A study of the hydration of calcium orthoplumbate. V.

The behavior of dehydration

Abstract

The retarding action of calcium hydroxide in dehydration of calcium orthoplumbate hydrates has been investigated by differential thermal analysis (DTA) in the previous paper. Authors found that the retarding action is closely correlated to the presence of the calcium hydroxide liberated. Such a phenomenon as the inhibition of dehydration is believed as that the crystals of calcium hydroxide produced by the hydration link together to form a three-dimensional network and that hydrated calcium orthoplumbate fills up the space existing in the calcium hydroxide network.
Above conclusion is based on both the activation energy of the hydrate and its loss of weight.
In this paper, the dehydration of calcium orthoplumbate hydrates will be identified by thermogravimetric analysis (TGA) pattern, and the information thus obtained will be used for comparison to analyzing the dehydration behavior of crystalline water and negative catalytic effects of calcium hydroxide by differential thermal analysis.
The activation energy (E) was obtained from the continuous record of weight change as a function of time and temperature. Authors used the Freeman's method derived from the theoretical consideration. The plot of Δlog (d_w/dt) /Δlog W_γvs.Δ (T^-1) /Δlog W_r should give a straight line; the slope of this line will be E. For the purpose of this plot, d_w and W_r can be determined directly from the thermogram. The reaction order (X) is obtained from the value of intercept of graph.
The results of present experiments will support the conclusion of the previous paper.
Thus, the method of thermogravimetric analysis can be used to obtain information about the kinetics and the reaction order of simple dehydration reactions.
Several factors which may influence the patterns have not been examined, such as pressure, aomosphere and particle size.
However, the present work appears to demonstrate that the dehydration of crystalline water in the hydrate mixture was retarded by the presence of the calcium hydroxide liberated; this conclusion has been based on both the activation energy of the hydrate and its loss of weight by thermogravimetric analysis and differential thermal analysis (in previous paper)