Water, stability field

The theoretical stability field of water in Eh and pH terms (see Fig. 11.2) bounds all theoretical redox reactions taking place in water. The upper boundary of the water stability field is defined by Eh and pH values for which liquid water is in equilibrium with 02(g) at 1 bar pressure. The lower boundary is defined by Eh and pH values for which liquid water is in equilibrium with H2(g) at 1 bar pressure. 

Figure 9.28 Pourbaix diagram for the AI-H2O system within the water stability field, for Question 9.24...

First, let us examine the completed Eh-pH diagram for Mn-HjO-Oj in Figure 12.7. There are typically four different types of boundaries shown on these diagrams. The top line, labeled O2/H2O, represents conditions for water in equilibrium with Oj gas at 1 atm. Above this line, a Pq greater than 1 atm is required for water to exist, so that because the diagram is drawn for a pressure of 1 atm, water is not stable above this line. Similarly, the bottom line H2O/H2 represents conditions for water in equilibrium with H2 gas at 1 atm. Below this line, Phj values greater than 1 atm are required for water to exist, that is, at 1 atm water is not stable. Therefore, the water stability field is between these two lines. 

Chromium, and similar transition metals such as vanadium and molybdenum, have high order oxidation states that form oxyanions and so can be extremely mobile in oxidizing conditions. Such oxyanions are soluble over a wide pH range (Fig. Ic shows general mobility fields), and are relatively weakly sorbed to solids. Chromium, vanadium and molybdenum are less mobile in reducing conditions as their lower oxidation states tend to form low-solubility oxides and hydroxides. Arsenic (a metalloid in Group V of the periodic table) also forms soluble oxyanions over most of the water stability field. However, it differs in that the (-1-3) oxidation... 

There has been much activity in this field of corrosion inhibition in recent years which appears to have been prompted by health and safety requirements. As with engine coolants, the use of nitrites, particularly where amines may also be present, needs to be considered carefully. Nitrites have been widely used in cutting, grinding, penetrating, drawing and hydraulic oils. Suggested replacements for nitrites and/or amines make use, inter alia, of various borate compounds, e.g. monoalkanolamide borates. Molybdates have also been proposed in conjunction with other inhibitors, e.g. carbox-ylates, phosphates, etc . Water-based metalworking fluids usually contain other additives in addition to corrosion inhibitors, e.g. for hard-water stability, anti-foam, bactericidal proderties and so on. Thus, claims are made for oil-in-water emulsions with bactericidal and anti-corrosion properties. 

Gangue minerals and salinity give constraints on the pH range. The thermochemical stability field of adularia, sericite and kaolinite depends on temperature, ionic strength, pH and potassium ion concentration of the aqueous phase. The potassium ion concentration is estimated from the empirical relation of Na+/K+ obtained from analyses of geothermal waters (White, 1965 Ellis, 1969 Fournier and Truesdell, 1973), experimental data on rock-water interactions (e.g., Mottl and Holland, 1978) and assuming that salinity of inclusion fluids is equal to ffZNa+ -h m + in which m is molal concentration. From these data potassium ion concentration was assumed to be 0.1 and 0.2 mol/kg H2O for 200°C and 250°C. 

Fig. 12.2. Redox-pH diagram for the Fe-S-H20 system at 100 °C, showing speciation of sulfur (dashed line) and the stability fields of iron minerals (solid lines). Diagram is drawn assuming sulfur and iron species activities, respectively, of 10-3 and 10-4. Broken line at bottom of diagram is the water stability limit at 100 atm total pressure. At pH 4, there are two oxidation states (points A and B) in equilibrium with pyrite under these conditions.

Uddin et al. (2008b) conducted several depressurization simulations for the Mallik 5L-38 well. Their results showed that the Mallik gas hydrate layer with its underlying aquifer could yield significant amounts of gas originating entirely from gas hydrates, the volumes of which increased with the production rate. However, large amounts of water were also produced. Sensitivity studies indicated that the methane release from the hydrate accumulations increased with the decomposition surface area, the initial hydrate stability field (P-T conditions), and the thermal conductivity of the formation. Methane production appears to be less sensitive to the specific heat of the rock and of the gas hydrate. 

The main usefulness of Eh-pH diagrams consists in the immediacy of qualitative information about the effects of redox and acid-base properties of the system on elemental solubility. Concerning, for instance, cerium, figure 8.20 immediately shows that, within the stability field of water, delimited upward by oxidation boundary curve o and downward by reduction boundary curve r, the element (in the absence of other anionic ligands besides OH groups) is present in solution mainly as trivalent cerium Ce and as soluble tetravalent hydroxide Ce(OH)2. It is also evident that, with increasing pH, cerium precipitates as trivalent hydroxide Ce(OH)3. 

Figure 8.21C shows the Eh-pH diagram for phosphorus at a solute total molality of 10 ". Within the stability field of water, phosphorus occurs as orthophos-phoric acid H3PO4 and its ionization products. The predominance limits are dictated by the acidity of the solution and do not depend on redox conditions. 

Figure 8.22A shows the Eh-pH diagram of iron in the Fe-O-H system at T = 25 °C and P = 1 bar. The diagram is relatively simple the limits of predominance are drawn for a solute total molality of 10 . Within the stability field of water, iron is present in the valence states 1+ and 3-I-. In figure 8.22A, it is assumed that the condensed forms are simply hematite Fe203 and magnetite Fe304. Actually, in the 3-1- valence state, metastable ferric hydroxide Fe(OH)3 and metastable goe-thite FeOOH may also form, and, in the 1+ valence state, ferrous hydroxide Fe(OH)2 may form. It is also assumed that the trivalent solute ion is simply Fe ", whereas, in fact, various aqueous ferric complexes may nucleate [i.e., Fe(OH), Fe(OH)2+, etc.]. 

Adding sulfur to the system (molality of solutes = 10 figure 8.22C), a wide stability field opens for pyrite FeS2 in reducing conditions and, almost at the lower stability limit of water, a limited field of pyrrhotite FeS is observed. 

Figure 8.23A shows a simplified Eh-pH diagram for the Mn-O-H system. Within the stability field of water, manganese occurs in three valence states (2+, 3 +, and 4+). Figure 8.23A shows the condensed phases relative to the three valence states as the hydroxide pyrochroite Mn(OH)2 (2+), multiple oxide haus-mannite Mu304 (2+, 3 + ), sesquioxide Mu203 (3 + ), and oxide pyrolusite Mn02 (4+). 

More recently, extension of experiments to high T has allowed the derivation of empirical equations valid over the entire stability field of water. The equation of Potter and Clynne (1978) for noble gases, for instance, is valid in the T range 298 < r (K) < 647 ... 

The four samples from each location were (1) nonfiltered, nonstabilized water for alkalinity testing, (2) field-filtered, nonstabilized water for nitrate and chloride testing, (3) field-filtered water stabilized with nitric acid for calcium, magnesium, and sodium testing, and (4) field-filtered water stabilized with sulfuric acid for ammonia and phosphate testing. 

Figure 8.2 depicts the stability fields of goethite and hematite as a function of temperature and water pressure using data from several sources. The graph shows clearly that as the temperature increases, the stability field for hematite widens (see also Chap. 14). The goethite stability field broadens as Pnp increases. At PH2O = 0, the equilibrium temperature is 100 °C and rises to 300 °C at PH2O = 2 MPa. 

The ranges of Eh and pH over which a particular chemical species is thermodynamically expected to be dominant in a given aqueous system can be displayed graphically as stability fields in a Pourbaix diagram,10-14 These are constructed with the aid of the Nernst equation, together with the solubility products of any solid phases involved, for certain specified activities of the reactants. For example, the stability field of liquid water under standard conditions (partial pressures of H2 and 02 of 1 bar, at 25 °C) is delineated in Fig. 15.2 by... 

Figure 15.2 Redox stability field of liquid water at 25 °C. Solid lines refer to gas partial pressures of 1 bar (standard state), broken lines to 10-6 bar.

For nonstandard partial pressures of the gases, these boundaries will be slightly displaced, but their slopes will remain the same. In Fig. 15.2, the stability field of water is only slightly narrowed by considering gas pressures a millionfold lower. 

In addition to the above information, Fig. 15.3 includes the standard-state stability field of liquid water itself (broken lines) from Fig. 15.2, as well as the I h-pH ranges found in natural waters (shaded area). The usefulness of the Pourbaix diagram now becomes apparent ... 

The passive range of typical stainless steels conveniently spans most of the Eh stability field of neutral water. This can be appreciated by examination of the E° or Eh values for reactions 16.20 to 16.25, with the caveat that these refer to pure iron and chromium metals rather than to stainless steels and that the conditions are standard ones rather than, for example, the very low [Cr3+] in equilibrium with the FeCr2C>4 film. The formation... 

Figure 3. Stability fields of some minerals in the Na20-Al203-Si02-H20 system at 25°C. as a function of Na+, H and dissolved silica. Points are from ground water analyses of siliceous rocks...

Figures 7.11a,b are arbitrary examples of the depths of hydrate phase stability in permafrost and in oceans, respectively. In each figure the dashed lines represent the geothermal gradients as a function of depth. The slopes of the dashed lines are discontinuous both at the base of the permafrost and the water-sediment interface, where changes in thermal conductivity cause new thermal gradients. The solid lines were drawn from the methane hydrate P-T phase equilibrium data, with the pressure converted to depth assuming hydrostatic conditions in both the water and sediment. In each diagram the intersections of the solid (phase boundary) and dashed (geothermal gradient) lines provide the lower depth boundary of the hydrate stability fields.

The Eh-pH predominance diagram for Asv and As111 species is shown in Fig. 5.9. The upper and lower boundaries represent the stability field for water. 

Several water compositions are plotted on the stability diagrams in Figure 8.2. It can be seen that at shallow Earth surface pressures and temperatures, seawater plots in the stability field of dolomite whereas solutions of average river water composition and most shallow groundwaters plot in the field of calcite. With burial of carbonate sediments and elevated P and T, the dolomite field shrinks, but subsurface fluid compositions evolve toward a composition in equilibrium with dolomite. This conclusion is probably one of the most important arguments for the formation of dolomite during deep burial diagenesis (see also Hardie, 1987). Thermodynamic considerations favor this reaction path, as well as the fact that... 

How such a EH-pH diagram can be determined analytically is explained below using the example of the Fe-02-H20 diagram shown in Fig. 15 left. In each Es-pH diagram the occurrence of the aqueous species is limited by the stability field of water. Above this field H20 converts to elementary oxygen, below this field to elementary hydrogen (also see Fig. 16). 

Fig. 19 Redox buffer and subdivision of natural ground waters into 4 redox ranges within the stability field of water black dashed lines indicate the boundaries of the four redox ranges (after Drever 1997)...

The program takes into account that aquatic species are limited in every pE-pH-diagram by the stability field of water. Therefore, the program deletes automatically all pE-pH combinations lying above the line of transformation 02-H20 or below the line of transformation H20-H2. Assistance to the program can be found within the menu HELP. 

When considering the increment of 1 for pE and pH, i.e. 15 pH values 31 pE values, the output file would comprise 465 jobs, numbered from SOLUTION 1 to SOLUTION 465, each containing different pE- and pH values. In fact, there will be only 377 jobs since the SOLUTIONS with pE-pH values above or below the stability field of water are missing. The water constituents defined under SOLUTION (e.g. Fe, Ca, Cl, C, S, etc.) are alike in all 377 jobs. Opening this input file takes about 30 seconds. Because files larger than 32 k cannot be opened in the Windows environment of PHREEQC either they have to be divided into smaller files or they have to be started directly with phreeqc.exe in the DOS prompt (phreeqc Input-File-Name Output-File-Name Database name). 

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