Solid–Liquid Metastable Equilibria for Solar Evaporation of Brines and Solubility Determination: A Critical Discussion

Abstract

Within the last two decades, a large number of articles were published that report, as the authors claim, 'metastable phase equilibria', or 'metastable solubilities'. The main objective of these studies was to carry out experiments under conditions closely meeting those in solar evaporation ponds or industrial evaporation-crystallization processes. Occasionally, such studies were subject to controversial discussion and criticism and the question was raised whether metastable equilibrium data are worth being published at all. This paper provides a critical discussion of such evaporation experiments. The thermodynamic background of stable and metastable solubility and typical experimental difficulties in solubility determinations are discussed in detail. We also demonstrate that the knowledge of metastable equilibria is very useful in different research areas such as geochemistry or industrial application of solubility equilibria. Finally, it is shown that so-called 'isothermal evaporation method' used in the above-mentioned studies does not yield stable or metastable solubilities.

Editorial: Guidelines for the Measurement of Solid–Liquid Solubility Data at Atmospheric Pressure

“…On the basis of the input of several JCED reviewers, manuscripts using this method are no longer accepted by this Journal. In a recent review, Steiger and Voigt critically discussed this method and concluded that the resulting data are not acceptable as thermodynamic measurements and, in particular, they do not correspond to valid metastable states.…”

“…Steiger and Voigt also listed typical experimental difficulties in solubility determinations, focusing chiefly on stable and especially metastable equilibria involving oceanic salts and related brine systems. In this editorial, special emphasis will be placed on experimental techniques that are acceptable for JCED publications with an aim to provide guidelines to authors and reviewers.…”

“…According to Ostwald’s Rule of Stages, the thermodynamically least stable phase generally precipitates first, followed by progressively more stable phases, some of which may persist for sufficient periods of time to attain metastable equilibrium with the solution. Crystallization of a certain solid phase often needs to be initiated by seeding (although this method does not always promote precipitation of the same phase, as has been found, for example, in the case of magnesium sulfate hydrates). Alternatively, the temperature dependence of solubility can be exploited by changing T in such a direction that more of the equilibrated phase dissolves, followed by a return to the desired equilibration temperature …”

“…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 …”

“…Precise pressure control is not necessary since the effect of small pressure changes on the solubility of solids in liquids near solids (commonly associated with a decrease in solubility) require unusually long periods of time. 4 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. 17 (c) Percolation cells (see refs 6 and 9 for a description of many different types) facilitate steps (iv) and (v) due to the confinement of the solid phase in a separate cell compartment, where it is repeatedly exposed to its solution until saturation is achieved.…”