Stability regions of water

It is considered useful to include here the potential-pH diagram for some redox systems related to oxygen (Fig. 2.1) [4]. Lines 11 and 33 correspond to the (a) and (b) dashed lines bounding the stability region of water, as depicted in all the subsequent Pourbaix diagrams. 

Figure 1.9 is the Pourbaix diagram for iron and some of its compounds in an aqueous system at 25°C. The equilibrium potential of the reaction Fe° = Fe2+ + 2e falls outside the stability region of water represented by dashed lines. Hence, measurement of the equilibrium electrode potential is complicated by the solvent undergoing a reduction reaction, while the iron is undergoing electrochemical oxidation. This is the basis of the mixed potential model of corrosion. 

Figure 1 demonstrates the drastic influence on the stability region of a lamellar liquid crystalline phase when an aromatic hydrocarbon is substituted by an aliphatic one. The lamellar phase formed by water and emulsifier is stable between 20 and 60 wt % water. Addition of an aromatic hydrocarbon (p-xylene) to the liquid crystalline phase increased the maximum amount of water from 45 to 85% (w/w) (Figure 1 left). Inclusion of an aliphatic hydrocarbon (n-hexadecane) gave the opposite result the maximum water content in the liquid crystalline state was reduced (right). Some of the factors which govern the association behavior of these surfactants and cause effects such as the one above are treated below. 

Mobility can also be severely affected by the redox environment. For example, Fe(II) and Mn(II) species are ordinarily soluble in natural waters deficient in oxygen, but their oxidized forms precipitate quite easily. The stability regions of insoluble Mn oxides can be seen in the Pourbaix diagram depicted in Figure 6.8. 

FIGURE 9.3-2 Potential-pH diagram showing region of water stability. (Reproduced from Poutbaix.1)... 

To develop the diagram, we first enter in Fig. 7-7 the stability limits of water that were established for Fig. 7-4. Next we refer to Table 7-3 that presents equations that interrelate all species and solids. We would find, from a detailed study of the diagram construction, that only a few of these equations describe significant boundaries moreover, these equations apply only in specific regions of the diagram. 

Most corrosion reactions of interest take place in the presence of water, and the area of the diagram in which water is present is indicated. This region, the stability field of water, is defined as the range of pH... 

The potential pH-diagram of magnesium is shown in Fig. 29. The region of stability of magnesium is below the stability line of water (a). Therefore, magnesium... 

The complexity of liquid water is continued into its solid phase also. Experimentalists and theoreticians have ohtained/predicted many different forms of ice at different temperatures and pressures [2]. The phase diagram displaying the stability regions of different forms of ice is shown in Figure 20.1. 

Wennerstrom (1981) and their calculation of stability regions of the different phases in soap-water systems and in the aerosol OT-water system are in good agreement with the experimental observations. A few additional aspects of structural relationships between phases will be considered here. 

In 1958, van der Waals and Platteenw developed a statistical mechanical theory for predicting the stability region of the clathrate hydrate phase with different guest molecules under different temperature and pressure conditions. The van der Waals-Platteeuw (vdWP) theory is based on the thermodynamic condition of equilibrium between a hydrate phase water/ice ()8) phase and i guest species encapsulated in the hydrate at the phase boundary ... 

Rhodium, Rh, like other noble metals, forms surface oxides upon anodic polarization even in the region of water stability, thus below the thermodynamic reversible potential of the oxygen evolution reaction, = 1.23 V, SHE 1-20). In aqueous H2SO4 solution, the oxide growth on Rh commences at 0.55 V, RHE (reversible hydrogen electrode), and up to ca. 1.40 V, RHE, a complete monolayer (ML) of Rh(OH)3 is formed as revealed by coupled CV and XPS measurements (27). 

Figure 4-4. Pourbaix diagram for the Mg-H20 system at 25 C (Makar and Kruger, 1993). The region of water stability lies between the lines marked (a) and (b). Circled numbers identify the reactions that separate the different regions of the diagram as follows ...

I in. 2-.. Pourbaix (Eh vs pH) diagram for Np (top) and Pu (bottom) in water that contains hydroxide, carbonate, fluoride ions as the main coordinating ligands. The shaded areas represent stability regions of solid phases that precipitate from a 10 mol L Np or Pu. solution. Solid lines represent the stability (lelds for solution. sivcics. The oxidation state stabilities are cnclo.sed by the area of water. stability above the top line, water is oxidized to oxygen and below the bottom line, water is reduced to hytlrogen. 

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