The use of massograms and voltammograms for distinguishing five basic combinations of charge transfer and mass transfer at electrode surfaces

The concept of massograms is described and their utility is demonstrated by the analysis of a wide range of interfacial reactions. Massograms are recorded on a quartz crystal microbalance, and show the rates of change of mass versus electrode potential. They are, therefore, the mass flux analogs of voltammograms, with which they can be productively compared. A Venn diagram is used to identify five different kinds of interfacial reaction which are distinguishable when massograms and voltammograms are recorded at the same time. What distinguishes the reactions is how charge transfer is coupled to mass transfer at the electrode surface. Examples of all five kinds of reaction are identified and discussed. The first section of the Venn diagram corresponds to faradaic processes that are associated with mass changes. Typical examples are electrodeposition, electrodissolution, and intercalation reactions. The second section of the Venn diagram corresponds to faradaic processes that are not associated with mass changes. Typical examples of these are gas evolution reactions. The third section of the Venn diagram corresponds to non-faradaic processes that are not associated with mass changes: capacitive charging is representative. The fourth section of the Venn diagram corresponds to non-faradaic processes that are associated with mass changes. An example of this is the specific adsorption of perchlorate ions on gold. The fifth and final section of the Venn diagram corresponds to mass changes that are associated with neither faradaic nor non-faradaic processes. An example is the Ostwald-like ripening of TCNQ microcrystals on gold.