Solubility of Rhodochrosite (MnCO₃) in NaCl Solutions

Abstract

The solubility of rhodochrosite (MnCO₃) at 25°C 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

$$\begin{gathered} {\text{MnCO}}_{\text{3}} ({\text{s}}) + 2{\text{H}}^{\text{ + }} = {\text{Mn}}^{{\text{2 + }}} + {\text{CO}}_{\text{2}} ({\text{g}}) + {\text{H}}_{\text{2}} {\text{O}} \hfill \\ {\text{ K}}_{{\text{so}}}^{\text{*}} = [{\text{Mn}}^{{\text{2 + }}} ]{\text{p(CO}}_{\text{2}} {\text{)/[H}}^{\text{ + }} {\text{]}}^{\text{2}} \hfill \\ \end{gathered} $$

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

$${\text{K}}_{{\text{sp}}}^{\text{*}} = [{\text{Mn}}^{{\text{2 + }}} ]_{\text{T}} {\text{[CO}}_{\text{3}}^{{\text{2}} - } {\text{]}}_{\text{T}}$$

These values have been fitted to the equation

$${\text{log K}}_{{\text{sp}}}^{\text{*}} = - 10.3 + 2.39\left| {^{0.5} - \;0.69} \right|$$

with a standard error of σ = 0.1 with Iand concentrations in molalities. The extrapolated value of log K ^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, γ_T(Mn²⁺) and γ_T(CO₃ ²⁻), have been used to estimate the stability constant for the formation of the MnCO₃ion pair, K ^*(MnCO₃ ⁰). The value of K ⁰(MnCO₃ ⁰) calculated from the values of K ^*(MnCO₃) by the Pitzer equation (σ = 0.1) in this study (4.8 ± 0.1) is in reasonable agreement with literature data.