CHAPTER XI

CARBON MONOXIDE

Carbon monoxide, because of its cheapness, accessibility and ease of manufacture, has been frequently considered as a possible war gas. Actually, it appears never to have been used intentionally for such purposes. There are several reasons for this. First, its temperature of liquefaction at atmospheric pressure is -139° C. This means too high a pressure in the bomb or shell at ordinary temperatures. Secondly, the weight of carbon monoxide is only slightly less than that of air, which keeps it from rolling into depressions, dugouts and trenches, as in the case of ordinary gases, and also permits of its rather rapid rise and dissipation into the surrounding atmosphere. A third reason is its comparatively low toxic value, which is only about one-fifth that of phosgene. However, as it can be breathed without any discomfort, and as it has some delay action, its lack of poisonous properties would not seriously militate against its use were it not for the other reasons given.

It is, nevertheless, a source of serious danger both in marine and land warfare. Defective ventilation in the boiler rooms of ships and fires below decks, both in and out of action, are especially dangerous because of the carbon monoxide which is produced. In one of the naval engagements between the Germans and the English, defective high explosive shell, after penetrating into enclosed portions of the ship, evolved large quantities of carbon monoxide and thus killed some hundreds of men. On shore, machine gun fire in enclosed spaces, such as pill boxes, and in tanks, liberates relatively large quantities of carbon monoxide. Similarly, in mining and sapping work, the carbon monoxide liberated by the detonation of high explosives constitutes one of the most serious of the difficulties connected with this work and necessitated elaborate equipment and extensive military training in mine rescue work.

The removal of carbon monoxide from the air is difficult because of its physical and chemical properties. Its low boiling point and critical temperature makes adequate adsorption at ordinary temperatures by the use of an active absorbent out of the question. Its known insolubility in all solvents similarly precludes its removal by physical absorption.

After extensive investigation two absorbents have been found.[24] The first of these consists in a mixture of iodine pentoxide and fuming sulfuric acid, with pumice stone as a carrier. Using a layer 10 cm. deep and passing a 1 per cent carbon monoxide air mixture at the rate of 500 cc. per minute per sq. cm. cross section, a 100%-90% removal of the gas could be secured for two hours at room temperature and almost as long at 0° C. The reaction is not instantaneous, and a brief induction period always occurs. This may be reduced to a minimum by the addition of a little iodine to the original mixture.

[Footnote 24: Complete details of this work may be found in _J. Ind. Eng. Chem._, =12=, 213 (1920).]

The sulfur trioxide given off is very irritating to the lungs, but by the use of a layer of active charcoal beyond the carbon monoxide absorbent, this disadvantage was almost completely eliminated. However, sulfur dioxide is slowly formed as a result of this adsorption and after prolonged standing or long-continued use of the canister at a high rate of gas flow gives serious trouble.

Considerable heat is given off in the reaction and a cooling attachment was required. The most satisfactory device was a metal box filled with fused sodium thiosulfate pentahydrate, which absorbed a very considerable amount of the heat.

Still a further disadvantage was the fact that the adsorbents became spent by use, even in the absence of carbon monoxide, since it absorbed enough moisture from the air of average humidity in several hours, to destroy its activity.

The difficulties mentioned were so troublesome that this absorbent was finally supplanted by the more satisfactory oxide absorbent described below.

[Illustration: FIG. 39.—Diagram of Carbon Monoxide Canister, CMA3.]

The metallic oxide mixture was the direct result of an observation that specially precipitated copper oxide with 1 per cent silver oxide was an efficient catalyst for the oxidation of arsine by oxygen. After a study of various oxide mixtures, it was found that a mixture of manganese dioxide and silver oxide, or a three component system containing cobaltic oxide, manganese dioxide and silver oxide in the proportion of 20:34:46 catalyzed the reaction of carbon monoxide at room temperature. The studies were extended and it was soon found that the best catalysts contained active manganese dioxide as the chief constituent. This was prepared by the reaction between potassium permanganate and anhydrous manganese sulfate in the presence of fairly concentrated sulfuric acid. It also developed that the minimum silver oxide content decreased progressively as the number of components increased from 2 to 4. The standard catalyst (Hopcalite) finally adopted for production consisted of 50 per cent manganese dioxide, 30 per cent copper oxide, 15 per cent cobaltic oxide and 5 per cent silver oxide. The mixture was prepared by precipitating and washing the first three oxides separately, and then precipitating the silver oxide in the mixed sludge. After washing, this sludge was run through a filter press, kneaded in a machine, the cake dried and ground to size. While it is not difficult to obtain a product which is catalytically active, it requires a vigorous control of all the conditions and operations to assure a product at once

## active, hard, dense and resistant as possible to the deleterious action

of water vapor.

[Illustration: FIG. 40.—Tanks and Press for Small Scale Manufacture of Carbon Monoxide Absorbent.]

Hopcalite acts catalytically and therefore only a layer sufficiently deep to insure close contact of all the air with the catalyst is needed. One and a half inches (310 gm.) were found ample for this purpose.

The normal activity of Hopcalite requires a dry gas mixture. This was secured by placing a three-inch layer of dry granular calcium chloride at the inlet side of the canister.

Because of the evolution of heat, a cooling arrangement was also used in the Hopcalite canisters.

The life of this canister was the same irrespective of whether its use was continuous or intermittent. The higher the temperature the longer the life because Hopcalite is less sensitive to water vapors at higher temperatures. Since, if the effluent air was sufficiently dried, the Hopcalite should function indefinitely against any concentration of carbon monoxide, the life of the canister is limited solely by the life of the drier. Therefore the net gain in weight is a sure criterion of its condition. After many tests it was determined that any canister which had gained more than 35 grams above its original weight should be withdrawn. The canisters, at the time of breakdown, showed a gain in weight varying between 42 and 71 grams, with a average of 54 grams. It is really, therefore, the actual humidity of the air in which the canister is used that determines its life.

[Illustration: FIG. 41.—Navy Head Mask and Canister.]

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