engleski

Voltammetry as a method of choice in the study of anion induced adsorption of metal ions

In the early seventies, anion induced adsorption of metal ions to the mercury electrodes was mainly studied by double potential step chronocoulometry. Moreover, it could be said that systems exhibiting such behaviour, especially cadmium(2+) in iodide solutions were important models in development of this method, i.e. its theory and practice. In polarography, however, effects produced by adsorption of metal ions from solutions of surface-active ligands were treated as problems in determination of stability constants, but not used as diagnostic criteria. By development of electrochemical instrumentation, theory of the corresponding electrode reactions and signal processing, things have changed. Nowadays, from pure voltammetric data it is possible to draw the conclusions about composition of the surface-active complex, its interaction with previously adsorbed free ligand ions, attractions and repulsions at the electrode surface etc. It could be said that sensitivity of individual techniques to the process of interest increases in order DC<DPP<NPP<SWV Cadmium adsorption from bromide media for example (at the ionic strength I=1 mol/L and metal concentration of 5ˇ10-5 mol/L) could be detected only by square wave voltammetry. Relatively high sensitivity of normal pulse polarography, in which adsorption maximum appear, could be confirmed only with very short pulses. It is because the effects of interest, generally become more pronounced as the technique becomes faster. In other words the increased speed of the method has the same consequences as the increased adsorption constant. Experimental results are additionally influenced by the concentrations of dissolved metal, ligand and total electrolyte. It is useful to combine measurements (in a wide ligand concentration range) at two different metal levels: a lower at which linear adsorption is nearly valid and a higher at which electrode surface is completely covered by adsorbed complex. In the later case reduction from dissolved state produces additional signal or change in the shape of the existing peak which should be additionally processed. Results obtained at the lower metal concentration could be correlated with its distribution in the solution whereas at the higher level they mainly reflect the interactions at the electrode surface. Voltammetric results are generally in good agreement with conclusions arising not only from double potential step chronocoulometry but also from other electrochemical and nonelectrochemical techniques. At the present state of the art it seems that more informations could be obtained from voltammetric than from chronocoulometric results. On the other side calculations of the surface concentration (gamma) are more straightforward in the later case what means that both techniques should be combine in real studies. With modern instruments it is not a problem because they offer both of them.

adsorption

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