CHLORINE
Chlorine is of interest in chemical warfare, not only because it was
the first poison gas used by the Germans, but also because of its
extensive use in the preparation of other war gases. The fact that,
when Germany decided upon her gas program, her chemists selected
chlorine as the first substance to be used, was the direct result of an
analysis of the requirements of a poison gas.
To be of value for this purpose, a chemical must satisfy at least the
following conditions:
(1) It must be highly toxic.
(2) It must be readily manufactured in large quantities.
(3) It must be readily compressible to a liquid and yet
be more or less easily volatilized when the pressure
is released.
(4) It should have a considerably higher density than
that of air.
(5) It should be stable against moisture and other
chemicals.
Considering the properties of chlorine in the light of these
requirements, we find:
(1_a_) Chlorine is fairly toxic, though its lethal
concentration (2.5 milligrams per liter of air) is
very high when compared with some of the later gases
developed. This figure is the concentration necessary
to kill a dog after an exposure of thirty minutes. Its
effects during the first gas attack showed that, with
no protection, the gas was very effective.
(2_a_) Chlorine is very readily manufactured by
the electrolysis of a salt (sodium chloride) solution.
The operation is described below. In 100-pound
cylinders, the commercial product sold before the War
for 5 cents a pound. Therefore on a large scale, it can
be manufactured at a very much smaller figure.
(3_a_) Chlorine is easily liquefied at the
ordinary temperature by compression, a pressure of 16.5
atmospheres being required at 18° C. The liquid which
is formed boils at -33.6° C. at ordinary atmospheric
pressure, so that it readily vaporizes upon opening
the valve of the containing cylinder. Such rapid
evaporation inside would cause a considerable cooling
of the cylinder, but this is overcome by running
the outlet pipe to the bottom of the tank, so that
evaporation takes place at the end of the outlet pipe.
(4_a_) Chlorine is 2.5 times as heavy as air,
and therefore the gas is capable of traveling over a
considerable distance before it dissipates into the
atmosphere.
(5_a_) The only point in which chlorine does not
seem to be an ideal gas, is in the fact that it is a
reactive substance. This is best seen in the success of
the primitive protection adopted by both the British
and the French during the days immediately following
the first gas attack.
At first, however, chlorine proved a very effective weapon. During the
first six months of its use, its value was maintained by devising new
methods of attack. When these were exhausted, phosgene was added (see
next chapter). With the decline in importance of cloud gas attacks, and
the development of more deadly gases, chlorine was all but discarded as
a true war gas, but remained as a highly important ingredient in the
manufacture of other toxic gases.
MANUFACTURE IN THE UNITED STATES
It was at first thought that the existing plants might be able to
supply the government’s need of chlorine. The pre-war production
averaged about 450 tons (900,000 pounds) per day. The greater amount
of this was used in the preparation of bleach, only about 60,000
pounds per day being liquefied. Only a few of the plants were capable
of even limited expansion. In an attempt to conserve the supply, the
paper mills agreed to use only half as much bleach during the war,
which arrangement added considerably to the supply available for war
purposes. It was soon recognized that even with these accessions, large
additions would have to be made to the chlorine output of the country
in order to meet the proposed toxic gas requirements.
After a careful consideration of all the factors, the most important
of which was the question of electrical energy, it was decided to
build a chlorine plant at Edgewood Arsenal, with a capacity of 100
tons (200,000 pounds) per day. The Nelson cell was selected for use
in the proposed plant. During the process of erection of the plant,
the Warner-Klipstein Chemical Company, which was operating the Nelson
cell in its plant in Charleston, West Virginia, agreed that men might
be sent to their plant to acquire the special knowledge required for
operating such a plant. Thus when the plant was ready for operation,
trained men were at once available.
[Illustration: FIG. 19.—Chlorine Plant, Edgewood Arsenal.]
[Illustration: FIG. 20.—Ground Plan of Chlorine Caustic Soda Plant,
Edgewood Arsenal.]
The following description of the plant is taken from an article by S.
M. Green in _Chemical and Metallurgical Engineering_ for July 1, 1919:
“The chlorine plant building, a ground plan of which is shown in
Figure 20, consisted of a salt storage and treating building, two
cell buildings, a rotary converter building, etc. In connection with
the chlorine plant, there was also constructed a liquefying plant for
chlorine and a sulfur chloride manufacturing and distilling plant.
“The salt storage and treating building was located on ground much
below the cell buildings, which allowed the railroad to enter the
brine building on the top of the salt storage tanks. These tanks were
constructed of concrete. There were seven of these tanks, 34 feet long,
28 feet wide and 20 feet deep having a capacity for storing 4,000 tons
of salt. There would have been 200 tons of salt used per day when the
plant was running at full capacity.
“On the bottom of each tank distributing pipes for dissolving-water
supply were installed, and at the top of each, at the end next to the
building, there was an overflow trough and skimmer board arranged so
that the dissolving-water after flowing up through the salt, overflowed
into this trough and then into a piping system and into either of two
collecting tanks. The system was so arranged that, if the brine was
not fully saturated, it could be passed through another storage tank
containing a deep body of salt. The saturated brine was pumped from the
collecting tanks to any one of 24 treating tanks, each of which had a
capacity of 72,000 gallons.
“The eighth storage bin was used for the storage of soda ash, used in
treating the saturated brine. This was delivered from the bin on the
floor level of the salt building to the soda ash dissolving tanks. From
these tanks it was pumped to any one of the 24 treating tanks. After
the brine was treated and settled, the clear saturated brine was drawn
from the treating tanks through decanting pipes and delivered by pumps
to any one of the four neutralizing tanks. These were located next
to a platform on the level of the car body. This was to provide easy
handling of the hydrochloric acid, which was purchased at first, though
later prepared at the plant from chlorine and hydrogen. The neutralized
brine was delivered from the tanks by a pump to a tank located at a
height above the floor so that the brine would flow by gravity to the
cells in the cell building.
“There were to be two cell buildings, each 541 feet long by 82 feet
wide, and separated by partitions into four sections, containing six
cell circuits of 74 cell units. Each section is a complete unit in
itself, provided with separate gas pump, drying and cooling equipment,
and has a guaranteed capacity of 12.5 tons of chlorine gas per 24 hours.
“Each Nelson electrolytic cell unit consists of a complete fabricated
steel tank 13 by 32 by 80 inches, a perforated steel diaphragm spot
welded to supporting angle irons, plate glass dome, fourteen Acheson
graphite electrodes 2.5 inches in diameter, 12 inches long and fourteen
pieces of graphite 4 by 4 by 17 inches, and various accessories.
(The cell is completely described in _Chemical and Metallurgical
Engineering_, August 1st, 1919.) Each cell is operated by a current
of 340 amperes and 3.8 volts and is guaranteed to produce 60 pounds
of chlorine gas and 65 pounds of caustic soda using not more than 120
pounds of salt per 24 hours, the gas to be at least 95 per cent pure.
[Illustration: FIG. 21.—Interior View of the Cell Building.]
“The salt solution from the cell feed tank, located in the salt
treating building, flows by gravity through a piping system located in
a trench running the length of each cell building, and is delivered to
each cell unit through an automatic feeding device which maintains a
constant liquor level in the cathode compartment.
“The remaining solution percolates from the cathode compartment through
the asbestos diaphragm into the anode compartment and flows from the
end of the cell, containing from 8 to 12 per cent caustic soda, admixed
with 14 to 16 per cent salt, into an open trough and into a pipe in the
trench and through this pipe by gravity to the weak caustic storage
tanks located near the caustic evaporator building.
[Illustration: FIG. 22.—Nelson Electrolytic Cell, showing the Interior
Arrangement of the Cell.]
“The gas piping from the individual cell units to and including the
drying equipment is of chemical stoneware. The piping is so designed
that the gas can be drawn from the cells through the drying equipment
at as near atmospheric pressure as possible in order that the gas can
be kept nearly free of air. When operating, the suction at the pump was
kept at ¹/₂₀ inch or less. The quality of the gas was maintained at a
purity of 98.5 to 99 per cent. The coolers used were very effective,
the gas being cooled to within one degree of the temperature of the
cooling water, no refrigeration being necessary. The drying apparatus
consisted of a stoneware tower of special design containing a large
number of plates, and thus giving a very large acid exposure. There
was practically no loss of vacuum through the drying tower and cooler.
The gas pumping equipment consisted of two hydroturbine pumps using
sulfuric acid as the compressing medium. The acid was cooled by
circulation through a double pipe cooler similar to those used in
refrigerating work. The gas was delivered under about five pounds
pressure into large receiving tanks located just outside the pump
rooms, and from these tanks into steel pipe mains which conducted the
gas to the chemical plant.”
The purity of the gas was such that it was not found necessary to
liquefy it for the preparation of phosgene.
PROPERTIES
Chlorine, at ordinary atmospheric pressure and temperature, is a
greenish yellow gas (giving rise to its name), which has a very
irritating effect upon the membranes of the nose and throat. As
mentioned above, at a pressure of 16.5 atmospheres at 18° C., chlorine
is condensed to a liquid. If the gas is first cooled to 0°, the
pressure required for condensation is decreased to 3.7 atmospheres.
This yellow liquid has a boiling point of -33.6° C. at the ordinary
pressure. If very strongly cooled, chlorine will form a pale yellow
solid (at -102° C.). Chlorine is 2.5 times as heavy as air, one liter
weighing 3.22 grams. 215 volumes of chlorine gas will dissolve in 100
volumes of water at 20°. It is very slightly soluble in hot water or in
a concentrated solution of salt.
Chlorine is a very reactive substance and is found in combination in a
large number of compounds. Among the many reactions which have proved
important from the standpoint of chemical warfare, the following may be
mentioned:
Chlorine reacts with “hypo” (sodium thiosulfate) with the formation of
sodium chloride. Hypo is able to transform a large amount of chlorine,
so that it proved a very satisfactory impregnating agent for the early
cloth masks.
Water reacts with chlorine under certain conditions to form
hypochlorous acid, HOCl. In the presence of ethylene, this forms
ethylene chlorhydrin, which was the basis for the first method of
preparing mustard gas. In the later method, in which sulfur chloride
was used, chlorine was used in the manufacture of the chloride.
Chlorine reacts with carbon monoxide, in the sunlight, or in the
presence of a catalyst, to form phosgene, which is one of the most
valuable of the toxic gases.
Chlorine and acetone react to form chloroacetone, one of the early
lachrymators. The reaction of chlorine with toluene forms benzyl
chloride, an intermediate in the preparation of bromobenzylcyanide.
In a similar way, it is found that the greater number of toxic gases
use chlorine in one phase or another of their preparation. One author
has estimated that 95 per cent of all the gases used may be made
directly or indirectly by the use of chlorine.
Chlorine has been used in connection with ammonia and water vapor
for the production of smoke clouds. The ammonium chloride cloud thus
produced is one of the best for screening purposes. In combination
with silicon or titanium as the tetrachloride it has also been used
extensively for the same purpose.
On the other hand one may feel that, whatever bad reputation chlorine
may have incurred as a poison gas, it has made up for it through the
beneficial applications to which it has lent itself. Among these we may
mention the sterilization of water and of wounds.
In war, where stationary conditions prevail only in a small number
of cases, the use of liquid chlorine for sterilization of water is
impractical. To meet this condition, an ampoule filled with chlorine
water of medium concentration has been developed, which furnishes a
good portable form of chlorine as a sterilizing agent for relatively
small quantities of water.
Chlorine has also been applied, in the form of hypochlorite, to the
sterilization of infected wounds. The preparation of the solution and
the technique of the operation were worked out by Dakin and Carrel.
This innovation in war surgery has decreased enormously the percentage
of deaths from infected wounds.