(4) Conversion of the thiodiglycol into mustard gas (dichlordiethyl-sulphide), using gaseous hydrochloric acid.
The thiodiglycol was produced at Ludwigshafen and provides one of the best examples of the adaptation of the German dye works for the purpose of producing war chemical. Technically, ethylene is a fairly difficult gas to produce in large quantities, but, for the Ludwigshafen works, these difficulties were a thing of the past. There were twelve big units before the war, and, by the time of the Armistice, these had been increased to seventy-two in connection with mustard gas manufacture. In a similar way, the number of the units for chlorhydrin, the next step, was increased from three to eighteen. These two processes had all been worked out very thoroughly in connection with the production of indigo. These new plants were identical with the peace-time units. The expansion was a mere question of repetition requiring no new designs or experiments and risking no failure or delay. Success was assured. The last step, the production of thiodiglycol, occurred in the causticising house, to which no substantial alterations or additions appear to have been made for the purpose. As sodium sulphide is used in large quantities as a raw material in the dye industry, and was already produced within the I.G., no difficulty was presented in connection with its supply.
The thiodiglycol was forwarded to two other factories, one of which was Leverkusen, where 300 tons of mustard gas were produced monthly. The reaction between thiodiglycol and hydrochloric acid was one which required very considerable care. At one stage of the war the Allies viewed with much misgiving the possibility of having to adopt this method. But the technique of the German dye industry solved this as satisfactorily and as steadily as other chemical warfare problems, bringing its technical experience to bear on the different difficulties involved.
Diphenychlorarsine.—This was the earliest and main constituent of the familiar Blue Cross shell. It was prepared in four stages:
(1) The preparation of phenyl arsinic acid.
(2) The conversion of the above to phenyl arsenious oxide.
(3) The conversion of the latter into diphenyl arsinic acid.
(4) The conversion of the latter into diphenyl-chlor-arsine.
This is another example of a highly complicated product which might have presented great difficulties of production, but the problem of whose manufacture was solved, almost automatically, by the German organisation.
The first step, that of the manufacture of phenyl arsinic acid, was carried out at Ludwigshafen in one of the existing azo dye sheds without any alteration of plant, just as a new azo dye might have been produced in the same shed. It is believed that another dye factory also produced this substance. At Ludwigshafen the conversion to diphenyl arsinic acid occurred. This was again carried out in the azo colour shed, with no more modification than that involved in passing, from one azo dye to another.