Lewisite environmental persistence

Sulfur mustard can be considered environmentally persistent because it is chemically stable and of low volatility. When protected from weathering conditions, it may persist in soil for years. VX is moderately persistent because of low volatility and slow rate of hydrolysis. The G-agents can be considered non-persistent on the basis of volatility and hydrolysis rates. Depending on environmental conditions, their half-lives may be measured in hours to days. Lewisite is rapidly hydrolyzed but the insoluble oxide formed is stable in the environment. In addition, arsenical degradation products of lewisite persist in the environment. Because of its extreme volatility and relatively rapid hydrolysis, cyanogen chloride is not persistent in the environment. 

The agents that can be considered environmentally persistent, in varying degrees, are VX, sulfur mustard, and lewisite. Agent VX exhibits very low volatility and a slow rate of hydrolysis its half-life in soil may be measured in weeks to months, depending on environmental conditions. Sulfur mustard may persist... 

Lewisite was first synthesized in 1904 and stockpiled by the United States, Germany, and Japan during World War II. Although Lewisite is considerably more potent than HD, it produces immediate symptoms of physical discomfort, which rendered it relatively ineffective as a stand-alone chemical warfare agent. However Lewisite was found to increase the environmental persistence of HD by depressing its freezing point at doses that had no apparent physiological effect, and therefore Lewisite stockpiles were retained for several decades. 

Lewisite, CHCl=CHAsCl2, 2-chlorovinyldichloroarsine, named after its discoverer W. L. Lewis, was produced by the USA and shipped to Europe in 1918, too late to be used in WWI. Between the wars it was also produced by Japan and the Soviet Union. It is relatively easily made from arsenic trichloride and acetylene, although the process is technically more difficult than the production of sulfur mustard. Lewisite is more volatile (bp 190 °C) than sulfur mustard and hence it is less persistent it also appears to be more sensitive to environmental moisture. In contrast to sulfur mustard, its initial effect (skin pain or irritation) is almost instant, and blisters appear within a few hours. There has been no confirmed instance of use, although Japan is suspected of having used lewisite in China in WWII. In addition to being stockpiled as a neat agent, lewisite was mixed with sulfur mustard to speed up the onset of action and to depress the freezing point of the latter. 

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