Relationship between the Morphological Character of 5-Nitroacetyl-salicylic Acid Crystals and the Decomposition Rate in a Humid Environment

Abstract

Two morphologically different crystals, column-shaped and ramified, were sorted under a microscope from a recrystallized batch of 5-nitroacetylsalicylic acid. Their degradation rates in the solid state were quantitatively studied and the progress of decomposition was observed under a polarizing microscope. Plots of the ratio decomposed against the reaction time were sigmoidal under all reaction conditions, and Avrami's kinetic equation was applicable to this reaction below 40% degradation. The reactivity of the ramified crystals was greater than that of the column-shaped crystals under all experimental conditions. Although the parameter B of Avrami's equation, which is assumed theoretically to be proportional to the product of the number of latent reaction nuclei and the voluminal growth rate varied by a factor of 10⁴ at maximum under various experimental conditions, the ratio of B between ramified and column-shaped crystals remained fairly constant under all the conditions tested, and was about five. The results of observation of the decomposing crystals under a polarizing microscope agreed well with Avrami's theory of the formation and growth of reaction nuclei.