Redox reactions in growing single ice crystals: A mechanistic interpretation of experimental results

Laboratory studies have demonstrated coupled reduction-oxidation (redox) re actions of included ions in free-falling ice crystals growing in a supercoo led liquid water cloud and during the linear freezing of dilute solutions. These reactions are hypothesized to occur as a consequence of the separatio ns of ions of opposite signs at the interfaces between the growing ice phas e and the liquid or liquidlike solution layer. The potential determining io ns in the ice-water system are hydronium. and hydroxyl ions (i.e., the sign of the charge in the growing ice phase is pH dependent). Changes in the so lution pH during the growth of single crystals or freezing of dilute soluti ons can occur by hydrolysis of reaction products and will change the magnit ude of the potential and/or cause a reversal of the charge orientation. The redox reactions occur by electron transfer from oxidizable ions in the sol ution phase to a positive ice phase, or from a negative ice phase to reduce able ions in the solution phase. They are concentration-dependent reactions , solutions with solute concentrations above about 10(-3) M do not develop potentials. This concentration is also that at which the electrochemical do uble layer at the ice-solution interface becomes compact. The reduction of carbonate and bicarbonate ions and of absorbed carbon dioxide during the gr owth of ice crystals containing strong base-weak acid salts was shown to yi eld formate ion and formaldehyde. (C) 2001 Academic Press.