PYROXYLIN (Formula H ; molecular weight, 252; synonyms, pyroxyline, gun cotton). This name is applied to a series of compounds obtained by acting on vegetable fibres with nitric and sulphuric acids. They are all more or less explosive, and are all what are termed substitution compounds, being derived from the vegetable fibre by the sub stitution of atoms of for an equal number of atoms of H in the fibre. The pyroxylin with which we have to deal with here is that known as the soluble kind, sometimes termed collodion wool, as is used in the manufacture of collodion for photographic purposes.
It can be purchased ready for use, and it is advisable to obtain it in this form, as its manu facture is attended with considerable danger.
The following directions are for preparing pyroxyline.* The most important part is the composition of the nitric-sulphuric acid in which the pyroxyline is prepared, as the slighest variation in the quantity of water, in the relative propor tions of the two constituent acids, or in the temperature at the time of putting in the cotton, will effect the result.
Sulphuric acid sp. gr. r.854 at 6o°F 18 fluid ounces Nitric acid sp. gr. 1.45 at 6o°F 6 fluid ounces Distilled water 4% fluid, ounces The water is poured into a dish, and the nitric and sulphuric acids added in the order written.
The mixture is then well stirred, and the temperature, which will have considerably increased on addition of the sulphuric acid, must be allowed to sink to 15o° Fahr., and at this tem perature it must be kept by means of a water-bath. The wool is first immersed in a strong solu tion of sodium or potassium carbonate to free it from the natural oil contained in it, and then well washed in clean water until the latter shows no trace of alkalinity or the salt used. It is then dried. The wool prepared in this manner should be weighed out in balls of about thirty grains each, and these are immersed one by one into the acid mixture, and well stirred in it, sc that each ball becomes thoroughly saturated with the acids. They are then left for ten minutes and removed and washed in a running stream of water for twenty-four hours, or until they give no acid reaction when tested with litmus. They are then dried in the sun or on a water-bath. When dry they form the collodion wool of commerce, and should be entirely soluble in a mixture of equal parts of alcohol and ether. They should also be inflammable. The nitric is the only acid that acts upon the cotton wool, the sulphuric merely increases the rapidity of its action.
Pyroxyline can be made from many varieties of cellulose. Cotton, straw, pith, flax, linen, calico, have all been used, giving more or less different results However, cotton wool is the sub stance generally employed, and sea islands cotton is generally preferred. Schering's celloidin is a very pure variety of collodion cotton. It occurs in horny faintly yellow masses, soluble in alco hol and ether and giving a very fine collodion, producing an even and structureless film.
Pyroxyline is insoluble in water, alcohol or ether alone, but is soluble in a mixture of the two latter, forming collodion.
Hardwich says that the temperature of the nitro-sulphuric acid at the time of immersing the cotton invariably affects the photographic properties of pyroxyline. At high temperatures a portion of the film is converted into a substance which has a bitter taste, and turns brown when treated with alkalies. This substance is believed to be nitro-glucose, formed by the action of strong nitric acid upon grape sugar; the grape sugar itself being prodnced by the cellulose by contact with the warm and diluted sulphuric acid. Its presence in the collodion tends to diminish the sensitiveness of the film to weak rays of light, but increases the rapidity and intensity of the development in negative images Pyroxyline is liable to decomposition, a partial liberation of oxides of nitrogen apparently taking place, forming in some cases, red fumes within the bottle. It can be kept perfectly, however, if the following precautions are taken : First, it should be thoroughly washed and thoroughly dried ; and, secondly, it should be kept in a well-ventilated vessel—a thin cotton bag, in which it can be loosely stowed away is a good receptacle. It is dangerous to press it too tight in a stoppered bottle. Light, heat and moisture are all agents facilitating decomposition, it should therefore be kept in a dark, dry and cool place.
was probably the first to introduce this substance for collodion making. It is a kind of pyroxyline made from paper instead of from cotton wool. The follow ing is a formula for washed emulsion processes : Nitric acid sp. g. 2 fluid ounces.
Sulphuric acid sp. g. i 845 3 fluid ounces.
White blotting paper* 145 grains.
The temperature should be about loo degs. Fahrenheit and the time of immersion about half an hour.