_MANUFACTURE OF NITRO-GLYCERINE._

Properties of Nitro-Glycerine--Manufacture of Nitro-Glycerine--Nitration-- The Nathan Nitrator--Separation--Filtering and Washing--The Waste Acids-- Treatment of the Waste Acid from the Manufacture of Nitro-Glycerine and Gun-Cotton.

~Properties of Nitro-Glycerine.~--Nitro-glycerol is a heavy oily liquid of specific gravity 1.6 at 15 C., and when quite pure is colourless. The commercial product is a pale straw yellow, but varies much according to the purity of the materials used in its manufacture. It is insoluble in water, crystallises at 10.5 C., but different commercial samples behave very differently in this respect, and minute impurities prevent or delay crystallisation. Solid nitro-glycerol[A] melts at about 12 C., but requires to be exposed to this temperature for some time before melting.

The specific gravity of the solid form is 1.735 at +10 C.; it contracts one-twelfth of its volume in solidifying. Beckerheim[B] gives the specific heat as 0.4248 between the temperatures of 9.5 and 9.8 C., and L. de Bruyn gives the boiling point as above 200.

[Footnote A: Di-nitro-mono chlorhydrin, when added to nitro-glycerine up to 20 per cent., is said to prevent its freezing.]

[Footnote B: _Isb., Chem. Tech._, 22, 481-487. 1876.]

Nitro-glycerine has a sweet taste, and causes great depression and vertigo. It is soluble in ether, chloroform, benzene, glacial acetic acid, and nitro-benzene, in 1.75 part of methylated spirit, very nearly insoluble in water, and practically insoluble in carbon bisulphide. Its formula is C_{3}H_{5}(NO_{3})_{3}, and molecular weight 227. When pure, it may be kept any length of time without decomposition. Berthelot kept a sample for ten years, and Mr G. M"Roberts, of the Ardeer Factory, for nine years, without their showing signs of decomposition; but if it should contain the smallest trace of free acid, decomposition is certain to be started before long. This will generally show itself by the formation of little green spots in the gelatine compounds, or a green ring upon the surface of liquid nitro-glycerine. Sunlight will often cause it to explode; in fact, a bucket containing some water that had been used to wash nitro-glycerine, and had been left standing in the sun, has in our experience been known to explode with considerable force. Nitro-glycerine when pure is quite stable at ordinary temperatures, and samples have been kept for years without any trace of decomposition. It is very susceptible to heat, and even when quite pure will not stand a temperature of 100 C.

for a longer period than a few hours, without undergoing decomposition. Up to a temperature of 45 C., however, properly made and purified nitro- glycerine will remain unchanged almost indefinitely. The percentage composition of nitroglycerine is as follows:--

Found. Theory for C_{3}H_{5}(N0_{2})_{3}.

Carbon 15.62 15.86 per cent.

Hydrogen 2.40 2.20 "

Nitrogen 17.90 18.50 "

Oxygen ... 63.44 "

The above a.n.a.lysis is by Beckerheim. Sauer and Adou give the nitrogen as 18.35 to 10.54 per cent. by Dumas" method; but I have never found any difficulty in obtaining percentages as high as 18.46 by the use of Lunge"s nitrometer. The decomposition products by explosion are shown by the following equation--

2C_{3}H_{5}(NO_{3})_{3} = 6CO_{2} + 5H_{2}O + 6N + O;

that is, it contains an excess of 3.52 per cent. of oxygen above that required for complete combustion; 100 grms. would be converted into--

Carbonic Acid (CO_{2}) 58.15 per cent.

Water 19.83 "

Oxygen 3.52 per cent.

Nitrogen 18.50 "

The volume of gases produced at 0 and 760 mm., calculated from the above, is 714 litres per kilo, the water being taken as gaseous. Nitro-glycerine is decomposed differently if it is ignited as dynamite (i.e., kieselguhr dynamite), and if the gases are allowed to escape freely under a pressure nearly equal to that of the atmosphere. Sarrau and Vieille obtained under these conditions, for 100 volumes of gas--

NO 48.2 per cent.

CO 35.9 "

CO_{2} 12.7 "

H 1.6 per cent.

N 1.3 "

CH_{4} 0.3 "

These conditions are similar to those under which a mining charge, simply ignited by the cap, burns away slowly under a low pressure (i.e., a miss fire). In a recent communication, P.F. Chalon (_Engineering and Mining Journal_, 1892) says, that in practice nitro-glycerine vapour, carbon monoxide, and nitrous oxide, are also produced as the result of detonation, but he attributes their formation to the use of a too feeble detonator.

Nitro-glycerine explodes very violently by concussion. It may be burned in an open vessel, but if heated above 250 C. it explodes. Professor C.E.

Munroe gives the firing point as 2O3-2O5 C., and L. de Bruyn[A] states its boiling point as 185. He used the apparatus devised by Horsley. The heat of formation of nitro-glycerine, as deduced from the heat of combustion by M. Longuinine, is 432 calories for 1 grm.; and the heat of combustion equals 1,576 cals. for 1 grm. In the case of nitro-glycerine the heat of total combustion and the heat of complete decomposition are interchangeable terms, since it contains an excess of oxygen. According to Dr W.H. Perkin, F.R.S.,[B] the magnetic rotation of nitro-gylcerine is 5,407, and that of tri-methylene nitrate, 4.769 (diff. = .638). Dr Perkin says: "Had nitro-glycerine contained its nitrogen in any other combination with oxygen than as -O-NO_{2}, as it might if its const.i.tution had been represented as C_{3}H_{2}(NO_{2})_{3}(OH)_{3}, the rotation when compared with propyl nitrate (4.085) would be abnormal."

[Footnote A: _Jour. Soc. Chem. Ind._, June 1896, p. 471.]

[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 -20C. 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 gla.s.sy ma.s.s, without any perceptible crystallisation. The ma.s.s when warmed to 0C. first rapidly liquefies and then begins to crystallise. The true freezing point of pure nitro- glycerine was found to be 12.3C. 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 const.i.tution, 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 pa.s.sing 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 a.n.a.lysis 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 t.i.trate with the tenth normal soda solution.

The nitric peroxide can be determined with a solution of pota.s.sium 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.

_Example._ N/10 permanganate 3.16 grms. per litre, 1 c.c. = O.0046 grm.

N_{2}O_{4}, 2 c.c. of sample of acid specific gravity 1.52 = 3.04 grms.

taken for a.n.a.lysis. Took 20 c.c. permanganate solution, O.0046 x 20 =.092 grm. N_{2}O_{4}, and (.092 x 100)/3.04 = 3.02 per cent. N_{2}O_{4}. The specific gravity should be taken with an hydrometer that gives the specific gravity directly, or, if preferred, the 2 c.c. of acid may be weighed.

A very good method of rapidly determining the strength of the sulphuric acid is as follows:--Weigh out in a small weighing bottle, as nearly as possible, 2.45 grms. This is best done by running in 1.33 c.c. of the acid (1.33 x 1.84 = 2.447). Wash into a large Erlenmeyer flask, carefully washing out the bottle, and also the stopper, &c. Add a drop of phenol- phthalein solution and t.i.trate, with a half normal solution of sodium hydrate (use a 100 c.c. burette). Then if 2.45 grms. exactly have been taken, the readings on the burette will equal percentages of H_{2}SO_{4} (mono-hydrate) if not, calculate thus:--2.444 grms. weighed, required 95.4 c.c. NaOH. Then--

2.444 : 95.4 :: 2.45 : _x_ = 95.64 per cent. H_{2}SO_{4}.