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$\ce{HCN}$ is notoriously unstable: It boils at room temperature, and it tends to polymerize when concentrated in unbuffered liquid form. Yet $\ce{HCN}$ was for decades widely used as a fumigant pesticide. At least one commercial formulation called Zyklon became notorious after it was employed for genocide by Nazi Germany.

I was wondering what chemistry allowed large masses of $\ce{HCN}$ to be bound (presumably adsorbed?) to a solid (Zyklon was known to consist of pellets), put in cans, and then released simply by opening the cans?

The best research I have found concludes of the solid substrate:

Zyklon-B is composed of calcium sulfate but not the soluble anhydrite of calcium sulfate. Due to its microcrystalline structure it is possible to determine that the Zyklon-B sample is either the natural form of anhydrous calcium sulfate, also known as the mineral anhydrite or, equally likely, the insoluble anhydrite resulting from heating gypsum at temperatures above 650 degrees centigrade. Both of these forms preserve the microcrystalline structure.

Is it known, or deducible, that calcium sulfate will adsorb large masses of $\ce{HCN}$, hold it in a "shelf-stable" (nonreactive) manner when enclosed, and then release it completely at STP?

(E.g., I don't know anything about adsorption of organics, but assuming that $\ce{CaSO4}$ has adsorbed $\ce{HCN}$ to saturation then, since calcium sulfate is a powerful desiccant, I can imagine that on exposure to a humid atmosphere it would preferentially absorb water and the resulting $\ce{CaSO4·2H2O}$ must not be capable of adsorbing $\ce{HCN}$?)

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  • $\begingroup$ Discussing this borders on distasteful, but are you sure Zyklon-B was released simply by opening the cans in the Nazi death camps, or was water added at some point? Once unsealed the pellets could get water from the air, but that seems like a very slow process. $\endgroup$
    – MaxW
    Nov 3, 2018 at 21:28
  • $\begingroup$ Also the Wikipedia article seems that Zyklon-B didn't have just one formulation. $\endgroup$
    – MaxW
    Nov 3, 2018 at 21:30
  • $\begingroup$ Poked around some on internet and it seems that the solid pellets were just dumped into the gas chambers. I'd guess that the OP's last paragraph is probably the gist for CaSO4. However it does seem that other adsorbents might have been used also. $\endgroup$
    – MaxW
    Nov 3, 2018 at 22:08
  • $\begingroup$ @MaxW – Indeed, it is hard to find answers to these simple chemistry questions because there is so much more material (and controversy) online covering the role of Zyklon in the Nazi genocide. E.g., it does appear that adsorbed HCN will outgas at STP (i.e., "just opening the cans"), and that process could be accelerated by heating. OTOH, p.234-238 of this reference suggest (unclearly) that humidity actually inhibits/slows the release of HCN as water binds to the substrate. $\endgroup$
    – feetwet
    Nov 4, 2018 at 17:15
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    $\begingroup$ I'll add that Cyanosil is the name of a very similar product that has been sold recently. Not sure if it is still being sold. $\endgroup$
    – MaxW
    Nov 4, 2018 at 17:54

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