Principle of unchanging total concentration and its implications for modeling unsupported transient voltammetry

The advantages and disadvantages of electrochemistry with little or no added supporting electrolyte are reviewed. Analysis of such systems is complicated by the increased importance of the migrational component of transport. Some computations and analyses can be simplified by invoking the principle that for any electrochemical experiment where the diffusivities of all the solute species do not differ significantly from a common value, the total solute concentration remains uniform and constant. This principle, which applies to both transient and steady-state regimes, holds independently of the cell and electrode geometries and is not compromised by migration and/or convection. The only constraint, additional to the assumption of uniform diffusivities, is the obvious one: that the electron-transfer reaction, and any homogeneous reactions that may participate, do not themselves perturb the number of solute particles (molecules and ions) present. Some consequences of the principle are explored, especially relating to three-ion systems. Using the principle, a framework of procedures is derived which may be followed to obtain a solution to many transient voltammetric problems at a macroelectrode when supporting electrolyte is absent.