Solubility of siderite (FeCO3) in NaCl solutions

The solubility of siderite (FeCO3) at 25degreesC under constant CO2 partial pressure [p(CO2)] was determined in NaCl solutions as a function of ionic strength. The dissolution of FeCO3(s) for the reaction FeCO3( s) + 2H(+) = Fe2+ + CO2( g) + H2O K (*)(so) = [Fe2+]\pCO(2)/[ H+](2) has been determined as a function of pH = - log[H+]. From these values we have determined the equilibrium constant for the stoichiometric solubility to FeCO3(s) in NaCl K-* (sp) = [Fe2+][ CO32-] These values have been fitted to the equation log [K-* (sp)] = -10.9 + 2.518\ I-0.5 - 0.657 I with a standard error of s = 0.15. The extrapolated value of log(K-sp(o)) - 10.9 in water is in good agreement with data in the literature (- 10.8 to - 11.2) determined in solutions of different composition and ionic strength. The measured values of the activity coefficient, gamma(T)(Fe2+) gamma(T)(CO32-), have been used to estimate the stability constant for the formation of the FeCO3 ion pair, K*(FeCO3). The values of K*(FeCO3) have been fitted to the equation (s = 0.09) log [K-*(FeCO3)] = 6.3 - 2.3135 I-0.5 + 0.7091 I The value of log[K-o(FeCO3)] in water found in this study (6.3 +/- 0.2) is slightly higher than the value found from extrapolations in 1.0 m NaClO4 solutions (5.9 +/- 0.2). These differences are related to the model used to determine the activity coefficients of the Fe(II) and carbonate species in the two solutions.