Effect of change in shape factor of a single crystal on its dissolution behavior

To study the effect of change in the shape factor of real crystals on their dissolution behavior using a potassium dichromate crystal as a model for particulates in general.

A model geometry (parallelepiped) has been suggested for a dissolving particle. Single crystals of potassium dichromate which are monoclinic prisms were grown individually from supersaturated solutions at 40 degrees C. Dissolution studies were carried out on five such crystals in 0.1N H2SO4 at 25 degrees C and a stirrer speed of 50 +/- 1 rpm. The five crystals had different degrees of non-isometricity. Initial dimensions of the crystals were measured using image analysis techniques. The shape factor of the dissolving crystal as a function of time was obtained indirectly from the dissolution data.

The shape factor of a single crystal changed significantly after about 50% dissolution. The nature of this change depended on the degree of non-isometricity of the crystal. The change in shape factor of the dissolving crystal was accounted for in the Hixson-Crowell cube root law, and a modified form of the cube root equation was developed. This equation for dissolution explained the observed upward curvature in the cube root law plot.

The shape factor for any non-isometric particle cannot be considered to be constant over the dissolution event, as is commonly assumed. This change has an appreciable effect on the dissolution behavior of crystals. This study is particularly of significance for elongated shapes like needles and platelets. By the methodology described here, it was possible to determine the initial shape factor of the crystal and the intrinsic dissolution rate constant.