[Footnote B: Jour. Chem. Soc., W.H. Perkin, 1889, p. 726.]
The solubility of nitro-glycerine in various solvents has been investigated by A.H. Elliot; his results may be summarised as follows:—
_______________________________________________________________________ | | Solvent. | Cold. | Warm. _____________________________|______________________|__________________ | | Water | Insoluble | Slightly soluble Alcohol, absolute | Soluble | Soluble " 93% | " | " " 80% | Slowly soluble | " " 50% | Insoluble | Slightly soluble Methyl alcohol | Soluble | Soluble Amyl " | " | " Ether, ethylic | " | " " acetic | " | " Chloroform | " | " Acetone | " | " Sulphuric acid (1.845) | " | " Nitric acid (1.400) | Slowly soluble | " Hydrochloric acid (1.200) | Insoluble, decomposed| Slowly soluble Acetic acid, glacial | Soluble | Soluble Carbolic acid | " | " Astral oil | Insoluble | Insoluble Olive " | Soluble | Soluble Stearine oil | " | " Mineral jelly | Insoluble | Insoluble Glycerine | " | " Benzene | Soluble | Soluble Nitro-benzene | " | " Toluene | " | " Carbon bi-sulphide | Insoluble | Slightly affected Turpentine | " | Soluble Petroleum naphtha, 71°-76° B.| " | Insoluble Caustic soda (1:10 solution) | Insoluble. | Insoluble. Borax, 5% solution | " | " Ammonia (.980) | " | " slightly | | affected. Ammonium sulph-hydrate | Insoluble, sulphur | Decomposed. | separates | Iron sulphate solution | Slightly affected | Affected. Iron chloride (1.4 grm. Fe | Slowly affected | Decomposed. to 10 c.c. N_{2}O) | | Tin chloride | Slightly affected | Affected. _____________________________|______________________|__________________
Many attempts have been made to prepare nitro-glycerine explosives capable of withstanding comparatively low temperatures without freezing, but no satisfactory solution of the problem has been found. Among the substances that have been proposed and used with more or less success, are nitro- benzene, nitro-toluene, di-nitro-mono-chlorhydrine, solid nitro derivatives of toluene,[A] are stated to lower the freezing point of nitro-glycerine to -20°C. without altering its sensitiveness and stability. The subject has been investigated by S. Nauckhoff,[B] who states that nitroglycerine can be cooled to temperatures (-40° to -50° C.) much below its true freezing point, without solidifying, by the addition of various substances. When cooled by means of a mixture of solid carbon, dioxide, and ether, it sets to a glassy mass, without any perceptible crystallisation. The mass when warmed to 0°C. first rapidly liquefies and then begins to crystallise. The true freezing point of pure nitro- glycerine was found to be 12.3°C. The technical product, owing to the presence of di-nitro-glycerine, freezes at 10.5° C. According to Raoult's law, the lowering of the freezing point caused by m grms. of a substance with the molecular weight M, when dissolved in 100 grms. of the solvent, is expressed by the formula: [Delta] = E(m/M), where E is a constant characteristic for the solvent in question. The value of E for nitro- glycerine was found to be 70.5 when calculated, according to Van't Hoff's formula, from the melting point and the latent heat of fusion of the substance. Determinations of the lowering of the freezing point of nitro- glycerine by additions of benzene, nitro-benzene, di-nitro-benzene, tri- nitro-benzene, p.-nitro-toluene, o.-nitro-toluene, di-nitro-toluene, naphthalene, nitro-naphthalene, di-nitro-naphthalene, ethyl acetate, ethyl nitrate, and methyl alcohol, gave results agreeing fairly well with Raoult's formula, except in the case of methyl alcohol, for which the calculated lowering of the freezing point was greater than that observed, probably owing to the formation of complex molecules in the solution. The results show that, in general, the capacity of a substance to lower the freezing point of nitro-glycerine depends, not upon its freezing point, or its chemical composition or constitution, but upon its molecular weight. Nauckhoff states that a suitable substance for dissolving in nitro- glycerine, in order to lower the freezing point of the latter, must have a relatively low molecular weight, must not appreciably diminish the explosive power and stability of the explosive, and must not be easily volatile at relatively high atmospheric temperatures; it should, if possible, be a solvent of nitro-cellulose, and in every case must not have a prejudicial influence on the gelatinisation of the nitro-cellulose.
[Footnote A: Eng. Pat. 25,797, November 1904.]
[Footnote B: Z. Angew. Chem., 1905, 18, 11-22, 53-60.]
~Manufacture of Nitro-Glycerine.~—Nitro-glycerine is prepared upon the manufacturing scale by gradually adding glycerine to a mixture of nitric and sulphuric acids of great strength. The mixed acids are contained in a lead vessel, which is kept cool by a stream of water continually passing through worms in the interior of the nitrating vessel, and the glycerine is gradually added in the form of a fine stream from above. The manufacture can be divided into three distinct operations, viz., nitration, separation, and washing, and it will be well to describe these operations in the above order.
~Nitration.~—The most essential condition of nitrating is the correct composition and strength of the mixed acids. The best proportions have been found to be three parts by weight of nitric acid of a specific gravity 1.525 to 1.530, and containing as small a portion of the oxides of nitrogen as possible, to five parts by weight of sulphuric acid of a specific gravity of 1.840 at 15° C., and about 97 per cent. of mono- hydrate. It is of the very greatest importance that the nitric acid should be as strong as possible. Nothing under a gravity of 1.52 should ever be used even to mix with stronger acid, and the nitration will be proportional to the strength of the acid used, provided the sulphuric acid is also strong enough. It is also of great importance that the oxides of nitrogen should be low, and that they should be kept down to as low as 1 per cent., or even lower. It is also very desirable that the nitric acid should contain as little chlorine as possible. The following is the analysis of a sample of nitric acid, which gave very good results upon the commercial scale:—Specific gravity, 1.525, N_{2}O_{4}, 1.03 per cent.; nitric acid (HNO_{3}), 95.58 per cent.
The amount of real nitric acid (mono-hydrate) and the amount of nitric peroxide present in any sample should always be determined before it is used for nitrating purposes. The specific gravity is not a sufficient guide to the strength of the acid, as an acid having a high gravity, due to some 3 or 4 per cent of nitric oxides in solution, will give very poor nitration results. A tenth normal solution of sodium hydroxide (NaOH), with phenol-phthalein as indicator, will be found the most convenient method of determining the total acid present. The following method will be found to be very rapid and reliable:—Weigh a 100 c.c. flask, containing a few cubic centimetres of distilled water, and then add from a pipette 1 c.c. of the nitric acid to be examined, and reweigh (this gives the weight of acid taken). Now make up to 100 c.c. at 15° C.; shake well, and take out 10 c.c. with a pipette; drain into a small Erlenmeyer flask, and add a little of the phenol-phthalein solution, and titrate with the tenth normal soda solution.
The nitric peroxide can be determined with a solution of potassium permanganate of N/10 strength, thus: Take a small conical flask, containing about 10 c.c. of water, and add from a burette 10 to 16 c.c. of the permanganate solution; then add 2 c.c. of the acid to be tested, and shake gently, and continue to add permanganate solution as long as it is decolourised, and until a faint pink colour is permanent.