Pitfalls in the determination of empirical dissolution rate equations of minerals from experimental data and a way out: an iterative procedure to find valid rate equations, applied to Ca-carbonates and -sulphates

Empirical rate equations such as R=k(1−c/ceq)n in the dissolution of minerals are common in nature, e.g. limestone. The quantity c is the concentration of a major ion contained in the mineral, and ceq its concentration at equilibrium. If experimental data obey such a rate equation, by plotting log(R) versus log(1−c/ceq) straight lines are found from which k and n can be determined. In many experiments, however, especially for natural minerals ceq is not known exactly. If one uses wrong values of ceq that deviate only a few percent from true equilibrium such plots are severely distorted and one may conclude that above some value cs the true order n changes to a new value, even when a rate equation as given above is valid. We present an iterative computational procedure, which allows to find the valid rate equation from experimental data, even when ceq is not known. The method is applied to limestone and synthetic calcite as well as to natural and synthetic gypsum. New experimental data are given for the dissolution rates of anhydrite (CaSO4). By use of our new method, we find that this mineral exhibits a surface controlled rate equation with k=5.0±1.0×10−6 mmol cm−2 s−1, n=4.5±0.2 and ceq=23.5±0.1 mmol/l at T=10 °C.