Potential pH diagrams

Thermodynamic data (4) for selected manganese compounds is given ia Table 3 standard electrode potentials are given ia Table 4. A pH—potential diagram for aqueous manganese compounds at 25°C is shown ia Figure 1 (9). 

In contrast with the Pb-HjO system, it can be seen in Figure 4.13 that in the presence of SO " the corrosion zone in the region of low pH no longer exists, owing to the thermodynamic stability of PbSO. The Pb-HjO-COj system has been expressed in a similar pH/potential diagram in which account has been taken of insoluble carbonates and basic carbonates of lead. 

The predictions of the pH/potential diagram are generally fulfilled, but in very concentrated acid solutions, attack may diminish, owing to the relative insolubility of the relevant salt in the acid. Thus, lead nitrate, although soluble in water, has (owing to common ion effect) only slight solubility in concentrated nitric acid, and the corrosion rate is reduced. Similarly, lead chloride is less soluble in moderately concentrated hydrochloric acid than... 

The (photo)electrochemical behavior of p-InSe single-crystal vdW surface was studied in 0.5 M H2SO4 and 1.0 M NaOH solutions, in relation to the effect of surface steps on the crystal [183]. The pH-potential diagram was constructed, in order to examine the thermodynamic stability of the InSe crystals (Fig. 5.12). The mechanism of photoelectrochemical hydrogen evolution in 0.5 M H2SO4 and the effect of Pt modification were discussed. A several hundred mV anodic shift of the photocurrent onset potential was observed by depositing Pt on the semiconductor electrode. 

Once one has a pH-potential diagram with lines drawn for the MB+ + ne M reaction and for the xA + mH+ + ne yD + zH2 reaction, all one has to do is to draw a line perpendicular to the pH axis at the particular value of pH corresponding to that of the solution (Fig. 12.10). If that line intersects the M"+ + ne M line at a more negative value of potential than the xA + mH+ + ne yD + zH20 line, then a simple conclusion follows. The M + + ne M reaction will tend to run spontaneously in the deelectronation direction and produce M + from M (i.e., dissolution), and the other reaction will tend to proceed spontaneously as an electronation reaction (and thus absorb electrons supplied during the deelectronation of the metal) if a path is provided for the electron flow from the sink for the deelectronation reaction to the source for the electronation reaction. The metal M will be said to tend to corrode spontaneously. 

Pourbaix diagrams, or pH-potential diagrams, have been constructed to facilitate the prediction of the various phases (reactions and reaction products) that are stable in an aqueous electrochemical system at equilibrium. " Boundary lines in such diagrams divide the areas of stability for different phases and are derived from the use of Nemst equation... 

Table 4.4 gives the pH and the static mixed potential for each of the NH3 slurries as well as a prediction of the copper stability from the Cu-HjO and Cu-NHj-HjO pH-potential diagrams. 

Fig. 6. Model pH-potential diagram calculated for the thallium oxide-water system with consideration of the existence of the mixed-valence oxide. Equilibrium potentials for the redox systems Tl" /Tl203 (1), Tl" /mixed oxide (2) and mixed oxide/Tl203 (3) are given for 1 mm TI2SO4 solution.

In Figs. 2.3 and 2.4, SiF is considered as the predominant species. These diagrams may vary somewhat when other silicon complexes, such as SiF and SiFf , are also considered. Further, in real systems, the boundaries of the stability domains in the pH-potential diagrams depend as well on the type of Si02, which may have different crystallinity and structures and thus different solubilities, as shown in Fig. 2.5. ... 

A thermodynamic analysis was conducted for corrosion of iron alloys in supercritical water. A general method was used for calculation of chemical potentials at elevated conditions. The calculation procedure was used to develop a computer program for display of pH-potential diagrams (Pourbaix diagrams). A thermodynamic analysis of the iron/water system indicates that hematite (Fe203> is stable in water at its critical pressure and temperature. At the same conditions, the analysis indicates that the passivation effect of chromium is lost. For experimental evaluations of the predictions, see the next paper in the symposium proceedings. 

The pH-potential diagram or Pourbaix diagram, the graphical presentation of a stable species within a pH-potential region, is a valuable tool for analyzing the electrochemical equilibria in aqueous solutions. With a diagram, the reaction products can be determined under given conditions. If a redox reaction product of a... 

FIGURE 2 pH-POTENTIAL DIAGRAM FOR Fe-H20 SYSTEM AT CRITICAL CONDITION... 

FIGURE 5 pH-POTENTIAL DIAGRAM FOR Cr-H20 SYSTEM AT AMBIENT CONDITION... 

A computer program was developed for electrochemical equilibria calculations and graphical pH-potential diagram presentation of a one-metal/one-nonmetal/water system. The program can be used for temperatures and pressures exceeding the supercritical point of water. The calculations show that hematite (Fe203) is the oxidation product of iron in supercritical water, and the oxidation product of chromium in supercritical water is an ionic species, Cr04 . Passivation effect of chromium is lost in supercritical water. 

Figure 1. pH potential diagram of CO2 and its related substances. pH potential relations for water are shown in broken lines. Reprinted with permission from Ref. 15, Copyright (1982) Chemical Society of Japan. 

Such pH potential diagrams may be given for all the products of CO2 reduction. Only the standard electrode potentials of CO2 reduction are given below for formation of CO, CH4, C2H4 C2H5OH, and C3H7OH for brevity. The values are estimated from thermodynamic data, in aqueous media at 25°C with respect to SHE. The standard potentials are conveniently given at pH 7.0, where most of actual CO2 reductions are measured. 

Fig- 7.39 pH-potential diagram for copper used in the analysis of corrosion in Brussels water. Shaded regions indicate pH-potential conditions for corrosion. Vertical bar defines corrosion potential limits for pitting at pH = 8. Source Ref 56... 

Write down the reaction equation for reduetion of O2 as well as Nemst s equation in eaeh of the two cases. Explain the similarity and the difference between eorresponding addends in the Nemst s equation in the two cases. Indieate the standard equilibrium potential of the reactions and the equilibrium potential as a function of the pH at p02 = 1 in a pH-potential diagram. 

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