Potential-pH plots

On the scale at which these plots are usually made, potential-pH plots for the... 

It can be noted that Pourbaix diagrams may be constructed for all elements. The diagrams subdivide the potential-pH plots into r ons of immunity, corrosion or passivation. These are very useful in prediction of tendency of metals to corrode. These diagrams, however, has several limitations which will be summarixed at the end. 

Monolayers at the surface of aqueous electrolyte solutions may be useful models for polyelectrolytes [11, 14] and for biological membranes [15]. The average distribution of the fixed charges, limited to a plane near the interface, is well defined and the variation of the interfacial potential may be measured. Thus the surface potential-pH plots resemble those of acid-base titrations of polyacids [16, 19]. From these curves the degree of dissociation of the fatty acid in the monolayer may be deduced [17, 18]. 

In this chapter, the voltammetric study of local anesthetics (procaine and related compounds) [14—16], antihistamines (doxylamine and related compounds) [17,22], and uncouplers (2,4-dinitrophenol and related compounds) [18] at nitrobenzene (NB]Uwater (W) and 1,2-dichloroethane (DCE)-water (W) interfaces is discussed. Potential step voltammetry (chronoamperometry) or normal pulse voltammetry (NPV) and potential sweep voltammetry or cyclic voltammetry (CV) have been employed. Theoretical equations of the half-wave potential vs. pH diagram are derived and applied to interpret the midpoint potential or half-wave potential vs. pH plots to evaluate physicochemical properties, including the partition coefficients and dissociation constants of the drugs. Voltammetric study of the kinetics of protonation of base (procaine) in aqueous solution is also discussed. Finally, application to structure-activity relationship and mode of action study will be discussed briefly. 

If the inner reference of the electrode is such that the iso-pH value (isotherm intersection point) lies at pH = 7 0.5 (according to a DIN standard), which may agree with the Ege vs. pH plot in Fig. 2.11, then the overall potential of this... 

At unit activities of the oxidant and reductant, the potential depends only on pH the slope of the line for a plot of potential versus pH is governed by the ratio m/n. Potential-pH diagrams are a concise means to display the redox properties of a system. We will take uranium as an example. The +6, +5, +4, and + 3 oxidation states are known in aqueous solution. The determination of +6 uranium by coulometric titration has been investigated by many workers and the lower oxidation states have all been used as coulometric titrants. Hydrolyzed uranium species exist in a noncomplexing solution, but the chemistry is simplified considerably if the discussion is limited to solutions more acidic than about pH 4. Some of the half-reactions to be considered are listed next with E° vs. NHE ... 

This is only possible if the factor k is known. Roy et al. obtained it from the slope of the voltage/pH plot above the inflection point whereas Bard et al. calculated it from the slope of the onset of photocurrent vs. pH (26). However, only poorly reproducible values were obtained by the former and latter method due to high voltage fluctuations and too low photocurrents, respectively. We have determined the k value by a new method through variation of the redox couple (A,2 +/A, + ). In this case a linear relation between the pH0 value and the redox potential of the pH-independent redox couple is expected [Eq. (4)]. 

Potential-pH diagram — Graphical display of electrode potentials (usually on the SHE-scale) vs. -+pH values for a selected element based on thermodynamic calculations. In the plot the chemically stable species are indicated taking into account all conceivable electrochemical as well as chemical reactions. Initially proposed and extensively researched by Pourbaix. 

Figure 45 shows plots of the d-band vacancy against quantity of electricity consumed in the reduction of the oxide layer formed under chlorine evolution conditions. This result shows that coverage of oxide on the electrode surface can be linearly related to the d-band vacancy because the quantity of electricity required for the formation of a monolayer on each metal is approximately 0.5 mC cm . The coverage by oxide is considered to be a function of electrode potential, pH, and concentration of Cr. If these... 

Figure 32. Amperometric response of rotating Cat/NiO modified GC electrode to H202, conditions -0.3 V constant potential, pH 7.0 and rotation speed is 2000 rpm, (A) successive addition of lOOpM and (B ) 1 liM insets plot of chronoamperometric current vs, H202 concentration and linear calibration curve for determination of KM. Reprinted from Biophysical Chemistry, 125, A.Salimi, E. Sharifi, A. NoorBakhash, S. Soltanian, Direct electrochemistry and electrocatalytic activity of catalase immobilized onto electrodeposited nano-scale islands of nickel-oxide, 546, Copyright( 2007), with permission from Elsevier.

It is not obvious to all scientists that the pH reported in (pH) or ao(pH) plots (from which the lEP or PZC is determined) is the equilibrium pH of the dispersion used for the measurements. The following description was found in [208], The authors equilibrated their particles in a solution 1 of pH 1, 1.9, 3, 5, 7, 8, 11, or 13. The particles were then separated from solution 1 and redispersed in pure water. The new dispersion (particles in solution 2) was used to measure the electrophoretic mobility. Obviously, the pH of the solution 2 formed by equilibration of pretreated particles with water was different from the pH of solution 1, and most scientists would have plotted the potential against the pH of solution 2 to... 

Scattered results are reported in [2979], and there are too few data points near the IEP. Also, in [2980,2981], too few data points are available near the IEP to make a reliable estimate. In [2982], at one pH value is reported. In [2983], potentials were measured only at pH 1,4, and 12. The IEP was obtained in [2984] from ESA measurements for two titanias (source or characterization of the powers or experimental details were not reported). The unusually high IEP reported in [2984] may be due to experimental errors the sohd concentration was too low, and the electrolyte background was not subtracted. Among the results from [1726], only the IEP for Nd(Ol I), was used. The other lEPs are based on arbitrary interpolations. The pH reported in the (pH) plot in [208] was not the pH of the dispersion. A home-made apparatus was used in [267], atypical shapes of electrokinetic curves... 

Figure 3 shows the Em vs. pH plot of procaine at the NB/W interface. Procaine gives a single wave in the pH range tested, indicating that the half-wave potentials corresponding to plots (a) and (b) in Fig. 1 [see Eqs. (15) (16)] should lie at such a positive potential that they are not observed within the potential window of the NB/W interface examined and that the E /2 vs. pH curve should correspond to plots (c)-(d) — (e)-(f) in Fig. 1 [see Eq. (21)]. Thus, the pH-independent half-wave potential of procaine corresponding to plots (d) = (e), Fi/2311. was determined to be = 0.266 V. Also, the experimental half-... 

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