“…Demonstrating the attainment of equilibrium, however, is indispensable for a solubility study to be thermodynamically meaningful. - Equilibration times generally need to be determined experimentally; for batch methods which, for example, use cells that are moved by rocking or rotating, this involves carrying out steps (iv)–(vi) periodically until constant composition has been achieved. However, this does not necessarily imply that stable equilibrium has been attained: sometimes phase transitions from metastable to stable solids (commonly associated with a decrease in solubility) require unusually long periods of time . Consistent with Ostwald’s Rule of Stages, the kinetics of crystallization and dissolution are generally slower for the stable solid phase than for the metastable phases of a particular system, which means that the latter (e.g., nesquehonite, MgCO 3 ·3H 2 O) usually attain metastable equilibrium faster than the former (e.g., magnesite, MgCO 3 ) attains stable equilibrium, even when these equilibria are approached from undersaturation
…”