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Updated: October 8, 2026
On projecting dry pulverized iodide of potassium into fused anhydrous phosphoric acid, a violent disengagement of iodine takes place, attended by a transient ignition; fused hydrate of phosphoric acid liberates iodine abundantly from iodide of potassium; this reaction is accompanied by the phenomenon of flame and formation of a considerable quantity of hydriodic acid. Iodide of Mercury.
We may wonder whether anything really analogous to this can be met with in the case of a gas, and we are thus led to consider the phenomena of dissociation. If we heat a body which, in a gaseous state, is capable of dissociation hydriodic acid, for example at a given temperature, an equilibrium is established between three gaseous bodies, the acid, the iodine, and the hydrogen.
The zinc and silver forming together a voltaic pair, with the salt water intervening, oxidation of the zinc takes place, and the silver surface commences to evolve hydrogen gas; while this is in a nascent condition it decomposes the film of iodide of silver, giving rise to the production of hydriodic acid, which is very soluble in water, and hence instantly removed.