- 1. Those which give a decided crystalline sublimate:
- (a.) Below 100°, e.g., cocaine, theine, thebaine, cantharidin.
- (b.) Between 100° and 150°, e.g., quinetum.
- (c.) Between 150° and 200°, e.g., strychnine, morphine, pilocarpine.
- 2. Those which melt, but give no crystalline sublimate:
- 3. Those which neither melt nor give a crystalline sublimate, e.g., saponin.
§ 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 | „ |