Potential pH, dependence

The selective net loss of a component such as zinc, aluminium or nickel from copper-base alloys sometimes occurs when these alloys corrode. Early studies of the phenomenon were done by simple immersion. More recently, however, the potential-pH dependence of de-alloying has been examined , and it appears that this approach can provide a much more detailed understanding of the mechanism. Future experimental work is expected to include potentiostatic and potentiodynamic techniques to a much greater extent. 

The pH at which the line with a -60 mV/pH slope intersects that with 0 m V/pH slope represents the pK value of reduced Qa. which, as noted, is 9.6. A similar midpoint potential-pH dependence has been reported for Chromatium vinosum, Rs. rubrum and Rp. viridis, with pK values of 8.0, 8.8 and 7.8 respectively. Interestingly, the equilibrium midpoint potentials for the chromatophores of these four different photosynthetic bacteria estimated from the point of intersection ofthe lines of 60 and 0 mV/pH slopes are rather similar, lying within a narrow 50 mV range centered at -175 mV. 

Obviously the potential/pH dependence here will be zero (on the RHE scale) as the numbers of electrons and OH ions in the overall redox reaction are equal. 

To confirm the hypothesis of the presence of BSA on the pore wall, measurements of the SP through the pores of both the ZIOOS and ZIOOS+BSA membranes were performed with a 0.001 M NaCl solution at different pHs (de Lara and Benavente 2009) Figure 2.12b shows a comparison of the results obtained with both membranes. Positive -potential values were obtained for the ZIOOS membrane in agreement with the electropositive character of both AI2O3 and Zr02, but the -potential/ pH dependence found for the ZlOOS+BSA-fouled membrane is completely different, with a zero -potential value at a pH of 5, that is, the isoelectric point obtained for the fouled membrane hardly differs from that for the protein (BSA isoelectric point -4.9 Nystrom and Jarvinen 1991), which is a clear indication of protein deposition on the membrane pore wall and a confirmation of the EIS results. These results further confirm the use of EIS, a nondestructive technique, to study membrane modifications caused by fouling. 

All these reactions are potential and/or pH dependent, so they can occur or not according to the actual conditions in, or close to, the pit embryo. This makes rather complex the interpretation of the pitting potential pH dependence when sulfides act as pitting sites (see below). Furthermore, they do not explicitly take into account the effect of chloride ions for the dissolution of Mn + they should then be slightly modified for this purpose, introducing the solution chloride content as the third critical parameter for the MnS dissolution kinetics. 

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