1911 Encyclopædia Britannica/Nitroglycerin
NITROGLYCERIN, C3H5(NO3)3 or CH2NO3⋅CH2NO3 glyceryl trinitrate, an explosive first obtained in 1846 by Ascanio Sobrero (Mem. Acad. Torino, 1847) by acting with a mixture of strong nitric and sulphuric acids on glycerin at the ordinary temperature. The reaction proceeds in several stages, mono-, di- and finally tri-nitrate being produced, the final stage requiring sulphuric acid as a dehydrator. When pure it is a very pale yellow oil of sp. gr. 1·614 at 4° C. and 1·60 at 15° C. One gram requires for solution between 800 and 1000 c.c. of water, 4 c.c. of absolute alcohol or 18 c.c. of wood spirit, and it is scarcely at all soluble in glycerin itself, but mixes in all proportions with ether, acetone, ethyl acetate and benzene.
In the manufacture glycerin is dropped in a very thin stream into a mixture of 3 parts of nitric (sp. gr. 1·5) and 5 parts of sulphuric acid (sp. gr. 1·84), the containing vessel being cooled by a water jacket and the acid mixture agitated by a stream of cooled air, the temperature being kept at about 15° C. A considerable excess of acids is necessary for the completion and safety of the reaction, usually about 8 parts of the acid mixture to 1 of glycerin. The higher the strength of the acids the higher the yield of nitroglycerin and the smaller the loss by solution in the waste acids. In recent practice some sulphin trioxide, or fuming sulphuric acid, is added, so that the mixture of acids contains less than 1% of water. The action is very rapid, and the product, which rises to the top of the acids, is separated and washed successively with cold and then tepid water, and finally with water made slightly alkaline with sodium carbonate or hydroxide, to remove all adhering or dissolved acids which would otherwise render the product very unstable. Nitroglycerin dissolves a little water and then appears thick or milky. Generally it is either dried, after being separated from the wash water, by means of common salt, upon a layer of which the moist nitroglycerin is gently run and allowed to drain or filter through, or it is filtered through a mass of dry sponge or similar dry and porous material.
Under ordinary pressure it boils at above 200° C. (L. de Bruyn). If gradually heated it begins to vaporize and decompose at about 130°, and as a rule it detonates when heated slightly above this temperature, previously giving off some red fumes. A little vapour is given off at ordinary temperatures and pressures, and when under a few millimetres pressure only it rapidly vaporizes below 100° C. The freezing-point is uncertain, owing perhaps to the existence of two modifications, as suggested by Kast (Zeits. f. ges. Schiess- u. Sprengstoff, 1·225; see also S. Nauckhoff, Zeits. f. ang. Chem., 18, Heft 1 and 2). It is frequently given as 43° to 46° F. (about 6° to 8° C.), and it is stated to be more sensitive to percussion when frozen (Beilstein). It crystallizes (in long needles) more easily when gently agitated during the cooling, or when mixed with such substances as kieselguhr. At one time it was transported all over America in a frozen condition without serious accidents, and according to Sir F. Nathan (Jour. Soc. Chem. Ind., 1908, 27, p. 5) it is safer to export in the frozen state. To prevent the freezing of nitroglycerin in dynamite it has been proposed to add various substances, such as chlordinitroglycerin, nitrated diglycerin or tetranitrodiglycerol, and also mono- and di-nitroglycerin. The latter two have been studied by C. W. Will (Ber., 1908, 7, p. 407), who obtained two isomeric dinitroglycerins, one of which is eminently crystallizable and the other fluid. Both are sensitive to percussion, but a little less so than nitroglycerin. The mononitroglycerin also exists in two forms, neither of which is strictly speaking explosive. It appears that an addition of dinitroglycerin to nitroglycerin would materially retard its freezing or lessen its sensitiveness (see also C. Claessen, Ger. Pat. 210990 (1909)).
| Mono. | Di. | Tri. | |
| Specific gravity | 1·40 | 1·47 | 1·6 |
| Melting-point | α 58° | α hydrate, 26° | labile, 2·2° |
| β 54° | β hydrate (fluid) | stable, 12·2° | |
| Boiling-point 18 mm. | 155-160° | 145° about | 160° |
| Solubility | 70% | 7·7% | ·16% |
The liquid when soaked into a porous combustible substance like blotting-paper burns rapidly and quietly, and when struck with a hammer on a hard surface violently detonates; when a little of the liquid is spread on an anvil and struck, the portion immediately under the hammer only will, as a rule, detonate, the remainder being scattered. Some solutions of nitroglycerin (in ether, acetone, &c.) burn quietly, and the same is the case when it is held in solution or suspension in a colloid substance, as gelatinized guncotton, &c.
Strong sulphuric acid dissolves nitroglycerin, and this solution on being poured into water yields dinitroglycerin (see Will, loc. cit.) and also some mononitroglycerin. When the solution in the strong acid is allowed to stand, some nitric acid is first evolved, and as the temperature rises this is followed by a general decomposition of the substance, though not necessarily an explosive one. Shaken with mercury and sulphuric acid, nitroglycerin yields its nitrogen as nitric oxide; the measurement of the volume of this gas is a convenient mode of estimating nitroglycerin. Ammonium hydroxide has no appreciable action at ordinary temperatures, but strong solutions of sodium or potassium hydroxides start a decomposition, with rise of temperature, in which some nitrate and always some nitrite is produced. Some glycerin may be re-formed, but with very strong alkaline solutions little of the glycerin molecule escapes destruction, oxalic acid and several other products resulting. Alcoholic solutions of the alkalis also produce much nitrite along with some formate and acetate. Calcium or potassium sulphides and potassium hydrosulphides completely reduce nitroglycerin to glycerin, some of the sulphur being oxidized and some precipitated. Hydriodic acid reduces it to glycerin and nitric oxide. Aniline and similar bases are oxidized and partially nitrated by nitroglycerin, with the production of non-explosive compounds.
The first attempts to utilize the explosive power of nitroglycerin were made by Nobel in 1863; they were only partially successful until the plan, first applied by General Pictot in 1854, of developing the force of gunpowder in the most rapid manner and to the maximum extent, through initiative detonation, was applied by Nobel to nitroglycerin. Even then, however, the liquid nature of the substance, though advantageous in one or two directions, constituted a serious obstacle to its safe transport and storage and to its efficient employment; it was therefore not until Nobel produced plastic solid preparations by mixing the liquid with porous substances, such as gunpowder, or carbon and sulphur, and finally kieselguhr in a fine state of division, capable of absorbing and retaining considerable quantities of it, that it could be employed as a blasting agent (see Explosives, Dynamite, Cordite). (W. R. E. H.)
Therapeutics.—Nitroglycerin has a sweet burning taste and is decidedly poisonous. Its vapour produces violent headache, and the same effect is often caused by handling compositions containing it. Prior to its use as an explosive, its alcoholic solution found application in medicine under the name of glonoin. Although a nitrate, its pharmacological actions resemble those of nitrites such as amyl nitrite, taken internally. The explanation is that in an alkaline medium at body heat nitroglycerin yields a nitrite, probably as a preliminary stage of resolution. Nitroglycerin shaken up with warm very dilute alkaline solutions, as sodium carbonate, for a few minutes only, always yields sufficient nitrite to give the diazoreaction; and, as stated, strong alkaline solutions always produce some nitrite as one of the decomposition products. This gradual conversion in the tissues is a valuable property of nitroglycerin, as its effects take longer to manifest themselves than is the case with amyl and other nitrites. Nitroglycerin is valuable as a preventive in cases of cardiac pain, such as angina pectoris, and it is also used in other conditions where it is desirable to reduce the arterial tension. The British Pharmacopoeia contains a liquor trinitrini (1%), and tablets made up with chocolate, each containing one-hundredth of a grain.