Proton Transfer in Solution

Abstract

The understanding of the role of proton transfer in the mechanism of transport processes in solution is based on three accepted concepts: 1. In a solution containing ions produced by dissociation of solvent molecules, rapid charge transfer may take place between these ions and solvent molecules when they collide. This charge transfer and the sudden charge displacement (jump) that is involved, e.g., between a hydrogen ion and a water molecule or a CH3O− ion and a methanol molecule, may contribute to the transport of current in solution. This possibility was recognized as early as 1905 by Danneel.1 2. Successive charge transfers in a particular direction (e.g., under the influence of an external electric field) are necessary for this contribution. A series of successive transfers can take place only if each jump is followed by structural rearrangement in the liquid. As an example of such rearrangements, rotation of solvent molecules was also mentioned by Danneel.1 3. No free protons exist permanently in aqueous solutions. The hydrogen ion is the monohydrate of the proton, i.e., the hydronium (or oxonium) ion, H3O +. Recognition of this ion was first substantiated by Goldschmidt and Udby2 in their theory of kinetics of acid-catalyzed esterification reactions. Recognizing the hydrogen ion as H3O+ also meant identifying Danneel’s charge transfer as proton transfer from H3O+ to H2O.