E vs. pH diagrams

The coordination chemistry of vanadium is strongly influenced by the oxidizing/reducing properties of the metallic centre, and the chemistry of vanadium ions in aqueous solution is limited to oxidation states +2, +3, +4 and +5, although V2+ can reduce water. Redox potentials are given in Table 1 and an E vs. pH diagram is shown in Figure 1. 

Since graphical displays of physicochemical relationships are convenient to use, much of the data in this paper are represented by e vs. pH diagrams. Redox potential and pH have been chosen as master variables only for convenience this does not mean that e and pH always can be regarded as independent of each other. 

Figure 6. Hypothetical e vs. pH diagram showing how the presence of iron silicate may affect the stability relations in the Fe-Si-C02-H20 system...

Figure 8. Hypothetical e vs. pH diagram showing how the presence of various iron silicates may affect the stability relations in the Fe-S-Si-H20 system. The full lines refer to iron species, heavy dashed lines to the system H2S-HS"-SO/ Thin dashed lines indicate how the presence of iron silicates may decrease the highest pH at which iron sulfides are stable...

It is frequently useful to look at the situation in another way by considering what combinations of potential and pH allow the stable existence of a particular species. This information is most usefully expressed by means of a E-vs.-pH diagram, also known as a Pourbaix diagram. 

Fig. 3.5 Potential vs. pH diagram for the CdTe-H20 system at 25 °C. Solid CdTe is thermodynamically stable over the entire pH range. Consequently, CdTe does not hydrolyze at any H and OH activities of practical interest. In acidic solutions, the only process accompanying cathodic CdTe polarization is hydrogen release. Therefore, in the region of cathode potentials, CdTe is a sufficiently stable electrode material from the electrochemical point of view. The -1.35 V potential is the lowest limit of stabihty. Below this limit, CdTe corrodes in the whole pH range e.g., for pH < 2.8, H2Te vapor is produced at -1.25 V. For pH > 2.8, diteUuride or telluride ions are formed with disintegration of the compound. (With kind permission from Springer Science+Business Media [82])...

In the discussion of E the vs pH diagram for iron in water depicted in Figure 1.70, we noted that, with application of high positive potentials, the system moves into a region of passivity and results in a reduced corrosion rate. The passive film formed should be coherent and insulating to withstand corrosion and mechanical breakdown. Upon formation of the passive state the corrosion rate is reduced. Thus by polarization and applying more positive potentials than corrosion potentials the metal attains passivity and is protected. This is the principle of anodic protection. It is necessary that the potential of passivation be maintained at all times, since deviations outside the range would result in severe corrosion. 

The line labeled (b)in Fig. 4.10 represents the behavior of E vs. pH for Eq. (4.41). The chemical behavior of water across all possible values of potential and pH as shown in Fig. 4.10 is divided into three regions. In the upper region, water can be oxidized to produce oxygen while in the lower region it can be reduced to form hydrogen gas. Water is therefore only thermodynamically stable between lines (a) and (b). It is common practice to superimpose these two lines (a) and (b) on all E-pH diagrams to mark the water stability boundaries. 

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 ... 

There is, however, a shortcut approach based on the potential vs. pH representation of equilibrium potentials. The approach is as follows Suppose the M"+ + ne M reaction does not involve proton transfer. Its equilibrium potential is then independent of pH and can therefore be represented on the potential-pH diagram as a straight line parallel to the pH axis (Fig. 12.10). Next, one considers the electron acceptor A present in the solution in contact with the metal M and calculates the equilibrium potential for its reactions. Suppose it involves a proton transfer as well, i.e., xA+ mH+ + ne yD + zH20. Since this reaction involves both electron and proton... 

Pourbaix diagrams are plots of (reversible) potential vs. pH for elements in pure water. They consist of regions of stability defined by lines as borders. Three types of lines exist on Pourbaix diagrams. Horizontal lines describe reactions that are dependent only on potential (e.g., Fe = Fe2+ + 2e ). Vertical lines describe reactions that are dependent only on pH (e.g., Fe2+ + 20H = Fe(OH)2). Angled lines correspond to reactions that depend on both potential and pH (e.g., 02 + 4H+ + 4e = 2H20). 

The principle of the cathodic protection may be elucidated for the case of carbon steel. The Pourbaix diagram for iron in water consisting of the plot E (potential) vs pH is shown in Figure 1.68. The regions of passivity, immunity and corrosion are seen in the figure. 

To draw these diagrams, one needs the E[J values. These are listed in data books or may be determined by the standard potentials for each of the components of the reactions using Eq. 7.16. The dissolution and chemical reaction equations are then written for aU possible oxidation states with suitable values of x as a function of pH. Using dilferent values of (M +(aq))/ MO t), one can then calculate Ejj and plot the results vs. pH. 

Oxidized nitrogen forms anions nitrous and nitric acids, i.e., nitrates NOj and nitrites NO2. As diagram Eh vs. pH shows, the nitrate form of nitrogen is positioned along the upper limit of H O stability. The equilibrium between aquaphilic NOj and may be evaluated from a reaction ... 

Figure 4.17 E-pH diagram of iron with the cathodic protection criterion at -053 V vs. SHE (-0.85 V vs. CCSRE).

The region of immunity [Fig. 1.15 (bottom)] illustrates how corrosion may be controlled by lowering the potential of the metal, and this zone provides the thermodynamic explanation of the important practical method of cathodic protection (Section 11.1). In the case of iron in near-neutral solutions the potential E = —0-62 V for immunity corresponds approximately with the practical criterion adopted for cathodically protecting the metal in most environments, i.e. —0-52 to —0-62V (vs. S.H.E.). It should be observed, however, that the diagram provides no information on the rate of charge transfer (the current) required to depress the potential into the region of immunity, which is the same (< —0-62 V) at all values of pH below 9-8. Consideration of curve//for the Hj/HjO equilibrium shows that as the pH... 

An optimization procedure for the separation of epinephrine bitartrate, L-DOPA, 3,4-dihydroxyphenylacetic acid, notepinephrine-HCl and dopamine-HCl (with 3,4-dihydroxybenzylamine-HCl as internal standard), was described by He et al. [1075]. A C,8 column was used in conjunction with an electrochemical detector (-1-0.6 V vs, Ag/AgCl). A window diagram of relative retention times for adjacent eluting solute pairs (i.e., lR2/tRi) resulted in three acceptable solvent composition windows. The optimal solvent conditions were found to be 2.5/97.5 acetonitrile/water (0.23% sodium acetate with 0.02% EDTA and 0.066% sodium heptanesulfonate adjusted to pH 3.9 with monochloroacetic acid). Elution was complete in <30 min and all peaks were well resolved. Detection limits for dog or human plasma samples were reported as 10pg/mL for epinephrine and norepinephrine. 

Pourbaix Diagrams. Plots of equilibrium pH vs electrochemical potential E describe the effects of aqueous corrosion on borosilicate and silicate glasses. They are applicable to weathering studies and to ground water attack on nuclear waste glasses. The diagrams display any immune zone between active corrosion... 

The second example shows the carbon steel polarization behavior when exposed to a deaerated solution maintained at 25°C and pH of five. The mixed potential diagram of this system is shown in Fig. 5.15. The shift of the E to a more negative value of -0.368 V vs SHE should be noted. The modeled projected lines provide an estimate of the corrosion current density of 4 pA cm in this case and this current translates into a penetration rate of 0.05 mm... 

---