Acidity and Phase Behavior of Frozen Hydrochloric\nAcid during Thawing

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