N.M.R. study of the dehydration of sodium bromide dihydrate

Abstract

The dehydration of polycrystalline NaBr,2H₂O is studied by observing the proton, sodium, and bromine nuclear magnetic resonances. Fully hydrated samples and samples which have been partly vacuum-dehydrated show the same behaviour. As the sample is gradually warmed from room temperature water is evolved more and more freely until at 50.8°C complete dehydration occurs. However, the proton resonance results indicate little, if any, preliminary reorientation or migration of the water molecule through the lattice, as this increasingly rapid loss of water occurs. With each stage of vacuum dehydration the sodium and bromine resonances, which are sensitive indicators of the anhydrous cubic phase, increase steadily in intensity. The observations are consistent with the classical mechanism of vacuum dehydration proceeding inwards from the surface of the crystallite. The outer layers are completely converted into the cubic phase while the interior of the crystallite is unaffected.