E-pH diagrams

de Zoubov published Atlas d equUibres electrochimiques, a collection of E-pH diagrams for 90 chemical elements. This volume was translated into English in 1966 by J. A. Franklin and published as Atlas of Electrochemical Equilibria in Aqueous Solutions. Subsequently other investigators published computer programs for constructing the diagrams L. Santoma  

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Figures 1.1 through 1.17 exemplify the general nature of E-pH diagrams. Each E-pH diagram is a plot of E against pH for aqueous solutions.

Figure 1.1 E-pH diagram for Na species. Soluble species concentrations (except ...

Figure 1.2 E-pH diagram for Na species. Soluble species concentrations (except H+) = 10° ° M. Soluble species and most solids are hydrated. No agents producing complexes or insoluble compounds are present other than HOH and OH . Species transformations take place at constant pH.

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.

Figure 1.8 is an E-pH diagram for Al with the soluble species at 0.10 M except for the hydrogen ion concentration. This changed concentration applies to both Al" " and A1(0H)4. By observation of Figure 1.9, these transition equations can be seen for the descending vertical lines at constant pH values of 0.0, 7.0, and 14.0 ... 

The first three equations above lend themselves to treatment by equilibria expressions and the last two by the Nernst expression. In all five cases it is the pH that is being solved for. The estimated values taken from the E-pH diagram are presented in brackets for comparison. 

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... 

Since E-pH diagrams involve the pH which is a representation of the hydrogen ion concentration, the first of these equations is preferred. 

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