Inhibiting effects of lanthanum ion on calcite formation from CaCL₂-NaHCO₃ solutions at 25°C

The influence of La³⁺ on calcite formation was studied using CaCl₂-NaHCO₃ solutions. We adopted a closed system, where the pH change of the solutions was monitored in a closed vessel. The closed system reduces a number of thermodynamic variables of the solution, thus enabling us to calculate the amount of CaCO₃ precipitate only from pH with the assumption of equilibria among dissolved components in solution. After the analytical inspection of the calculation and assumption on which the calculation is based on, it was applied to experiments of the calcite formation in a Teflon vessel. The mole concentration of lanthanum added in the experiments was smaller than 1/3000 of that of calcium ion in the solutions. The addition of lanthanum oxide, chloride, and nitrate caused the difference in the pH change of the solution. The decrease of pH was smaller when lanthanum reagents were added, indicating that the amount of CaCO₃ precipitate decreased. The smallest decrease was observed when lanthanum chloride or nitrate salt was added. The X-ray diffraction study showed that the CaCO₃ precipitated in any solutions were calcite. If an equilibrium is assumed, which is considered to be likely from the pH change, the ion activity product (IAP) calculated in this study increased about 6 times when lanthanum ion, whose concentration is only 1/14 000 mole of calcium ion, was present.