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 deg.  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 analysis 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 deg.  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 titrate with the tenth normal soda solution.

The nitric peroxide can be determined with a solution of potassium 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 analysis.  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.