Because of the use of arsenic trichloride in the manufacture of organic arsenic compounds, a method of preparation was developed from arsenic trioxide and sulfur chloride or hydrogen chloride. It was also shown experimentally that the phosgene of the tail gas of phosgene plants might be converted into arsenic trichloride by reaction with arsenic trioxide. Charcoal is the catalyzer of this reaction. Arsenic trichloride is also of interest because it was one of the constituents of the mixture vincennite, early used by the French. This was a mixture of hydrocyanic acid, stannic chloride, arsenic trichloride and chloroform. While extensively used at first, it was gradually replaced by phosgene. Arsenic triflouride was also prepared by the action of sulfuric acid upon a mixture of calcium flouride and arsenic trioxide. The compound is very easily decomposed by the moisture of the air, and furthermore is not very toxic. Organic arsenic derivatives are the most important compounds from the military point of view. The first substance used was diphylchloroarsine, a white solid, which readily penetrated the canister and caused sneezing. This was used alone, and in solution in phenyl dichloroarsine. Later methyl and ethyl dichloroarsines were introduced. [Illustration: FIG. 38.—Apparatus for the Manufacture of Methyldichloroarsine.] METHYLDICHLOROARSINE The Germans apparently used ethyldichloroarsine because they had no suitable method for the preparation of methyl dichloroarsine, which is a more satisfactory material. The Chemical Warfare Service developed the following method of preparation of the methyl derivative. Sodium arsenite (Na₃AsO₃) is prepared by dissolving arsenic trioxide in sodium hydroxide solution. The action of methyl sulfate at 850 C. gives disodium methyl arsenite, Na₂CH₃AsO₃. Sulfur dioxide reduces the arsenite to methyl arsine oxide, CH₃AsO, which is then reacted with hydrochloric acid to give methyl dichloroarsine. The final product is distilled from the mixture and condensed. This material costs from two to two and a half dollars per pound for chemicals (war prices). Methyldichloroarsine is a colorless liquid of powerful burning odor, which boils at 132° C. It is somewhat soluble in water and is soluble in organic solvents. The specific gravity is 1.838 at 20° C. The vapor pressure at 25° was found to be 10.83 mm. mercury. Not only is the material toxic but it has remarkable vesicant properties, comparing favorably with mustard gas in this respect. Ethyldichloroarsine, which was used by the Germans, was prepared by the method given above, using ethyl sulfate, but the yield was never over 20 per cent. In general this has properties similar to the methyl derivative. DIPHENYLCHLOROARSINE