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Solubility of Rhodochrosite (MnCO3) in NaCl Solutions

Authors:

Institution:

(1) Rosenstiel School of Marine and Atmospheric Science, University of Miami, 4600 Rickenbacker Causeway, Miami, Florida, 33149

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 sgr

= 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, gamma

_T(Mn²⁺) and gamma

_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 ( sgr

= 0.1) in this study (4.8 ± 0.1) is in reasonable agreement with literature data.

Keywords:

Rhodochrosite solubility activity coefficients sodium chloride Pitzer equations

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