Quasichemical Approach to pH Shifts in Frozen Phosphate Buffers

Abstract

Shifts in the pH of buffer solutions upon freezing have received much attention particularly in the life sciences, as such pH variations during freezing may damage biological samples, biomolecules, and pharmaceuticals. The understanding and prediction of the said pH changes upon freezing are essential to utilize buffer solutions in a wide temperature range, including subzero temperatures. Phosphate is of particular importance for aqueous buffer preparation as it covers a wide pH range and displays high biocompatibility. However, the phase behavior and pH changes in phosphate buffers at subzero temperatures are very complex due to the variety of species involved in the phase and acid–base equilibria. This paper focuses on the interpretation of the pH changes in phosphate buffers under freezing conditions using an extended universal quasichemical (EUQ) model. This model has been previously applied to understand the phase behavior (solid precipitation) in phosphate buffers at subzero temperatures but not to the calculation of the pH of the liquid phase (freeze-concentrated solution, FCS) that coexists with ice and salt precipitate. Since the EUQ model provides activity coefficients for all of the components in the system, not only the phase behavior but also the pH value can be estimated. The calculated pH values are compared to experimental values and are interpreted from the viewpoints of salt deposition, salt enrichment in the FCS, and supercooling effects.