Gypsum solubility in water at 25°C

Abstract

The analysis of published data and our experimental results showed that the existing estimates of gypsum solubility in water (C_m^↑) at 25°C range from 0.0147 to 0.0182 M. Such a scatter (more than 20%) is the result of a combined influence of experimental conditions and ability of gypsum to form supersaturated solutions. This influence appears most noticeable during gypsum dissolution in a dispersed state. Under such conditions, equilibrium is determined with a precision of no better than ±4.5%; if a flat surface similar to natural one is dissolved, the (C_m^↑) value is characterized by the minimum scatter (±1%). According to experimental data (25°C), the solubility of gypsum particles in water is inversely proportional to their size at r < 1 μm and is r-invariant for larger grains. The invariance of (C_m^↑) at r > 1–5 μm is supported by the data of analytical calculations using the approximate Ostwald–Freundlich equation. It is supposed that the difference of the concentrations of dissolved gypsum at the boundaries of the field of metastable state of gypsum-saturated groundwater can be 1–5%. The results of our study can be used for the description of gypsum dissolution in the models of mass transfer between gypsum-bearing rocks and groundwater.