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528 CHEMISTRY AND TECHNOLOGY OF EXPLOSIVES

should be regarded as the quinonoid form. Hale [15] found that the colour of ammo-

nium picrate was darker (more red), the more concentrated the ammonia used

for the neutralization of the picric acid. By heating to 150°C the red form changed

into the yellow one.

T. Urbanski, Hackel and Galas [16] found that it was the pH of the solution

in which picric acid was neutralized that was responsible for the colour of ammonium

picrate crystals. The higher the pH the more readily the red crystals were formed.

By neutralizing picric acid with an ammonium carbonate solution the authors

always obtained a yellow product, whereas by using a 25% ammonia solution for

neutralization, they obtained a red product. Both forms were found to have

equal specific gravities, 1.6715, whereas they differed somewhat in their densities

of loading when pressed. At low pressures, densities of loading of the red form

were slightly higher. At high pressures the yellow form was observed to have the

higher density (Table 120).

This difference in density values should be ascribed to the physical properties of

crystals. At low pressures the crystals of the red form, due to their shape, can be packed

more easily in a given space. At high pressures a lower mechanical strength of the yellow

crystals plays a part, viz. being disintegrated they more readily fill up the space.

TABLE 120

DENSITY OF AMMONIUM PICRATE FORMS

Pressure, kg/cm^2

Density, g/cm^3

yellow form red form

0 (loose) 0.891

110 1.244

330 1.396

660 1.490

1100 1.553

1540 1.593

1910 1.596

2540 1.616

1.045

1.299.

1.392

1.490

1.529

1.562

1.569

1.582

The above authors, in a detailed study of the explosive properties of the two

forms of ammonium picrate, found no difference in their explosive power. However,

the red form is slightly more difficult to bring to explode and it is to this property

that differences in the figures obtained in determination of their explosive prop-

erties may be ascribed.

Here are the principal data obtained by T. Urbanski, Hackel and Galas [16]:

Sensitiveness to impact: 10% explosions from a 10 kg weight falling from a height of

ca. 20 cm, which is in agreement with Kast’s results [7]

sensitiveness to flame: yellow ammonium picrate ignites after 8 swings; the red form

after 12.5 swings

initiation temperature (20°/min) 257-259°C

specific pressure, f (experimentally) 7200 m

expansion in the lead block test 330-335 cm^3