Acidity and Phase Behavior of Frozen Hydrochloric Acid during Thawing

Freezing and its application is growing in popularity, yet the understanding of the nonequilibrium transformations and acidity changes that occur in frozen solutions upon thawing have remained relatively unexplored. By contrast to other acids such as nitric acid and sulfuric acid, not even the phase diagram is known fully for hydrochloric acid. Even more importantly, the nonequilibrium transformations upon heating glassy freeze-concentrated solution (FCS) are also not well understood and freeze concentration lacks quantification. This work rectifies the knowledge gap by providing the freeze-concentration factors on the example of hydrochloric acid. For this purpose, we have used differential scanning calorimetry to reveal phase changes upon heating. UV–vis spectroscopy of acid–base indicators is employed to elucidate acidity changes. All the samples reach negative values of the Hammett acidity function from −2.5 to −0.25 after freezing, showing that aqueous HCl can freeze concentrate 7–250,000 times depending on its initial concentration. We observe the glass-to-liquid transition of the freeze-concentrated glassy solution above −140 °C and cold crystallization of the ultraviscous FCS to HCl hydrates above −110 °C. Cold crystallization leads to basification, whereas acidification accompanies the subsequent melting of the eutectic ice/HCl-hexahydrate. Finally, melting of the ice immersed in solution shows basification caused by the dilution with meltwater. High (1 M) and low (<10 mM) concentrations freeze homogeneously, whereas intermediate concentrations reveal the presence of freeze-concentrated regions of higher and lower concentrations having distinct glass transition and melting temperatures.