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

The advantages and disadvantages of electrochemistry with little or no adde d supporting electrolyte are reviewed. Analysis of such systems is complica ted 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 s olute concentration remains uniform and constant. This principle, which app lies to both transient and steady-state regimes, holds independently of the cell and electrode geometries and is not compromised by migration and/or c onvection. The only constraint, additional to the assumption of uniform dif fusivities, is the obvious one: that the electron-transfer reaction, and an y 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. U sing the principle, a framework of procedures is derived which may be follo wed to obtain a solution to many transient voltammetric problems at a macro electrode when supporting electrolyte is absent.