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Published April 20, 2006 | public
Journal Article

Acidity of Frozen Electrolyte Solutions


Ice is selectively intolerant to impurities. A preponderance of implanted anions or cations generates electrical imbalances in ice grown from electrolyte solutions. Since the excess charges are ultimately neutralized via interfacial (H^+/HO^-) transport, the acidity of the unfrozen portion can change significantly and permanently. This insufficiently recognized phenomenon should critically affect rates and equilibria in frozen media. Here we report the effective 19F NMR chemical shift of 3-fluorobenzoic acid as in situ probe of the acidity of extensively frozen electrolyte solutions. The sign and magnitude of the acidity changes associated with freezing are largely determined by specific ion combinations, but depend also on solute concentration and/or the extent of supercooling. NaCl solutions become more basic, those of (NH_4)_2SO_4 or Na_2SO_4 become more acidic, while solutions of the 2-(N-morpholino)ethanesulfonic acid zwitterion barely change their acidity upon freezing. We discuss how acidity scales based on solid-state NMR measurements could be used to assess the degree of ionization of weak acids and bases in frozen media.

Additional Information

© 2006 American Chemical Society. Received: February 23, 2006; In Final Form: March 13, 2006. C.R. was a MURF student. C.S.B. acknowledges support from the Betty and Gordon Moore Foundation. This work was financed by NSF grants ATM-0228140 and ATM-0534990.

Additional details

August 19, 2023
October 23, 2023