Soda-Lime
Charcoal is not a satisfactory all-round absorbent because it has too little capacity for certain highly volatile acid gases, such as phosgene and hydrocyanic acid, and because oxidizing agents are needed for certain gases. To overcome these deficiencies the use of an alkali oxidizing agent in combination with the charcoal has been found advisable. The material actually used for this purpose has been granules of soda-lime containing sodium permanganate. Its principal function may be said to be to act as a reservoir of large capacity for the permanent fixation of the more volatile acid and oxidizable gases.
The development of a satisfactory soda-lime was a difficult problem. The principal requirements follow: Its activity is not of vital importance, as the charcoal is able to take up gas with extreme rapidity and then later give it off more slowly to the soda-lime. Absorptive capacity is of the greatest importance, since the soda-lime is relied upon to hold in chemical combination a very large amount of toxic gas. Both chemical stability and mechanical strength are difficult to attain. The latter had never been solved until the war made some solution absolutely imperative.