A discontinuity in the dehydration of certain salt hydrates

Several investigators have studied the rate of loss of water from salt hydrates exposed in dry atmospheres at ordinary temperatures. The process for salts forming more than one hydrate consists of a series of stages, during each of which the evaporation proceeds at an approximately uniform rate. The existence of sudden changes in the rate of dehydration has been used as evidence of the existence of definite hydrates, even in cases where the hydrate concerned has not been isolated. Thus Richards (1) found a sudden change in rate with BaCl2. 2H2O at a point corresponding to the composition BaCl2. 1H2O. Davis and Eyre (2), in a recent paper on the “rehydration” of fibres and certain salts, state that the weight-time curve consists of a random series of parabolic segments and straight lines. The Odén-Keen Automatic Recording Balance (3) has rendered it possible to follow the progress of the dehydration and hydration of salt hydrates under conditions that were not possible by the methods of intermittent weighing hitherto adopted. The rehydration of anhydrous copper sulphatelu moist or saturated atmospheres showed no discontinuities similar to those found by Davis and Eyre. During the rapid dehydration of copper sulphate and barium chloride crystals, however, a characteristic break has been found. When these materials were dried at a high temperature it was found that the evaporation was either completely arrested or extremely slow for some minutes, when the composition of the residue could be represented by a definite hydrate such as CuSO4. 3H2O, CuSO4. 1H2O, or BaCl2. 1H2O. This phenomenon does not appear to have been observed previously, and its interpretation should provide an interesting problem in the kinetics of heterogeneous reactions.
RESP-00391