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Solubility Product of Belle Fourche and Colony Montmorillonites in Acid Aqueous Solutions¹

ABSTRACT

Published solubility studies for Belle Fourche and Colony montmorillonites were re-examined to help answer the question as to whether or not montmorillonite can come to equilibrium with acid aqueous solutions. Values of pH-pAl³⁺ for both montmorillonites were plotted against pH₄SiO₄ after adjusting to common values of pH-pMg²⁺ and pH. Three lines of reasoning indicate that selected Belle Fourche montmorillonite samples came to equilibrium with acid aqueous solutions during a 3-to-4-year study period. First, the slope of the solubility line for Belle Fourche montmorillonite was in agreement with that predicted for equilibrium with a solid phase of the same Al/Sl ratio as Belle Fourche montmorillonite. Second, the stability of the solubility line in relation to the stability of other minerals agreed with known natural relationships, and third, the precision of the pK values is well within experimental error. Similarly adjusted pH-pAl³⁺ values for Colony montmorillonite have a wide scatter when plotted against pH₄SiO₄. This scatter indicates that collectively these samples had not come to equilibrium with any solid phase during a study period of approximately 1 year or less. By examining comparable data for gibbsite (previously unpublished) it is shown that variation in the apparent pK values for Colony montmorillonite may be a consequence of pH variation as the system approaches equilibrium. It is argued that statistical relationships between nonequilibrium components for noncongruent dissolution do not permit conclusions with regard to equilibrium relationships for either Colony montmorillonite in particular, or solid phases in general.

¹ Scientific Paper no. 4821. College of Agric. Res. Center, Washington State Univ., Pullman, Wash. Project 1885.

² Professor of Soils, Washington State Univ., Pullman, WA 99164.

Received for publication June 30, 1977. Accepted for publication January 19, 1978.