engleski

Δ Ep vs. pH Plot as a Source of Information in Polarographic Studies of Kinetically Labile Metal Complexes at a Constant Ligand to Metal Ratio

Taking into account that during polarographic measurements at a constant ligand to metal ratio and gradually increasing pH value, the slope of Δ Ep vs. pH plot is sometimes used for identification of the major complex species in the solution, characteristics of the mentioned relationship were studied in more detail. Assuming reversibility of the electrode process, kinetic lability of all the complexes and absence of possible complications, such as surface activity of dissolved species, it was demonstrated (by model calculations) that the shape, slope(s) and position of Δ Ep vs. pH plot are highly affected by different factors. Even in very simple examples such as systems of HL - ML type numerous limitations (concerning the acceptable values of β 1[L]t product, relative concentrations of free ligand and metal etc.) appear which can sometimes completely prevent the successful application of this diagnostic method for identification of the major complex species because experimental results obtained under usual laboratory conditions do not meet the criteria. If, however, the mentioned limitations are ignored completely erroneous results could be obtained. In more complex systems, containing different species of HnL, MLp and MHL types together with OH- ion as a competing ligand, (with or without formation of ternary complexes) additional factors such as relationship between stability constants of successive complexes or of the two series of coordination species (MLp and M(OH)r)) become important and define the final characteristics of Δ Ep vs. pH plot i.e. the information that can be extracted from it.

Polarography; Δ Ep vs. pH plot; Complex species; HL-ML; H(2)L-ML-ML2; HnL-ML systems

nije evidentirano