The penta-nitrate may be obtained in a pure state by the following process, devised by Eder:—The gun-cotton is dissolved in concentrated nitric acid at 90° C., and reprecipitated by the addition of concentrated sulphuric acid. After cooling to 0° C., and mixing with a larger volume of water, the precipitated nitrate is washed with water, then with alcohol, dissolved in ether-alcohol, and again precipitated with water, when it is obtained pure. This nitrate is soluble in ether-alcohol, and slightly in acetic acid, easily in acetone, acetic ether, and methyl-alcohol, insoluble in alcohol. Strong potash (KOH) solution converts into the di-nitrate C_{12}H_{18}O_{8}(NO_{3})_{2}. The hexa-nitrate is not soluble in acetic acid or methyl-alcohol.
The lower nitrates known as the tetra- and tri-nitrates are formed together when cellulose is treated with a mixture of weak acids, and allowed to remain in contact with them for a very short time (twenty minutes). They cannot be separated from one another, as they all dissolve equally in ether-alcohol, acetic ether, acetic acid, methyl-alcohol, acetone, amyl acetate, &c.
As far as the manufacture of explosive bodies is concerned, the two forms of nitro-cellulose used and manufactured are gun-cotton or the hexa- nitrate (once regarded as tri-nitro-cellulose), which is also known as insoluble gun-cotton, and the soluble form of gun-cotton, which is also known as collodion, and consists of a mixture of several of the lower nitrates. It is probable that it chiefly consists, however, of the next highest nitrate to gun-cotton, as the theoretical percentage of nitrogen for this body,. the penta-nitrate, is 12.75 per cent., and analyses of commercial collodion-cotton, entirely soluble in ether-alcohol, often give as high a percentage as 12.6.
We shall only describe the manufacture of the two forms known as soluble and insoluble, and shall refer to them under their better known names of gun-cotton and collodion-cotton. The following would, however, be the formulæ[A] and percentage of nitrogen of the complete series:—
Hexa-nitro-cellulose C_{12}H_{14}O_{4}(NO_{3}){6} 14.14 per cent.
nitrogen.
Penta-nitro-cellulose C{12}H_{15}O_{5}(NO_{3}){5} 12.75 per cent.
nitrogen.
Tetra-nitro-cellulose C{12}H_{16}O_{6}(NO_{3}){4} 11.11 per cent.
nitrogen.
Tri-nitro-cellulose C{12}H_{17}O_{7}(NO_{3}){3} 9.13 per cent.
nitrogen.
Di-nitro-cellulose C{12}H_{18}O_{8}(NO_{3}){2} 7.65 per cent.
nitrogen.
Mono-nitrocellulose C{12}H_{19}O_{9}(NO_{3}) 3.80 per cent.
nitrogen.
[Footnote A: Berthelot takes C_{24}H_{40}O_{20} as the formula of cellulose; and M. Vieille regards the highest nitrate as (C_{24}H_{18}(NO_{3}H){11}O{9}). Compt. Rend., 1882, p. 132.]
~Properties of Gun-Cotton.~—The absolute density of gun-cotton is 1.5. When in lumps its apparent density is 0.1; if twisted into thread, 0.25; when subjected, in the form of pulp, to hydraulic pressure, 1.0 to 1.4. Gun-cotton preserves the appearance of the cotton from which it is made. It is, however, harsher to the touch; it is only slightly hygroscopic (dry gun-cotton absorbs 2 per cent. of moisture from the air). It possesses the property of becoming electrified by friction. It is soluble in acetic ether, amyl acetate, and acetone, insoluble in water, alcohol, ether, ether-alcohol, methyl-alcohol, &c. It is very explosive, and is ignited by contact with an ignited body, or by shock, or when it is raised to a temperature of 172° C. It burns with a yellowish flame, almost without smoke, and leaves little or no residue. The volume of the gases formed is large, and consists of carbonic acid, carbonic oxide, nitrogen, and water gas. Compressed gun-cotton when ignited often explodes when previously heated to 100° C.
Gun-cotton kept at 80° to 100° C. decomposes slowly, and sunlight causes it to undergo a slow decomposition. It can, however, be preserved for years without undergoing any alteration. It is very susceptible to explosions by influence. For instance, a torpedo, even placed at a long distance, may explode a line of torpedoes charged with gun-cotton. The velocity of the propagation of the explosion in metallic tubes filled with pulverised gun-cotton has been found to be from 5,000 to 6,000 mms. per second in tin tubes, and 4,000 in leaden tubes (Sebert).
Gun-cotton loosely exposed in the open air burns eight times as quickly as powder (Piobert). A thin disc of gun-cotton may be fired into from a rifle without explosion; but if the thickness of the disc be increased, an explosion may occur. The effect of gun-cotton in mines is very nearly the same as that of dynamite for equal weights. It requires, however, a stronger detonator, and it gives rise to a larger quantity of carbonic oxide gas. Gun-cotton should be neutral to litmus, and should stand the Government heat test—temperature of 150° F. for fifteen minutes (see page 249). In the French Navy gun-cotton is submitted to a heat test of 65° C. (= 149° F.) for eleven minutes. It should contain as small a percentage of soluble nitro-cotton and of non-nitrated cotton as possible.
The products of perfectly detonated gun-cotton may be expressed by the following equation:—