagner. The fluid is acidulated with sulphuric acid, and the alkaloid precipitated by a solution of iodine in iodide of potassium. The precipitate is collected and dissolved in an aqueous solution of hyposulphite of soda. The filtered solution is again precipitated with the iodine reagent, and the precipitate dissolved in sulphurous acid, which, on evaporation, leaves behind the pure sulphate of the base.
It is also very useful for quantitative purposes to combine an alkaloid with gold or platinum, by treating the solution with the chlorides of either of those metals—the rule as to selection being to give that metal the preference which yields the most insoluble and the most crystallisable compound.
The following table gives the percentage of gold or platinum left on ignition of the double salt:—
| Gold. | Platinum. | |||||
|---|---|---|---|---|---|---|
| Atropine, | 31 | ·57 | ... | |||
| Aconitine | 20 | ·0 | ... | |||
| Amanitine, | 44 | ·23 | ... | |||
| Berberine, | 29 | ·16 | 18 | ·11 | ||
| Brucine, | ... | 16 | ·52 | |||
| Cinchonine, | ... | 27 | ·36 | |||
| Cinchonidine, | ... | 27 | ·87 | |||
| Codeine, | ... | 19 | ·11 | |||
| Coniine, | ... | 29 | ·38 | |||
| Curarine, | ... | 32 | ·65 | |||
| Delphinine, | 26 | ·7 | ... | |||
| Delphinoidine, | 29 | ·0 | 15 | ·8 | ||
| Emetine, | ... | 29 | ·7 | |||
| Hyoscyamine, | 34 | ·6 | ... | |||
| Morphine, | ... | 19 | ·52 | |||
| Muscarine, | 43 | ·01 | ... | |||
| Narcotine, | 15 | ·7 | 15 | ·9 | ||
| Narceine, | ... | 14 | ·52 | |||
| Nicotine, | ... | 34 | ·25 | |||
| Papaverine, | ... | 17 | ·82 | |||
| Pilocarpine, | 35 | ·5 | 23 | ·6 to 25·2. | ||
| Piperine, | ... | 12 | ·7 | |||
| Quinine, | 40 | ·0 | 26 | ·26 | ||
| Strychnine, | 29 | ·15 | 18 | ·16 | ||
| Thebaine, | ... | 18 | ·71 | |||
| Theine, | 37 | ·02 | 24 | ·58 | ||
| Theobromine, | ... | 25 | ·55 | |||
| Veratrine, | 21 | ·01 | ... | |||
THE ALKALOIDS OF HEMLOCK—NICOTINE—PITURIE—SPARTEINE.
§ 318. The Conium maculatum, or spotted hemlock, is a rather common umbelliferous plant, growing in waste places, and flowering from about the beginning of June to August. The stem is from three to five feet high, smooth, branched, and spotted with purple; the leaflets of the partial involucres are unilateral, ovate, lanceolate, with an attenuate point shorter than the umbels; the seeds are destitute of vittæ, and have five prominent crenate wavy ridges. The whole plant is fœtid and poisonous. Conium owes its active properties to a volatile liquid alkaloid, Coniine, united with a crystalline alkaloid, Conhydrine.
§ 319. Coniine (conia, conicine), (C8H17N)—specific gravity 0·862 at 0°; melting-point, -2·5°; boiling-point, 166·6°. Pure coniine has been prepared synthetically by Ladenburg, and found to be propyl-piperidine C5H10NC3H7, but the synthetically-prepared piperidine has no action on polarised light. By uniting it with dextro-tartaric acid, and evaporating, it is possible to separate the substance into dextro-propyl-piperidine and lævo-propyl-piperidine. The former is in every respect identical with coniine from hemlock; it is a clear, oily fluid, possessing a peculiarly unpleasant, mousey odour. One part is soluble in 150 parts of water,[341] in 6 parts of ether, and in almost all proportions of amyl alcohol, chloroform, and benzene. It readily volatilises, and, provided air is excluded, may be distilled unchanged. It ignites easily, and burns with a smoky flame. It acts as a strong base, precipitating the oxides of metals and alkaline earths from their solutions, and it coagulates albumen. Coniine forms salts with hydrochloric acid (C8H15N.HCl), phosphoric acid, iodic acid, and oxalic acid, which are in well-marked crystals. The sulphate, nitrate, acetate, and tartrate are, on the other hand, non-crystalline.
[341] The saturated watery solution of coniine at 15°, becomes cloudy if gently warmed, and clears again on cooling.
If coniine is oxidised with nitric acid, or bichromate of potash, and diluted sulphuric acid, butyric acid is formed; and since the latter has an unmistakable odour, and other characteristic properties, it has been proposed as a test for coniine. This may be conveniently performed thus:—A crystal of potassic bichromate is put at the bottom of a test-tube, and some diluted sulphuric acid with a drop of the supposed coniine added. On heating, the butyric acid reveals itself by its odour, and can be distilled into baryta water, the butyrate of baryta being subsequently separated in the usual way, and decomposed by sulphuric acid, &c.
Another test for coniine is the following:—If dropped into a solution of alloxan, the latter is coloured after a few minutes an intense purple-red, and white needle-shaped crystals are separated, which dissolve in cold potash-lye into a beautiful purple-blue, and emit an odour of the base.[342] Dry hydrochloric acid gives a purple-red, then an indigo-blue colour, with coniine; but if the acid is not dry, there is formed a bluish-green crystalline mass. This test, however, is of little value to the toxicologist, the pure substance alone responding with any definite result.
[342] Schwarzenbach, Vierteljahrsschr. f. prakt. Pharm., viij. 170.
The ordinary precipitating agents, according to Dragendorff, act as follows:—
Potass bismuth iodide.
Phosphomolybdic acid gives a strong yellow precipitate; limit, 1 : 5000.
Potass. mercuric iodide gives a cheesy precipitate; limit, 1 : 1000 in neutral, 1 : 800 in acid, solutions.
Potass. cadmic iodide gives an amorphous precipitate, 1 : 300. The precipitate is soluble in excess of the precipitant. (Nicotine, under similar circumstances, gives a crystalline precipitate.)
Flückiger recommends the following reaction:[343]—“Add to 10 drops of ether in a shallow glass crystallising dish 2 drops of coniine, and cover with filter paper. Set upon the paper a common-sized watch-glass containing bromine water, and invert a beaker over the whole arrangement. Needle-shaped crystals of coniine hydro-bromine soon form in the dish as well as in the watch-glass.” Hydrochloric acid, used in the same way, instead of bromine water, forms with coniine microscopic needles of coniine hydrochlorate; both the hydro-bromide and the hydrochlorate doubly refract light. Nicotine does not respond to this reaction.