Voltammetric investigation of the complexation equilibria in the presence of a low level of supporting electrolyte part 1: Steady-state current-potential curves for inert complexes

Abstract

The use of microelectrodes for voltammetric investigations of the complexation equilibria at very low concentrations of supporting electrolyte 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 results) and finally. allows the concentrations of the species to be varied over a broader range. This paper presents the calculations of the steadystate currents for a wide range of complexes that are inert on the experimental time scale, and reports the influence of the concentration of the electroinactive ionic species on the limiting currents. Also, for a number of cases the variation of halfwave potential with the ligand concentration, resulting from changes in the ohmic drop, is given. It is assumed that only one species (the complex or the uncomplexed form) is electroactive; if this is the complex, it may or may not change the number of ligands. The theoretical results were obtained either employing the Myland-Oldham theory extended in this paper or by digital simulation. The results of calculations show that the magnitude of the changes in the steady-state limiting current on complexation depends on the type of complexation equilibrium, the type of the change in the reactant charge number in the electrode process, and the complex formation constant. In a number of situations migrational effects are negligibly small and no special treatment is necessary, despite the lack of supporting electrolyte. In other cases, where migration is significant, the relations between the measured steady-state limiting current and the complex formation constant :? are given in the form of fitted equations that can be used to obtain p from appropriate experimental data.

Amperometric determination of the water content in acetone, butter and margarine using a wall-tube platinum microelectrode in a flow injection system

“…The number of additions was seven and the correlation coefficient (r) was 0.9984. The standard deviations for the slope and intercept were 0.33 mA (% v/v) 21 and 0.05 mA, respectively. The limit of detection was determined as about 0.003% v/v, based on a signal-to-noise ratio of three.…”

“…In the latter case, the equation was found to be i = 13.4c + 0.11 (n = 6, r = 0.9996). The standard deviation for the slope and intercept were 0.20 mA (% v/v) 21 and 0.04 mA, respectively. For an LiClO 4 concentration of 1.0 3 10 24 m, the range of detection potentials used for water additions in the range 0.05-0.3% v/v was 3.3-4.0 V.…”

“…In these experiments the acetone contained 2.0 3 10 23 m LiClO 4 as supporting electrolyte. The flow rate of the mobile phase, i.e., acetone containing the same amount of LiClO 4 , was 0.1 ml min 21 . Some of the peaks obtained upon injection are shown in Fig.…”

“…2,[7][8][9][10][11][12][13][14][15] Studies performed with microelectrodes in low conductivity media have shown that the migrational mass transport of ionic species can be significant under such experimental conditions. 13,[16][17][18][19][20][21][22][23][24] Migrational effects can, however, also be observed for uncharged species in solutions containing small amounts of added supporting electrolyte since the generation of a charged product requires the build-up of an ionic layer in the vicinity of the electrode. 13,[21][22][23]25 Recently, it has been shown 26 that the positioning of a wall tube microelectrode with respect to the capillary end and also the flow rate, potential and ionic strength influence this build-up of the ionic layer significantly.…”

“…13,[16][17][18][19][20][21][22][23][24] Migrational effects can, however, also be observed for uncharged species in solutions containing small amounts of added supporting electrolyte since the generation of a charged product requires the build-up of an ionic layer in the vicinity of the electrode. 13,[21][22][23]25 Recently, it has been shown 26 that the positioning of a wall tube microelectrode with respect to the capillary end and also the flow rate, potential and ionic strength influence this build-up of the ionic layer significantly. The results of this study also indicated that such a wall-tube based flow injection system can be used for, e.g., determinations of water in various organic solvents and samples dissolved in organic solvents.…”