Studies In The Acceleration Effect Of Thiourea On The Reduction Of Zn(ii) In Mixed Water-organic Solvents At The Mercury Electrode
STUDIES IN THE ACCELERATION EFFECT OF THIOUREA ON THE REDUCTION OF Zn(II) IN MIXED WATER-ORGANIC SOLVENTS AT THE MERCURY ELECTRODE
B. Marczewska
Faculty of Chemistry, Maria Curie Skłodowska University,
PL 20-031 Lublin, Poland
The phenomenon of the acceleration of electrode processes in aqueous solutions on the mercury electrode was first noted by Heyrovsky in 1947. In 1978 Sykut et al. defined the general conditions of the occurrence of this, later on called the “cap-pair” rule.
The acceleration mechanism was partly explained yet it still remained unknown which of the two processes – adsorption or complex formation – has the dominant influence on changing the rate of the electrode process. In order to estimate this influence it seems proper to analyse the course of the acceleration process in solutions in which the structure of the active complex changes depending on the composition of the solution. This condition is fulfilled by mixed water-organic and organic solutions.
The aim of the study was an examination of the mechanism of accelerating the electroreduction of zinc ion on the mercury electrode by thiourea (TU).
Methods. Zn (II) has been selected as a depolarizer, TU – as the accelerating agent, while water-organic solvents contained acetone (AC), MeOH, EtOH, DMSO, DMF, of varied solvating properties in respect to the Zn (II) ion. The studies involved: 1. Determination of adsorption (differential capacity of the double layer; zero charge potential measurement; electrocapillary measurements; measurements of surface tension at zero charge potentials); 2. Kinetic studies (dc polarography; differential pulse polarography; cyclic chronovoltamperometry; faradaic impedance).
Summary of results:
1. The acceleration of the process of the electroreduction of Zn (II) ions by TU in mixed water-organic solutions has been demonstrated thus broadening the scope of the applicability of the cap-pair rule.
2. On the basis of determined values of surface excesses of TU it was possible to ascertain the adsorption of the accelerating substance at Zn (II) reduction potential in all the examined systems: H2O+MeOH, H2O+EtOH, H2O+AC, H2O+DMSO, H2O+DMF.
3. By applying water-organic solvents, it has been shown that in the acceleration of the process of Zn (II) electroreduction the dominant role is played by the composition of the active complex being reduced on the electrode surface.
4. It has been found that the overall rate of the electrode process is determined by the rate of the first electron transfer which rises in the presence of the accelerating substance, at the same time having no influence on the rate constant of the second electron.
5. A decrease in the value of the activation energy of the first electron transfer and lack of effect on the value of the activation energy of the second electron transfer.
6. A mechanism of the acceleration of the process of Zn(II) ion electroreduction on the mercury electrode in mixed water-organic solvents has been proposed; the mechanism involves the following stages:
– Displacement of solvent molecules from the electrode surface by the accelerating substance being adsorbed.
– Formation of the active complex is consequence of the replacement of solvent molecules from the first solvation shell of the cation by adsorbed molecules of the accelerating substance.
– Transfer of the first electron whose rate increases in the presence of the accelerating substance.
– Desolvation of the ion from the molecules of the co-ordination sphere (molecules of water, DMF or DMSO) and the second electron transfer.
– Formation of zinc amalgamate.
The presented results seems to contribute to our better knowledge of the mechanisms of electrode processes.
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