Dissolution of carboxylic acids. III: The effect of polyionizable buffers.

The dissolution behavior of three carboxylic acids of variable aqueous solubility but with approximately equal pKa values into aqueous buffered solutions has been studied as a function of pH and of buffer properties. The dissolution from constant-surface-area compressed disks of benzoic acid, 2-naphthoic acid, and indomethacin into solutions of constant ionic strength (mu = 0.5 with potassium chloride) and constant pH (maintained by pH stat) at 25 degrees C using a rotating disk apparatus was evaluated. Models for dissolution of these weak acids into diprotic and triprotic buffering media are developed to predict the flux of the acid as a function of bulk solution pH and the physical and chemical properties of the buffer and acid. The models assume that mass transfer can be represented by a single second order diffusive term and that instantaneous equilibrium between all reactive species exists. Values of flux and pH at the solid-liquid interface are calculated and the fluxes compared to experimentally determined values. Reasonable correlation was found between values predicted by the models and experimental flux values. Major influences on model accuracy are the Ka and physical properties of the buffer.