Abstract
AbstractThe effect of pH on the passivity of Sn has been investigated in buffered (phthalate‐borate, and citrate phosphate buffer series), and in unbuffered phosphate solutions, using the cyclic voltammetric technique. Two anodic peaks; the first being more pronounced than the second, and on cathodic peak have been observed in the two buffer series at different scan rates and pH values. The anodic and cathodic peak current densities (Ip) and potentials (Ep) are functions of the scan rate (v) and the pH value. Plots of Ip against v1/2 yield straight lines at each pH value. The effect of pH on Ip for the first anodic peak shows a shallow minimum in the near neutral and slightly alkaline range at all scan rates. Straight lines are also observed between Ep and v1/2, the extrapolation of which to v = 0 gives the spontaneous (no polarization) oxidation or reduction potentials (E′ and E″). The absence of polarization effects in E′ and E″ make them the most suitable values for comparison with thermodynamic data. Therefore, the two straight lines obtained between E′ and E″ on one hand and the pH on the other hand to give an estimate of (i) the slope (dE°/dpH), and (ii) the value of E° at pH ‐ 0 (from intercept). Comparison of the two experimental values with all available thermodynamic data for Sn(II) and Sn(IV) oxides and hydroxides shows that: (i) the first anodic peak represents the formation of Sn(OH)2 from Sn and OH−, (ii) the second anodic peak represents the formation of Sn(OH)4 directly from Sn and OH−, and (iii) the cathodic reduction peak correponds to the reduction of Sn(IV) oxidized species to Sn. The results in citratephosphate buffer have been treated in the same manner. However, the result of (dE°/dpH) and E° at pH = 0 deviate from thermodynamic data, because of the possible participation of other anodic reactions such as the formation of soluble Sn compounds, Sn complexes, and incorporation of anions in the anodic film.
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