Voltammetry with cathode stripping is widely used for determination of a number of heavy metals in various objects of the environment. This paper is focused towards the application of methods developed within the study of electrochemical behavior of heavy metals (Cu, Cd, Pb V) on the mercury electrode in the presence of a series of complexing agents. Using the adsorptive cathodic stripping voltammetry for dosage of V (V), Cu (II), Pb (II) and Cd (II), 2,3-dihydroxy-benzaldehide (2,3-DHBA), thiosemicarbazide diacetic acid (TSCDA), the reagents semicarbazone 8 - quinolinaldehyde (SCQA) and 4-phenyl-thiosemicarbazide (4- FTSC) respectively have served as agents of accumulation. Metals form complex compounds with those reagents that are adsorbed on the dropping and stationary mercury electrodes. In the case of Cu (II) the simplest concentration process of Mn+ can be expressed by the equations (1) and (2), for Pb the process according to scheme 3 is assumed, for vanadium and cadmium a process of stepwise complexation is observed: pMn+ + qL- MpLq pn-q (sol) (1) Reaction (1) occurs in solution and is considered as a chemical stage (R) of the process. The formed complex are adsorbed on the electrode: MpLq pn-q (sol) MpLq pn-q (ads) (2) Reaction (2) takes place on the electrode and has an adsorption nature (A). The overall scheme of the process can be depicted as R → A. The mechanism consists in adsorption of L and it is an adsorption stage (A): qL- (sol) qL- (ads) (3). At the second stage (R), the adsorbed ligand reacts with metal ions Mn+: pMn+ + qL- (ads) MpLq pn-q (ads) (4). Contrasting the reaction (1), the process (4) occurs on the electrode surface, but leads to formation of the complex with the same composition. The general scheme of the process can be depicted as A → R. The adsorptive nature of the recorded reduction peaks has been proved by the speed coefficients and the variation of dropping time of capillary in function of the potential applied to electrodes in solutions with separated and mixed components. The mixing ratio Me:L in solution as well as the species that is reduced on the electrode surface, have been determined by amperometric titration. For each metal separately the effect of pH and the impact of other species Zn(II), Fe(III), Ni(II), Cr(VI), Co(II), CH3COO-, CO3 2-, SO4 2- , Cl-, Br-, I-, CNS-, CH3COO-, CO3 2-, SO4 2-) on the analytical signal intensity have been investigated. The time of accumulation of analyzed species on the suspended mercury drop is dictated by the interference of other species, is dependent on the metal nature and varies within 10-180 s. The developed methods have been applied for determination of Cu (II), Pb (II), V (v) and Cd (II) in waters of the Prut River.
|