The stability field of water

The starting point in plotting the stability field of water is the Nemst equation, in the form  

This line, is plotted on the diagram. It means that any combination of voltage and pH lying below it will be sufficient to decompose water according to the half-reaction given. 

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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.]. 

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+). 

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. 

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). 

An electrolytic cell is similar to a voltaic cell except the electrochemical reactions involved do not occur spontaneously but require the input of current from an external source. Wires connected to each end of a battery and submerged in a suitable electrolyte can represent an electrochemical cell. As with voltaic cells, the creation and/or removal of ions at the electrodes facilitates the transfer of current into and out of solution. If the electrolytes in solution are redox-inert within the stability field of water (e.g., Na and Cf) and the voltage is over 1.2 volts, the hydrolysis of water may transfer current at the electrodes ... 

Figure 11.2 The stability field of water as a function of Eh and pH at 25 C and 1 bar pressure. Contours showing partial pressures of hydrogen and oxygen at intermediate Eh values have been computed with Eqs. (11.17) and (11.23). The crosshatched area is the locus of Eh values computed assuming the reaction 4H + 02(g) +...

Be able to draw an Eh-pH diagram showing schematically the stability field of water, and plot on it and label the Eh-pH conditions typical of some example surface-waters and groundwaters. 

As the first example, we will calculate the two boundaries for the stability field of water. 

Next the reduction of water to hydrogen gas must be considered as the lower bound of the stability field of water. 

The third type of bounds separate stability areas of two water-insoluble minerals in the field of water Eh and pH values. Within the boimdaries of the stability fields of water-insoluble minerals migratory forms of the component B, are missing. At pH and Eh values of the stability fields of water-insoluble minerals component Bi is removed from the solution. 

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... 

Figure 9.26 A Pourbaix diagram showing the stability field of water (shaded)...

Sketch these onto the stability field of water on the Pourbaix diagram of Figure 9.29 [sketch not shown at the end of this book.]. What copper species will be present in each of these ... 

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