Potential-pH relation

The effect of proton donors on the reduction of aromatic hydrocarbons has been discussed earlier in this chapter. The importance of potential-pH relations to the understanding and utilization of the redox behavior of these systems has long been recognized extensive potentiometric data have been obtained for only a limited number of organic compounds.50... 

The required solubility equations and the corresponding potential-pH relations of FeO, Fe203, and Fe304 in an acidic solution are summarized in Table 12.1 (see also Ref. [8]). 

Oxide, Metal Solubility Equation Potential—pH Relation... 

This reproducible Flade potential and its 0.059 pH dependence is characteristic of the passive film on iron. A similar potential-pH relation is found for the passive film on chromium, for Cr-Fe alloys, and for nickel, for which the stan-... 

The polarographic half-wave reduction potential of 4-nitroisothiazole is -0.45 V (pH 7, vs. saturated calomel electrode). This potential is related to the electron affinity of the molecule and it provides a measure of the energy of the LUMO. Pulse radiolysis and ESR studies have been carried out on the radical anions arising from one-electron reduction of 4-nitroisothiazole and other nitro-heterocycles (76MI41704). 

It is considered useful to include here the potential-pH diagram for some redox systems related to oxygen (Fig. 2.1) [4]. Lines 11 and 33 correspond to the (a) and (b) dashed lines bounding the stability region of water, as depicted in all the subsequent Pourbaix diagrams. 

It is important to recognize some of the limitations of the Pourbaix diagrams. One factor which has an important bearing on the thermodynamics of metal ions in aqueous solutions is the presence of complex ions. For example, in ammoniacal solutions, nickel, cobalt, and copper are present as complex ions which are characterized by their different stabilities from hydrated ions. Thus, the potential-pH diagrams for simple metal-water systems are not directly applicable in these cases. The Pourbaix diagrams relate to 25 °C but, as is known, it is often necessary to implement operation at elevated temperatures to improve reaction rates, and at elevated temperatures used in practice the thermodynamic equilibria calculated at 25 °C are no longer valid. 

Potentiometric EDTA titrations are best carried out with a mercury pool electrode (Figure 5.6) or a gold amalgam electrode. When this electrode dips into a solution containing the analyte together with a small amount of added Hg-EDTA complex, three interdependent reactions occur. For example, at pH = 8 the half cell reaction (a) which determines the electrode potential is related to the solution equilibrium by (b) and (c). 

The ffo/pH relation involves two parameters, <75 and apL in addition to the pHpzc. However, it will be legitimate to linearize Equation 18 whenever the second term on the right hand side is small compared to the first term. This depends on the magnitude of p, as for the potential/pH curves, and the linearization is thus legitimate for most oxides whose P is large. The linear equation is ... 

Linkson, Nobbs and Lake (56) Potential -pH diagrams relating to reduction of Cu (II) from acid leach solutions temp, up to 100°C. 

At equilibrium there is a zero free-energy change, AG=0, that takes place between compartments separated by a membrane, with the free-energy change being dependent on the difference in concentration of various ions and the electrical potential difference that exists across the membrane. The relationships among sodium, potassium, and chloride ions, pH, and electrolytic potential have become known as Donnan equilibria. The concentrations and electrolytic potentials are related by the following equation ... 

The potential/pH diagram for some redox systems related to oxygen is presented in Fig. 3. 

Selective ion electrodes (SIE). Selective ion electrodes are essentially variants of the well-known pH meter. They are membrane indicator types of electrodes in which a potential is developed across a membrane in the presence of the ion the size of the potential is related to the concentration and hence can be used to quantitatively detect and measure the species. However, instead of a glass membrane, as in the pH meter, the membranes consist of organics that are immersible in water. For example, anion-sensitive electrodes use a solution of an anion exchange resin in an organic solvent the liquid can be held in the form of a gel, for example, in polyvinyl chloride. The ion reacts with the organic membrane, setting up an equilibrium between the free ion in solution and the ion bound to the membrane, generating a potential difference, which is measured. 

The composition of bronze artifact corrosion products excavated from several lacustrine sites could be explained in relation to some characteristics of the environment. In one study, a combination of potential pH diagrams for copper and those defining different types of natural soil was used to explain the composition of corrosion products of copper artifacts excavated from different locations. While sulphides are predominant in anaerobic conditions, mostly carbonates are... 

It is common practice to utilize forced degradation studies to accelerate degradation of the drug substance or drug product to get an understanding of its degradation profile. Potential environmental conditions that can be used include 40°C and 75% relative humidity (RH), 50°C and 75% RH, 70°C and 75% RH, or 80°C and 75% RH. Oxidation, reduction, and pH-related degradations are... 

Surface pressures, potentials, and viscosities of stearic acid films on hydroxides of Li, Na, K, Rb, Cs, and NH4 have been determined to characterize the specific cation effect. The first observation was that of expansion of the isotherms at high pH related to the nature of the cation. In addition, unexpected results were evident with NH4OH substrates, notably high viscosity and reversal in the sign of the surface potential with compression. 

There is a second group of metals like Fe, Cr, Ni and their alloys, which do not follow all predictions of their potential-pH diagrams. As an example, the Pourbaix Diagram for iron of Fig. 3 predicts corrosion for all potentials in strongly acidic electrolytes. However, experiments show that it is passive for potentials above a potential of Ep = 0.58 — 0.059 pH. For these conditions the passive layer is far from any dissolution equilibrium and its protecting properties have to be related to its slow dissolution kinetics. The same arguments hold for the passivation of Cr, Ni and their alloys. 

Potential-pH diagram and SCC. Critical potentials for SCC of a system metaFsolution can be related to its -pH diagram, because these diagrams describe the conditions at... 

Another type of the facilitated transfer is that involving molecules with acid-base properties. A construction of ion partition diagram of the solute, which is analogous to Pourbaix s -> potential-pH diagrams widely used in electrochemistry of corrosion, has been proposed as a convenient tool that makes it possible to relate the voltammetric behavior to the partition of the neutral and the ionic species, as well as to acid-base equilibria in the adjacent liquid phases [xi]. 

Standard reduction potentials also relate to the commercial methods of production of the elements, to the methods of production and use of 0x0 anions as oxidizing agents, and to the oxidation states in which the elements will be found in natural waters and soils. Many of these tendencies depend both on the reduction potential and the pH of the system in which the elements are found, and can be shown graphically in Pourbaix... 

The Eh-pH and Eh-Es2- relations in the H2S-H2O system. The Eh values in the H2S-H20 system were found to be slightly dependent upon stirring ( 10 mV]. At 298 K the potentials of the platinum electrodes were established quite slowly [1 to 2 hr] at pH > 5. At pH < 5 it generally takes less than one hour. The potentials were independent of such electrode pretreatment as cathodic or anodic polarization. In Figure 4 we present the Eh-pH relations obtained in the H2S-H20-NaCl(0.7 j4] system. 

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 45 shows plots of the d-band vacancy against quantity of electricity consumed in the reduction of the oxide layer formed under chlorine evolution conditions. This result shows that coverage of oxide on the electrode surface can be linearly related to the d-band vacancy because the quantity of electricity required for the formation of a monolayer on each metal is approximately 0.5 mC cm . The coverage by oxide is considered to be a function of electrode potential, pH, and concentration of Cr. If these... 

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