VOLTAMMETRIC INVESTIGATION OF THE COMPLEXATION EQUILIBRIA IN THE PRESENCE OF A LOW-LEVEL OF SUPPORTING ELECTROLYTE .1. STEADY-STATE CURRENT-POTENTIAL CURVES FOR INERT COMPLEXES

The use of microelectrodes for voltammetric investigations of the comp lexation equilibria at very low concentrations of supporting electroly te allows the risk of competitive complexation or contamination to be avoided, makes the activities of the species involved closer to their concentrations (which facilitates comparisons with the spectroscopic r esults) and finally, allows the concentrations of the species to be va ried over a broader range. This paper presents the calculations of the steady state currents for a wide range of complexes that are inert on the experimental time scale, and reports the influence of the concent ration of the electroinactive ionic species on the limiting currents. Also, for a number of cases the variation of halfwave potential with t he ligand concentration, resulting from changes in the ohmic drop, is given. It is assumed that only one species (the complex or the uncompl exed form) is electroactive; if this is the complex, it may or may not change the number of ligands. The theoretical results were obtained e ither employing the Myland-Oldham theory extended in this paper or by digital simulation. The results of calculations show that the magnitud e of the changes in the steady-state limiting current on complexation depends on the type of complexation equilibrium, the type of the chang e in the reactant charge number in the electrode process, and the comp lex formation constant. In a number of situations migrational effects are negligibly small and no special treatment is necessary, despite th e lack of supporting electrolyte. In other cases, where migration is s ignificant, the relations between the measured steady-state limiting c urrent and the complex formation constant beta are given in the form o f fitted equations that can be used to obtain beta from appropriate ex perimental data.