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Lead (II) iodide film formation on the lead anode

by NG, Ah Chin

Abstract (Summary)
Restricted Item. Print thesis available in the University of Auckland Library or available through Inter-Library Loan. The kinetics and mechanism of the anodic process on lead metal in aqueous iodide solution at pH 7.0and 22°C were studied. The technique used was cyclic voltammetry, employing a rotating lead disc electrode. The potential of the lead electrode was swept periodically at known sweep rates and the resulting current/potential polarisation curves were obtained. The current density, charge density and potential of the observed process were measured as a function of the independent variables: electrode rotation rate, potential sweep rate, pH and iodide concentration. A monolayer of PbI2 was observed as a distinguishable intermediate stage proceeding bulk phase PbI2 formation. The monolayer charge density was 0.17 ± 0.01 mC/cm2 and it was laid down at an underpotential of 60 ± 3 mV, in contrast to the overpotentials normally observed for electrode processes. The anodic and cathodic current peaks for the formation and reduction of the monolayer occurred at the same potential, and the charge densities were independent of sweep rate, electrode rotation rate, iodide concentration and pH. The underpotential formation was explained in terms of lower surface free energy of the monolayer compound compared with the free lead surface. A model for the structure of the monolayer was postulated consisting of iodide bridges between the lead cations (4 bridges per lead cation) giving a more open structure than crystalline PbI2. A reversible electrochemical adsorption was proposed to explain the experimental results, and assuming a Langmuir isotherm it was possible to quantitatively explain the observed current peaks. On increasing the potential, a porous film of crystalline PbI2 formed. The film passivated the metal, as manifested by a sharp anodic current peak. Examinations of the PbI2 film with the scanning electron microscope showed that the film was porous at all potentials. The PbI2 crystals were formed mainly by a dissolution-precipitation mechanism. The peak current density was proportional to the square root of potential sweep rate. These results were explained by the Mueller-Arvia model in which the electrolyte resistance in the pores is the rate controlling factor. As the film grows, the pores become narrower and when the electrode surface is about 99.8% covered, the electrolyte resistance in the pores becomes very large and consequently the current decreases rapidly. After the anodic peak, the PbI2 film continued to grow and dissolve simultaneously. It is believed that the kinetics of growth in this region was controlled by the diffusion of aqueous iodide ions down the pores. The rate of PbI2 film dissolution was determined and it was found to depend on the square root of the electrode rotation rate and the iodide concentration, but was independent of potential sweep rate. The rate of dissolution at electrode rotation rate of 16.5 Hz is (6.8 ± 0.5) x 10-8 mol/cm2/s in 2.0 mol/l iodide solution. The dissolution rate was mass transport controlled and could be calculated satisfactorily from the Levich theory of the rotating disc electrode.
Bibliographical Information:

Advisor:

School:The University of Auckland / Te Whare Wananga o Tamaki Makaurau

School Location:New Zealand

Source Type:Master's Thesis

Keywords:

ISBN:

Date of Publication:01/01/1976

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