Unifying Batch-Dissolution Kinetics for Salts: Probing the Back Reaction for Gypsum and Calcite by Means of the Common-Ion Effect

Recent success in fitting the shrinking object model for dissolution kinetics to biogenic silica, silica gel, simple salts, sucrose and gypsum prompted this study of the effects of common ions upon gypsum dissolution kinetics. Middle-ground dissolutions were mainly studied, in which shrinkage of the surface area, S, is significant, and the system approaches, but does not reach, saturation, c _sat. Dissolution was monitored by conductimetry. At a constant ionic strength of 0.060 M, the net rate for gypsum dissolution is given by textNet textRate = k_textb ·S ·(c_textsat - c ) {text{Net}},{text{Rate}} = k_{text{b}} cdot S cdot (c_{text{sat}} - c ) , where k _b is a rate constant, and c can be expressed alternatively in terms of either [Ca²⁺], [SO₄ ²⁻] and [ε_±], that part of the electrolyte concentration contributed by gypsum dissolution, or as the equivalent total concentrations of these species, for example, [SO₄ ²⁻]_T. The presence of either calcium or sulphate as a common ion slows dissolution, and the effect of this upon c _sat, k _b and k _f, the forward rate constant, is discussed. Contrary to previous experience, it is emphasised that each fitting of the shrinking object model demands its own value of the Solubility of gypsum, c _sat, which can be derived from the Solubility Product. This experience with gypsum is aligned with previous work on calcite, to develop a unified approach to the batch dissolution of salts. It highlights serious deficiencies in the way earlier common-ion experiments were conceived and enacted, and in particular with the rate equation of Sj?berg (Geochim Cosmochim Acta 40:441–447, 1976) for calcite above a pH of 7. Common-ion experiments are shown to be crucially important for probing the back reaction to dissolutions and might be applied to the far bigger problem of silicate-mineral dissolution, where ‘non-linear kinetics’ are often observed.