Phosgene is a colorless gas at room temperatures, but becomes a liquid at 8°. The odor of phosgene is suggestive of green corn or musty hay. One liter of phosgene vapor weighs 4.4 grams (chlorine weights 3.22 grams). At 0° C., the liquid is heavier than water, having a specific gravity of 1.432. At 25°, the vapor exerts a pressure of about 25 pounds per square inch. Phosgene is absorbed by solid materials, such as pumice stone and celite. Pumice stone absorbs more than its own weight of phosgene. Thus 5.7 grams of pumice absorbed 7.4 grams phosgene, which completely evaporated in 60 minutes. German shell have been found which contained such a mixture (phosgene and pumice stone). While the apparent reason for their use is to prevent the rapid evaporation of the phosgene, it is a question whether such is the case, for a greater surface is really present in the case of pumice stone than where the phosgene is simply on the ground. Phosgene is slowly decomposed by cold water, rapidly by hot water. This reaction is important because there is always moisture in the air, which would tend to lower the concentration of the gas.
Phosgene is absorbed and decomposed by hexamethylenetetramine (urotropine). This reaction furnished the basis of the first protection used by the British. Later the catalytic decomposition of phosgene into carbon dioxide and hydrochloric acid by the charcoal in the mask furnished protection.
For most purposes a trace of chlorine in phosgene is not a disadvantage; for example, when it is used in cylinders or projectors. Under certain conditions, as when used as a solvent for sneezing gas, the presence of chlorine must be avoided, since it reacts with the substance in solution, usually producing a harmless material. Chlorine may be removed from phosgene by passing the mixture through cotton seed oil.
Protection
It was mentioned above that hexamethylenetetramine (urotropine) was used in the early pads (black veil and similar masks) and flannel helmets. This was found to be satisfactory against chlorine and phosgene, in the concentrations usually found during a cylinder attack. The mixture used consisted of urotropine, sodium thiosulfate (“hypo”), sodium carbonate and glycerine. The glycerine tended to keep the pads moist, while the other chemicals acted as protective agents against the mixture of phosgene and chlorine.
The introduction of the Standard Box Respirator with its charcoal-soda lime filling increased very materially the protection against phosgene. In this filling, the charcoal both absorbs the phosgene and catalyzes the reaction with the moisture of the air with which the phosgene is mixed, to form hydrochloric acid and carbon dioxide. Soda-lime absorbs phosgene but does not catalyze its decomposition. This shows the advantage of the mixture, since the hydrochloric acid, which is formed through the action of the charcoal, is absorbed by the soda-lime. Experiments seem to indicate that it does not matter which material is placed in the bottom of the canister, but that an intimate mixture is the best arrangement. Using a concentration of 5,000 parts per million (20.2 mg. per liter) a type H canister ([see page 217]) will give complete protection for about 40 minutes; when the air-gas mixture passes at the rate of 16 liters per minute the efficiency or life of a canister increases with a decrease in temperature, as is seen in the following table (the concentration was 5,000 parts per million, the rate of flow 16 liters per minute)
| Temperature ° C. | Efficiency (Time in minutes) |
|---|---|
| -10 | 223 |
| 0 | 172 |
| 10 | 146 |
| 20 | 130 |
| 30 | 125 |
| 40 | 99 |
From these figures it is seen that at -10° C. the life is about 50 per cent greater than at summer temperature. As would be expected the life of a canister is shortened by increasing the concentration of phosgene in the phosgene air mixture. This is illustrated by the following figures:
| Concentration p.p.m. | Life (Time in minutes) |
|---|---|
| 5,000 | 177 |
| 10,000 | 112 |
| 15,000 | 72 |
| 20,000 | 58 |
| 25,000 | 25 |
(25,000 p.p.m. is equal to 101.1 mg. per liter.)