CHLOROPICRIN
During the spring of 1917, strange reports came from the Italian front
that the Germans were using a new war gas. This gas, while it did
not seem to be very poisonous, had the combined property of being a
lachrymator and also of causing vomiting. Large number of casualties
resulted through the men being forced to remove their masks in an
atmosphere filled with lethal gases. The gas had the additional and
serious disadvantage of being a very difficult one to remove completely
in the gas mask. The first American masks were very good when chlorine
or phosgene was considered but were of no value when chloropicrin was
used.
One of the interesting facts of chemical warfare is that few if any
new substances were discovered and utilized during the three years of
this form of fighting. Chlorine and phosgene were well known compounds.
And likewise, chloropicrin was an old friend of the organic chemist.
So much so, indeed, that several organic laboratories prepared the
compound in their elementary courses.
Chloropicrin was first prepared by the English chemist, Stenhouse,
in 1848, by the action of bleaching powder upon a solution of picric
acid. This was followed by a careful study of its physical and chemical
properties, few of which have any connection with its use as a poison
gas. The use of picric acid as an explosive made it very desirable that
other raw materials should be used. Chloroform, which is the ideal
source theoretically (since chloropicrin is nitro-chloroform, Cl₃CNO₂),
gave very poor yields. While it may be prepared from acetone, in fair
yields, acetone was about as valuable during the war as was picric
acid. Practically all the chloropicrin used was prepared from this acid
as the raw material.
MANUFACTURE
In the manufacture of chloropicrin the laboratory method was adopted.
This consisted simply in passing live steam through a mixture of picric
acid and bleaching powder. The resulting chloropicrin passes out of the
still with the steam. There was a question at first whether a steam
jacketed reaction vessel should be used, and whether stirrers should be
introduced. Both types were tested, of which the simpler form, without
steam jacket or stirrer, proved the more efficient.
[Illustration: FIG. 27.—Interior of Chloropicrin Plant.]
The early work was undertaken at the plant of the American Synthetic
Color Company at Stamford, Connecticut. Later a large plant was
constructed at Edgewood Arsenal. At the latter place ten stills, 8 by
18 feet, were erected, together with the necessary accessory equipment.
The following method of operation was used:
The bleach is mixed with water and stirred until a cream is formed.
This cream is then pumped into the still along with a solution of
calcium picrate (picric acid neutralized with lime). When the current
of live steam is admitted at the bottom of the still, the temperature
gradually rises, until at 85° C. the reaction begins. The chloropicrin
passes over with the steam and is condensed. Upon standing, the
chloropicrin settles out, and may be drawn off and is then ready for
filling into the shell. The yield was about 1.6 times the weight of
picric acid used.
PROPERTIES
Chloropicrin is a colorless oil, which is insoluble in water, and which
can be removed from the reaction by distillation with steam. It boils
at 112° C. and will solidify at -69° C. At room temperature it has a
density of 1.69 and is thus higher than chloroform (1.5) or carbon
tetrachloride (1.59). At room temperature it has a vapor pressure of
24 mm. of mercury. It thus lies, in persistency, between such gases as
phosgene on the one hand, and mustard gas on the other, but so much
closer to phosgene that it is placed in the phosgene group.
Chloropicrin is a very stable compound and is not decomposed by water,
acids or dilute alkalies. The reaction with potassium or sodium
sulfite, in which all the chlorine is found as potassium or sodium
chloride, has been used as an analytical method for its quantitative
determination. The qualitative test usually used consists in passing
the gas-air mixture through a heated quartz tube, which liberates free
chlorine. The chlorine may be detected by passing through a potassium
iodide solution containing starch, or by the use of a heated copper
wire gauze, when the characteristic green color is obtained.
An interesting physiological test has also been developed. The eye
has been found to be very sensitive to chloropicrin. The gas affects
the eye in such a way that its closing is practically involuntary. A
measurable time elapses between the instant of exposure and the time
when the eye closes. Below 1 or 2 parts per million, the average eye
withstands the gas without being closed, though considerable blinking
may be caused. Above 25 parts, the reaction is so rapid as to render
proper timing out of the question. But with concentrations between 2
and 25 parts, the subject will have an overpowering impulse to close
his eye within 3 to 30 seconds. The time may be recorded by a stop
watch and from the values thus determined a calibration curve may be
plotted, using the concentration in parts per million and the time to
zero eye reaction. Typical figures are given below. It will be noted
that different individuals will vary in their sensitivity, though the
order is the same.
+--------+---------+---------+
| Conc. | A | B |
| p.p.m. | Seconds | Seconds |
+--------+---------+---------+
| 20.0 | 4.0 | 5.0 |
| 15.0 | 5.4 | 5.4 |
| 10.0 | 7.5 | 7.5 |
| 7.5 | 9.0 | 10.0 |
| 5.0 | 13.0 | 15.0 |
| 2.5 | 18.0 | 30.0 |
+--------+---------+---------+
[Illustration: FIG. 28.—Calibration Curve of Eyes for Chloropicrin.]
PROTECTION
Because of the stability of chloropicrin, the question of protection
resolves itself into finding an absorbent which is very efficient in
removing the gas from air mixtures. Fortunately such an agent was found
in the activated charcoal used in the American gas mask. The removal of
the gas appears to take place in two stages. In the first, the gas is
adsorbed in such a way that the long-continued passage of air does not
remove it. In the second, the gas is absorbed, and this, really excess
gas, is removed by pure air passing over the charcoal. The relation of
these two factors has an important bearing on the quality of charcoal
to be used in gas masks. It appears that up to a certain point an
increase of the quality is desirable: beyond this, it is of doubtful
value.
Unlike phosgene, chloropicrin is absorbed equally well at all
temperatures. Moisture on the other hand has a very decided effect.
It appears that charcoal absorbs roughly equivalent weights of
chloropicrin and of water; the presence of water in the charcoal thus
displaces an approximately equal amount of chloropicrin.
In the study of canisters it has been found that the efficiency
time is approximately inversely proportional to the concentration.
Formulas have been calculated to express the relation existing between
concentration and life of the canister, and also between the rate of
flow of the gas and the life.
While water seems to have a decidedly marked effect upon the life of a
canister, this is not true of other gases, and the efficiency of the
canister for each gas is not decreased when used in a binary mixture.
TACTICAL USES
Because of the high boiling point of chloropicrin it can only be used
in shell. The German shell usually contained a mixture of superpalite
(trichloromethyl chloroformate) and chloropicrin, the relative
proportions being about 75 to 25. These were called Green Cross Shell,
from the peculiar marking on the outside of the shell. Mixtures of
phosgene and chloropicrin (50-50) have also been used.
The Allies have used a mixture of 80 per cent chloropicrin and 20 per
cent stannic chloride (so-called N. C.). This mixture combines the
advantages of a gas shell with those of a smoke shell, since the
percentage of stannic chloride is sufficiently high to form a very good
cloud. In addition to this, it is believed that the presence of the
chloride increases the rate of evaporation of the chloropicrin. It has
been claimed that the chloride decreases the amount of decomposition
of the chloropicrin upon the bursting of the shell, but careful
experiments appear to show that this decomposition is negligible and
that the stannic chloride plays no part in it. This mixture was being
abandoned at the close of the war.
This N. C. mixture has also been used in Liven’s projectors and in
hand grenades. The material is particularly fitted for hand grenades,
owing to the low vapor pressure of the chloropicrin, and the consequent
absence of pressures even on warm days. As a matter of fact, it was the
only filling used for this purpose, though later the stannic chloride
was changed, owing to the shortage of tin, to a mixture of silicon and
titanium chlorides.
While chloropicrin is sufficiently volatile to keep the strata of air
above it thoroughly poisonous, it is still persistent enough to be
dangerous after five or six hours.