Gun Cotton

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Pictures taken 167ms apart of me on my Graduation day with a very small piece of gun cotton.

 

Cellulose - Gun cotton. Taken from my Delights of Chemistry site.

In 1833 Braconnot observed that when starch, wood and similar substances were treated with strong nitric acid a solution was obtained which, on the addition of water, yielded a white powder [1]. This material, which burned vigorously when ignited, was called xyloidin.

Pelouze, in 1838, by subjecting cotton to the same treatment produced a similar combustible substance, and C.F. Schönbein, towards the end of 1845, announced the discovery of a new explosive substance (guncotton) which he subsequently stated had been obtained by treating cotton with a mixture of nitric and sulphuric acids [1,2]. Böttger is stated to have discovered guncotton independently of Schönbein, in 1846, but entered into arrangement with the latter to develop the new explosive.

Although in his letter to Dumas, in 1846, Schönbein stated that the manufacture [of guncotton] is not attended with the least danger...[3], the early attempts to manufacture this explosive on a large scale were, however, attended by a number of disastrous explosions, and it was not until methods had been developed for freeing the nitrated cotton from dangerous impurities that the material could be manufactured and stored with safety.

In cellulose molecules three OH-groups are present in each six-carbon units, (C6H10O5)n. During the nitration reaction, all of these hydroxyl-groups can be nitrated forming trinitrocellulose, (C6H7O5(NO2)3)n, which contain approximately 14.4% oxygen.

The degree of nitration depends on the time of treatment in the nitrating mixture, and on a laboratory scale there is an empirical rule that approximately 15 minutes are required for making the trinitro-derivative whilst ca. 10 minutes will produce dinitrocellulose.

Demonstration.

Show that the ordinary cotton wool burns slowly and the combustion in air is incomplete. Then take a small piece of gun cotton and demonstrate that it looks very similar to the ordinary cotton wool. Then place a sheet of filter paper on a tripod and put the gun cotton on the paper. Touch the guncotton with a burning splint. The guncotton burns in an instant flash without setting fire or damaging the filter paper. Alternatively a small piece of fully nitrated, guncotton can be burned on the palm of your hand. Cellulose trinitate contains sufficient oxygen to decompose completely to gaseous products. When in open conditions it does so non-explosively.

Each unit of the trinitrated polymer decomposes to => 9/2 CO + 3/2 CO2 + 7/2 H2O + 3/2 N2

At present it becomes increasingly difficult to get hold of real cotton wool. Remember that modern synthetic substitutes cannot be used in this demonstration.

Safety.

Gun cotton is likely to decompose explosively if heated or ignited in a confined space.

Concentrated nitric and sulphuric acids are oxidising agents and can cause severe burns. Sulphuric acid is also a powerful dehydrating agent and generates considerable heat when diluted with water. If spilled on to the skin, care must be exercised to avoid excessive heating when flushing with water. If you have an ice bath to hand use the water from that or you can quickly remove the bulk of the liquid with a dry cloth or tissue. Flush with plenty of water and then treat the area with sodium hydrogen carbonate NaHCO3. Spills on the floor or bench should be neutralised with NaHCO3 and rinsed thoroughly.

Considerable heat is evolved when mixing these acids and it is recommended that the process be carried out in an ice bath.

References

1.    J.F. Thorpe and M.A. Whiteley, Thorpe’s Dictionary of Applied Chemistry, 4-th ed., vol. IV, London - New York - Toronto, Longmans, Green and Co., 1940, p. 501.

2.    G.W. MacDonald, Historical Papers on Modern Explosives, London, Whittaker & Co., 1912, p. 10.

3.    Ref. 2, p. 15.

4.    M.A. Ivanova and M.A. Kononova, Chemical Lecture Experiment, Moscow, Vyschaya Shkola, 1984, p. 143 (in Russian).