3. Separation should be sharp within half an hour, without the separation of flocculent matter, nor should any white flocculent matter (due to fatty acids) be formed when the nitrated glycerine is thrown into water and neutralised with carbonate of soda.
4. Should be free from lime and chlorine, and contain only traces of arsenic, sulphuric acid, &c.
5. Should not leave more than 0.25 per cent. of inorganic and organic residue together when evaporated in a platinum dish without ebullition (about 160° C.) or partial decomposition.
6. Silver test fair.
7. The glycerine, when diluted one-half, should give no deposit or separation of fatty acids when nitric peroxide gas is passed through it. (Nos. 1, 2, 3, and 5 are the most essential.)
The white flocculent matter sometimes formed is a very great nuisance, and any sample of glycerol which gives such a precipitate when tried in the laboratory should at once be rejected, as it will give no end of trouble in the separating house, and also in the filter house, and it will be very difficult indeed to make the nitro-glycerine pass the heat test. The out- turn of nitro-glycerine also will be very low. The trouble will show itself chiefly in the separating operation. Very often 2 or 3 inches will rise to the surface or hang about in the nitro-glycerine, and at the point of contact between it and the mixed acids, and will afterwards be very difficult to get rid of by filtration. The material appears to be partly an emulsion of the glycerine, and partly due to fatty acids, and as there appears to be no really satisfactory method of preventing its formation, or of getting rid of it, the better plan is not to use any glycerine for nitrating that has been found by experiment upon the laboratory scale to give this objectionable matter. One of the most useful methods of testing the glycerine, other than nitrating, is to dilute the sample one-half with water, and then to pass a current of nitric peroxide gas through it, when a flocculent precipitate of elaïdic acid (less soluble in glycerine than the original oleic acid) will be formed. Nitrogen peroxide, N_{2}O_{4}, is best obtained by heating dry lead nitrate (see Allen, "Commercial Organic Analysis," vol. ii., 301).
When a sample of nitro-glycerine is brought to the laboratory from the filter house, it should first be examined to see that it is not acid.[A] A weak solution of Congo red or methyl orange may be used. If it appears to be decidedly alkaline, it should be poured into a separating funnel, and shaken with a little distilled water. This should be repeated, and the washings (about 400 c.c.) run into a beaker, a drop of Congo red or methyl orange added, and a drop or so of N/2 hydrochloric acid added, when it should give, with two or three drops at most, a blue colour with the Congo red, or pink with the methyl orange, &c. The object of this test is to show that the nitro-glycerine is free from any excess of soda, i.e., that the soda has been properly washed out, otherwise the heat test will show the sample to be better than it is. The heat test must also be applied.
[Footnote A: A. Leroux, Bul. Soc. Chim. de Bel., xix., August 1905, contends that experience does not warrant the assumption that free acid is a source of danger in nitro-glycerine or nitro-cellulose; free alkali, he states, promotes their decomposition.]
Upon leaving the filter house, where it has been washed and filtered, and has satisfactorily passed the heat test, it is drawn off from the lowest tank in indiarubber buckets, and poured down the conduit leading to the precipitating house, where it is allowed to stand for a day, or sometimes longer, in order to allow the little water it still contains to rise to the surface. In order to accomplish this, it is sufficient to allow it to stand in covered-in tanks of a conical form, and about 3 or 4 feet high. In many works it is previously filtered through common salt, which of course absorbs the last traces of water. It is then of a pale yellow colour, and should be quite clear, and can be drawn off by means of a tap (of vulcanite), fixed at the bottom of the tanks, into rubber buckets, and is ready for use in the preparation of dynamite, or any of the various forms of gelatine compounds, smokeless powders, &c., such as cordite, ballistite, and many others.
Mikolajezak (Chem. Zeit., 1904, Rep. 174) states that he has prepared mono- and di-nitro-glycerine, and believes that the latter compound will form a valuable basis for explosives, as it is unfreezable. It is stated to be an odourless, unfreezable oil, less sensitive to percussion, friction, and increase of temperature, and to possess a greater solvent power for collodion-cotton than ordinary nitro-glycerine. It can thus be used for the preparation of explosives of high stability, which will maintain their plastic nature even in winter. The di-nitro-glycerine is a solvent for tri-nitro-glycerine, it can therefore be mixed with this substance, in the various gelatine explosives in order to lower the freezing point.