Diffusion and Reaction of Water in Additively Colored KCl

Additively colored KCl single crystals heated in the presence of water vapor developed a colorless region around the surface of the crystal. This bleaching effect is found to be described by a model in which water, or fragments from the dissociation of water, diffuses into the crystal and reacts with the excess potassium to form a colorless, photochemically sensitive reaction product, possibly KH or KOH. Using this model, an order‐of‐magnitude estimate of the diffusion coefficient of water in colored KCl is $${\text{D=(7.2±3)×10}}^{\text{−6}}\exp \text{[−(0.56±0.07)}\mathrm{eV}\mathrm{/kT]}{\mathrm{cm}}^2/\mathrm{h}.$$