Dissolution of acidic and basic compounds from the rotating disk: influence of convective diffusion and reaction

A mass transfer model was developed to describe the dissolution and reaction of acidic and basic compounds from a rotating disk in unbuffered water. Dissolution of two carboxylic acids, 2-naphthoic acid (1) and naproxen [(+)-6-methoxy-alpha-methyl-2-naphthaleneacetic acid, 2], and the free base, papaverine (6,7-dimethoxy-1-veratrylisoquinoline, 3), in aqueous solutions (mu = 0.1 with KCI) at 25 degrees C were investigated. An automated dissolution apparatus, which consisted of microcomputer-controlled autoburets, was constructed to monitor and adjust the pH of the aqueous solutions during the experiments. Unique features of the mass transfer model include treatment of mass transfer as a convective diffusion process rather than a stagnant film diffusion only process; treatment of ionization and acid-base reactions as heterogeneous reactions; use of experimental diffusion coefficients for all species, particularly H+ and OH-; and application of boundary conditions that specify flux for surface ionization produced species. The model accurately predicted the dissolution rate assuming the solubility, pKa, and diffusion coefficient of the compound were independently known. The model also predicted pH at the solid-liquid surface, the flux of H+ from the surface, and the contribution of A- to the total acid flux as a function of bulk pH of the aqueous solution.