CYCLIC VOLTAMMETRY AT GOLD, PLATINUM AND CARBON MICROELECTRODES IN ICE WITHOUT ADDED SUPPORTING ELECTROLYTE - EVIDENCE FOR LIQUID MICROPHASES AT TEMPERATURES WELL BELOW THE FREEZING-POINT OF WATER

The voltammetry of the Fe3+/2+ redox couple has been investigated in water without added electrolyte at temperatures considerably below the freezing point of the solvent. In order to minimize problems with (ohmic) IR drop, and to investigate the possible role of the electrode material, gold, platinum and glassy carbon microdisc rather than conventionally sized working electrodes were employed. Reduction of ferric sulphate, nitrate and chloride solutions was examined to ascertain the role of the anion, and data were compared with results of studies in perchloric acid to examine the role of added electrolyte. Irrespective of the electrode material, the anion, or the presence or absence of perchloric acid, the nature of the mass transport changes, from steady-state diffusion (sigmoidal shaped curves) in the liquid phase to a surface-confined process (peak shaped curves) in the ice phase. Under these surface-confined conditions, reduction of iron(III) and oxidation of iron(II) exhibits surface-confined or thin-layer type electrochemistry. Additionally, an increase in the peak current of the surface-confined process occurs over the temperature range -10-degrees-C to -40-degrees-C. Data suggest that the change in mass transport mechanism is a function of the solvent properties rather than the electrode material, anion or added electrolyte and that a liquid microphase which concentrates solutes in the microphase exists in the aqueous solvents at temperatures well below the freezing point of water. Presumably, at sufficiently low temperatures, precipitation of insoluble material also occurs onto the electrode surface. The observations in frozen water are also consistent with the previously published idea of a liquid microphase contributing to the voltammetry in frozen dimethylsulphoxide.