Predominance-region diagram

FIGURE 11-8 Predominance-region diagrams illustrating calmagite behavior as a function of pH and pMg (left) and pCa (ri t). (Color transitions occur over a range the lines are not meant to imply that they take place at unique values.)... 

Figure 1.93. /02-pH diagram with the stability fields of aqueous species in Na-K-H-S-Se-0 system for the conditions SS = 10 mol/kg H2O, ESe = 10 mol/kg H2O, ionic strength = 1, and temperature = 150°C. Dashed lines are the ratio ho-a i- /a i- in logarithmic units. Stability fields for native sulfur and native selenium and the boundaries between predominance regions of oxidized and reduced selenium species are omitted for clarity (Shikazono, 1978b). 

Figure 5 Redox predominance diagrams for iron (a) and manganese (b) boundaries represent the standard reduction potential for reduction of the (thermodynamically-stable) species above the boundary to the (thermodynamically-stable) species below the boundary. If the redox predominance regions of two species (e.g., the gray regions of Fe + and Mn04 ) do not overlap along the y-axis when the two diagrams are superimposed, reaction between the two species is thermodynamically favored...

The relation between potential and pH often is depicted graphically in the form of Pourbaix or predominance-region or potential-pH diagrams. To illustrate... 

FIGURE 12-1 Simplified predominance-region or potential-pH diagram for the system Fe, Fe(II), Fe(III). Left, predominant species present right, type of reaction occurring in passing from one region to another. 

We will state these equations and draw the lines they represent on a pCt.coj- pH predominance area diagram then by deduction we will select the areas of predominance for each species. Our approach will be to first examine the equilibria between Fe " " and the various hydroxo complexes, since these are independent of Cx.cog and will allow the establishment of pH regions of the predominance of the various hydroxo species. The task of deciding where to draw the lines that describe boundaries between hydroxo species and FeCOgts) will then be made easier. Selection of the first relationship to plot is arbitrary in this case we begin with Fe(OH)2(s) - Fe(OH)g-,... 

Figure 15.5 shows that the oxidation reaction of iodide ions is easier than that of the bromide ions, because the region where they predominate is farther from that of the dichromate ions than is the region where the bromide ions predominate. The diagram also indicates that when bromide ions are oxidized, iodide ions are also oxidized. The converse is not true. From thermodynamic and analytic standpoints, it is interesting to know the conditions in which the selective oxidation of iodide ions is possible (see below). 

In Pourbaix diagrams, the frontier straight lines between the predominance regions are the lines /pH. For example, in the general case above, the equation of the frontier line is... 

The stability and solubility constants derived at 25 °C for zero ionic strength have been used to create a predominance speciation diagram for vanadium(V). The diagram is illustrated in Figure 11.22. The diagram shows that both monomeric and polymeric species have regions of predominance. It is clear from the diagram... 

A predominance speciation diagram for zinc(II) has been created from the stability and solubility constants derived at 25 C for zero ionic strength. The diagram is illustrated in Figure 11.98. Zincite, ZnO(s), has been used as the dominant soUd crystalline phase. A region does not exist where the polymeric species,... 

This diagram indicates that there are no regions where the polymeric alu-minium(III) hydrolysis species are found to predominate, as has been found for many other metal ions. Low ionic strength, as well as high temperature, favours the formation of the monomeric species of metal ions. The diagram also shows that there is no predominance region for Al(OH)2 due to the enhanced stability of AlfOHljfaq). 

From these stability diagrams it is possible to predict the predominant species Pu(OH)3+ will exist in the regions bounded by - 0.60 < Eh < 0.20 at pH 4.0 to... 

Pourbaix diagrams (Pourbaix, 1963) indicate graphically the conditions of redox potential (Eh) and pH under which different types of corrosion behaviour may be expected. These plots of potential vs. pH indicate the phase and species in equilibrium with iron under various conditions (see Chap. 8). The solid phases indicated are those that are thermodynamically the most stable owing to kinetic factors other phases may be present during the initial stages of the corrosion process. What the different regions show, however, are the predominant oxidation states to be expected. 

Figure 1.5 is an E-pH diagram for Al under standard conditions. This means that all soluble species are at LOOM, the species being Al+ and Al(OH)4. The labels for the four species identify the regions in which they predominate under differing E and pH conditions. These predominance conditions may be seen by examination of the three vertical lines in Figure 1.6 and the three horizontal lines in Figure 1.7. Start at the top of the vertical line at a constant pH of 0.0. As one goes down the line, the predominant species Al+ gives way to the predominant species Al at an E value of about —1.7 v. The reduction halfreaction is written as follows with the E° value as obtained from appropriate tables attached.

At this juncture, four different E-pH diagrams have been empiricaiiy examined in detaii. Severai important points are to be noted. First, an E-pH diagram shows the regions of species predominance under conditions of E and pH. Second, the regions of predominance are separated by iines which show the transformations of predominant species. Third, these iines are reflections of transformation reactions which can be represented by equations. Fourth, when the transformation equation does not show the hydrogen ion, the... 

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