pH-Buffering Capacity of Ice

The pH of frozen solutions is a key parameter for some environmentally important reactions that occur in the cryosphere including atmospheric ice clouds because the pH affects the reaction equilibria and kinetics. Several studies reported interesting behavior of acidic or basic compounds on the ice surface and discussed them in relation to the quasi-liquid layer (QLL). We have found that the peculiar acid–base behavior in frozen systems is not unique to the QLL but is also seen in the freeze-concentrated solution (FCS), which is separated from ice upon the freezing of an aqueous solution. For example, when a dilute HCl prepared in aqueous NaCl is frozen, the pH value measured is larger than that predicted from the freeze concentration ratio. Similarly, the pH of buffer solutions shifts toward the neutral pH upon freezing, and this effect is more pronounced at higher freeze concentration ratios and lower buffer concentrations. These findings suggest that the ice/FCS system has a pH-buffering capacity at around neutral pH. The formation and elimination of the Bjerrum defects in the ice crystal near the FCS interface are discussed as the origin of the pH buffering.