Electrochemical studies of thallium and phosphate electrodes
Abstract (Summary)Restricted Item. Print thesis available in the University of Auckland Library or available through Inter-Library Loan. Part 1 The behaviour of the polycrystalline thallium anode in aqueous perchlorate, chloride, bromide and iodide solutions was investigated at 25°C. In 1M perchlorate, the metal dissolved directly to form Tl+(aq). Steady-state current/potential curves were measured at a rotating disk electrode. By extrapolation to high rotation rates, it was possible to eliminate the effect of mass transport and the reverse reaction (reduction of Tl+(aq)), and the kinetics of the pure anodic process could be determined. It was found that the anodic Tafel slope, ba = ?E/?logia = 59 ± 8mV.The kinetic results do not support a rate determining charge transfer step, but are consistent with rate determining desorption of the Tl+ intermediate Tl = Tl+(ads) + e-, charge transfer Tl+(ads) = Tl+(aq), r d s Addition of ethylenediaminetetra-acetic acid at pH 9.2 was found to accelerate the dissolution process, presumably by complexing Tl+(aq) at the electrode surface. The electrocrystallization of TlCl, TlBr and TlI films on polycrystalline Tl, Tl + X-(aq) = TlX(s) + e-, X = Cl, Br and I in lM HCl, KCl, KBr and KI was studied using cyclic voltammetry and galvanostatic techniques. It was found that thallium is passivated by surface films; the amount of film formed depending on the polarising conditions and the halide ion. Typical film thickness of 6.6, 1.2 and 1.0 nm were found for TlCl, TlBr and TlI respectively, under the same polarisation conditions. A simple model for passivation is presented, based on nucleation and spreading of patches of film. Simultaneous spreading and thickening of halide nuclei occurred. Vertical growth was favoured by slow potential sweep rates or small applied currents and by the lower stability of solid TlCl. The thickness of the continuous layer increased linearly with the applied electric field and the rate constant for film growth was closely similar for all three cases; TlCl, TlBr and TlI. A mechanism for film growth involving field assisted anion vacancy transfer through the film was successful in accounting for the results. Data derived from single crystal conductance studies on TlCl gave quantitative agreement with the results. When thallium halide films are reduced, the reduction reaction spreads rapidly to the film/solution interface before the bulk of the film is reduced. Part 2 Insoluble inorganic phosphates were examined in the form of a membrane electrodes for the response of potential to changes in phosphate activity. In recent years there has been reports of this occurring. BiPO4, BiPO4 doped with Ca2+, (UO2)3(PO4)2, UO2HPO4, LaPO4, Ag3PO4 and Ag3PO4/Ag2S were all examined but found to be sensitive to cations. The potential response of BiPO4 to changes in mixtures of Na+ and H+, and Na+ and K+ was examined more closely, and found to be reasonably predicted by the equation. E = E° + (nRT/F) ln (a1l/n + K12l/n a2l/n) Where K12 is the selectivity constant for ion 2 over ion 1, and n is a constant. For example, when ion 1 is Na+ and ion 2 is H+, n = 1.4 and K12 = 4 (i.e. the electrode is more sensitive to H+). The overall results demonstrate that it is not possible to make a phosphate sensitive membrane electrode. Reinterpretation of previous results and a literature survey correlate these findings.
School Location:New Zealand
Source Type:Master's Thesis
Date of Publication:01/01/1974