A study of dehydration and hydration of oxalic acid by means of the deuterium exchange reaction

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<jats:title>Abstract</jats:title> <jats:p>The deuterium exchange reaction in the course of the hydration of anhydrous oxalic acid with heavy water vapor and subsequent dehydration of the product has been studied.</jats:p> <jats:p>Two exchange mechanism have been suggested to be operative; the hydrate formation mechanism and the surface exchange mechanism. A distinction may be made between the two by the dependence of exchange reaction on the molar ratio of the reactants.</jats:p> <jats:p>Anhydrous oxalic acid prepared by dehydration at 100°C takes part in the exchange reaction through hydrate formation; while, with the anhydrous acid prepared by dehydration in vacuo at ordinary temperatures, the surface exchange mechanism comes into play.</jats:p> <jats:p>For the specimens of anhydrous acid prepared in vacuo, the molar ratio of oxalic acid in the surface to that of the bulk is estimated to be 0.6∼0.7 from the results of the deuterium exchange reaction. Dehydration of oxalic acid dihydrate in the vacuum of electron microscope yields amorphous anhydrous acid.</jats:p> <jats:p>The recrystallization of amorphous anhydrous oxalic acid was studied by the use of deuterium exchange reaction as a measure of surface area. It has been shown that adsorbed water exerts catalytic effect on the recrystallization. The process is associated with small or zero activation energy. Anhydrous oxalic acid recrystallized in the presence of water has been identified with α-form.</jats:p> <jats:p>Deuterium exchange reaction between unhydrated oxalic acid and hydrate water occurs in anhydrous oxalic acid partially hydrated with heavy water vapor. The rate of this exchange is practically independent of temperature. The reaction probably consists in the deuteration of the molecules in the interior of the anhydrate lattice.</jats:p> <jats:p>Treatment of amorphous anhydrous acid with a small amount of water vapor causes a peculiar change in the rate of subsequent rehydration. There is a considerable retarding effect on the rehydration rate for a particular range of the amount of water vapor.</jats:p>

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