engleski

Adsorption of PbBr2 complex on mercury electrodes

Adsorption of PbBr2 complex to the surface of mercury electrode was measured by the d.c. polarography. Polarograms of Pb(II) were recorded at various bulk concentrations of bromide ions and the relationship between the adsorption parameters and the surface concentration of the adsorbed bromide ions was investigated in order to define the proper adsorption mechanism. The theoretical model was developed and the adsorption parameters were calculated by fitting the experimental data. The d.c. polarogram of Pb(II) in bromide medium consisted of the main wave and the postwave, which were separated by the minimum. The main wave is a consequence of the reduction of Pb(II) at the electrode surface totally covered by the adsorbed complex. Its relative height followed the distribution of the PbBr2 complex in the bulk of the solution. This relationship identified the complex PbBr2 as the only surface active complex species. Theoretically it was shown that the minimum separating diffusion and adsorption waves indicates the existence of strong lateral attractions in the adsorbed layer. Hence, the adsorption of PbBr2 follows Frumkin isotherm. This process can be regarded either as direct adsorption, without participation of adsorbed bromide ions, or as the surface complexation mechanism during which additional coordination bonds are formed between the neutral complex and the initially adsorbed bromide ions. Assuming the first mechanism, in the presence of 3.872 M NaClO4, 0.117 M NaBr, 10-2 M HClO4 and 1 ´ 10-3 M Pb(II), the adsorption parameters are: Gmax = 1.35 ´ 10-9 mol cm-2, b = 6.6 ´ 103 M-1 and a = -3.9 (or g = -3.6 ´ 1012 J cm2 mol-2). However, it was noticed that the maximum surface concentration of the adsorbed PbBr2 was linearly correlated to the surface concentration of initially adsorbed bromide ions, which is the main difference between two possible adsorption mechanisms. Hence, it is most probable that the adsorption of PbBr2 occurs according to the surface complexation mechanism with strong lateral attractions in the adsorbed layer. The surface complexation constant is l = (7.5 ą 1.5) ´ 103 M-1, Frumkin coefficient is approximately g = -(4.1 ą 0.7) ´ 1012 J cm2 mol-2, and the maximum coordination factor is f-1 = 5.0 ą 0.5. The following reaction scheme is proposed: x PbBr2 + (Br-)ads " ((PbBr2)xBr-)ads where 5 ł x. This is a modification of the original concept. The proposed mechanism implies that the lowering of the surface coverage means a decrease of the average number of PbBr2 molecules which are bound to a single, adsorbed bromide ion. The results show that PbBr2 can be attached only to the initially adsorbed bromide ion and not anywhere else at the electrode surface. In other words, each initially adsorbed bromide ion serves as a center of crystallization for a maximum of five molecules of PbBr2. The model can be applied generally to all reversible redox reactions complicated by the reactant adsorption following Langmuir and Frumkin isotherms. A d.c. polarogram using a static mercury drop electrode may consist of a main wave and a postwave which are separated by a minimum. The minimum is a part of the postwave. It appears in the potential range in which the maximum surface coverage is diminished below 0.8. It is the consequence of the change of current-time relationship during the life-time of a single drop.

Adsorption; PbBr2 complex; mercury electrodes

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