Study of complexation equilibria using polarized metallic electrodes

Abstract

The response of a metallic electrode depends on its chemical nature and on the characteristics of the environment. The response time and the repeatability of the measurements depend on the previous history of the electrode and are determined by its superficial structure. In a very general sense, it can be stated that a metallic electrode seldom acquires the potential predicted by the Nernst equation, because the equilibrium M⁰⇌ M^z++ ze^– is not easily established at the electrode surface. Here, the response of an anodically polarized electrode was studied in relation to the response time and the repeatability of the measurements; the electrode was then employed to determine stability constants by a procedure described in the literature. In order to be suitable for analytical purposes, an electrode must give fast and reproducible measurements. By polarizing a metallic electrode with low anodic current intensities a fast response time and good repeatability were achieved. The response time was less than 10 s. This procedure enables base metal electrodes to be employed in situations where platinum, mercury or gold electrodes are preferred. In order to determine stability constants, a metallic electrode was chosen (according to the cation of which the complexes were to be studied) and was polarized anodically with different current intensities. The potential of the electrode was recorded for increasing ligand concentrations. This particular technique has some advantages with respect to the most common methodologies currently known. The problem of the dependence of the shape and the position of the polarization curves on the stability constants of the complex ions and the influence of the electrode surface in voltammetric techniques is solved by working at low anodic current intensities and by taking measurements when the electrochemical equilibrium has been reached. The equilibrium M⁰⇌ M^z++ ze^– is readily achieved at the electrode surface when the electrode is polarized; this represents a considerable advance with respect to potentiometric techniques in general. The total cation concentration does not change during the determination, and depends on the magnitude of the polarizing current. The free ligand concentration virtually coincides with the total ligand concentration because the concentration of cation generated at the electrode/solution interface is about 1 × 10^–5 mol dm^–3.