Heterogeneous kinetics: from solid–gas reaction to solid–liquid dissolution

The extension of the formal kinetics of solid–gas reactions to solid–liquid dissolution is investigated theoretically. The work is based on modelling the dissolution of a solid in a closed system by a stoichiometrically simple chemical reaction Asolid⇄Aliquid proceeding up to either an equilibrium state if the solid phase is initially in excess or exhaustion in the reverse situation. The initial amount of solid compared to the capacity of the liquid phase appears as a major factor of complexity. First, the rate equation is formulated in terms of the intensive variables without any assumption about the mechanism and the rate-limiting step. The resulting equations are compared to those derived from Nersnt boundary-layer theory of dissolution. Only when the solid phase is initially in deficit does the kinetic approach turn to be in the wrong if the rate of dissolution is assumed to depend on the maximum of the concentration gap. At this stage, the dissolution mechanism has to be taken into consideration and the rate equation adjusted accordingly. Finally, algebraic expressions for the fractional dissolution curves can be obtained. Except for particular solid liquid ratios they are not superimposable by an affinity relation.