Imbalance between Anion and Cation Distribution at Ice Interface with Liquid Phase in Frozen Electrolyte As Evaluated by Fluorometric Measurements of pH

When an aqueous electrolyte is frozen, anions and cations are distributed between liquid and ice phases in different fashions. This partition imbalance is relaxed by the transfer of H⁺ and OH^– to each phase, resulting in the acidification of the liquid phase when the cation is better distributed in the ice phase than the anion and in the basification in the opposite situation. In this work, a pH change in the liquid phase has been precisely evaluated by fluorescence ratiometry with pyranine as the pH probe. For frozen alkali chlorides (LiCl, NaCl, and KCl), the liquid phase is always basified by freezing due to the preferential partition of Cl^– over the alkali metal cations. Changes in pH are quantitatively analyzed by a partition model, in which the distribution of an ion between the liquid and ice phases is determined by the partition coefficient. Since the concentration of a salt (i.e., ions) in the liquid phase in contact with ice becomes higher as freezing proceeds, the concentration of the ions in the ice phase is higher near the interface with the liquid phase and decreases toward the interior of ice. When the temperature of a frozen electrolyte increases, the ionic imbalance is relaxed to some extent by melting of ice near the interface.