Solubility of rhodochrosite (MnCO3) in NaCl solutions

The solubility of rhodochrosite (MnCO3) at 25degreesC under constant carbon dioxide partial pressure p(CO2) was determined in NaCl solutions as a function of ionic strength I. The dissolution of MnCO3(s) for the reaction MnCO3(s) + 2H(+) = Mn2+ + CO2(g) + H2O K-so* = [Mn2+]p(CO2)/[H+](2) has been determined as a function of pH. From these values, we have determined the equilibrium constant for the stoichiometric solubility of MnCO3(s) in NaCl solutions K-sp* = [Mn2+](T)[CO32-](T) These values have been fitted to the equation log Ksp* = -10.3 + 2.39 I-0.5 -0.69 I with a standard error of sigma = 0.1 with I and concentrations in molalities. The extrapolated value of log K-sp(o) sp (-10.3) in water is in good agreement with literature data (-10.1 to 10.8) determined in solutions of different composition and ionic strength. The measured values of the activity coefficient, gamma(T)(Mn2+) and gamma(T)(CO32-), have been used to estimate the stability constant for the formation of the MnCO3 ion pair, K*(MnCO30). The value of K-0(MnCO30) calculated from the values of K*(MnCO3) by the Pitzer equation (sigma = 0.1) in this study (4.8 +/- 0.1) is in reasonable agreement with literature data.