Thermal investigation of the role of outer surfaces in the decomposition of crystalline solids

Abstract Recent studies have evidenced different decomposition behaviour between stored and freshly recrystallized samples of both NaHCO 3 and Na 2 WO 4 ·2H 2 O. To verify whether the above findings could be a general property of crystalline materials undergoing thermal decomposition we compared the thermal behaviour of aged and freshly recrystallized samples of a number of crystal hydrates of different nature (e.g. CuSO 4 ·5H 2 O, Na 3 C 6 H 5 O 7 ·2H 2 O, H 2 C 2 O 4 ·2H 2 O, Ni(C 2 H 3 O 2 ) 2 ·4H 2 O, CoSO 4 ·7H 2 O, etc.) The results suggest that the compounds examined can be divided into three main classes namely: i) those for which fresh surfaces retard the decomposition; ii) those for which the decomposition remains unaltered; iii) those for which fresh surfaces enhance the reactivity. It appears that the three classes can be characterized by different kinetic behaviour also. These findings can be interpreted in terms of a three-stage mechanism recently proposed for the dehydration of tetragonal nickel sulphate hexahydrate.