Equilibrium point, oxide-solution

In other cases, however, and in particular when sublattices are occupied by rather immobile components, the point defect concentrations may not be in local equilibrium during transport and reaction. For example, in ternary oxide solutions, component transport (at high temperatures) occurs almost exclusively in the cation sublattices. It is mediated by the predominant point defects, which are cation vacancies. The nearly perfect oxygen sublattice, by contrast, serves as a rigid matrix. These oxides can thus be regarded as models for closed or partially closed systems. These characteristic features make an AO-BO (or rather A, O-B, a 0) interdiffusion experiment a critical test for possible deviations from local point defect equilibrium. We therefore develop the concept and quantitative analysis using this inhomogeneous model solid solution. 

At this point the dissolution rate of Zn is equal to that of hydrogen ions production. The corresponding current density icorr is called corrosion current of zinc in hydrochloric acid. Therefore, on the contact surface zinc-acidic solution microscopic galvanic cells are formed of the type shown in Fig. 13.3 in which the electrons made available by zinc that oxidizes reduce the acid determining a new equilibrium point in which anode and cathode have the same potential Econ The variation of the electrode potentials with respect to their equilibrium values that leads to the common value E orr is called polarization. The thermodynamic driving force that sustains corrosion is given by the difference... 

The second chapter is by Aogaki and includes a review of nonequilibrium fluctuations in corrosion processes. Aogaki begins by stating that metal corrosion is not a single electrode reaction, but a complex reaction composed of the oxidation of metal atoms and the reduction of oxidants. He provides an example in the dissolution of iron in an acidic solution. He follows this with a discussion of electrochemical theories on corrosion and the different techniques involved in these theories. He proceeds to discuss nonequilibrium fluctuations and concludes that we can again point out that the reactivity in corrosion is determined, not by its distance from the reaction equilibrium but by the growth processes of the nonequilibrium fluctuations. ... 

Because of the adsorption equilibrium for H+ and OFT ions between the surface of semiconductors and an aqueous (aq) solution, the semiconductor surface attains the point of zero charge (PZC). The flat-band potential U[h of most semiconductors including all oxides and also other compounds such as n- and p-type GaAs, p-type GaP, and n- and p-type InP in an aqueous solution is determined solely by pH and shifts proportionately with pH with a slope of -59 mV/decade, that is, pH, for example,... 

Other portions of weathering profiles will establish, at any given point, an equilibrium between the fluid or aqueous solution and the silicate-oxide solids in the soil. Each portion of the profile will represent a different series of chemical conditions, i.e., the total relative masses of the various components will change with, for example, K O increasing downwards in the profile. However, the phase equilibria are such that the different portions of the profile can be analyzed on a P constant, T constant, X diagram for any small segment of the profile. 

The discussion of the high-temperature properties and thermal decomposition of carbonates in this chapter, a plan which will be followed for other classes of compounds in subsequent chapters, is given for each element. A brief discussion is followed by quantitative data on phase transitions, densities, and thermodynamic parameters for the carbonate, the corresponding oxide, and the decomposition reaction MC03 = MO + C02. If the carbonate and oxide are solids and form no solid solution, the equilibrium constant is the pressure of C02 which would be obtained if one begins with an evacuated container and lets the system come to equilibrium. In this case, the pressure is a unique function of the temperature. As pointed out in Chapter 1, this is no longer true... 

All the partial pressures calculated from the equilibrium constants assume unit activity for the condensed-phase components. This assumption is good when they are solid. Above the melting points of the salts, however, continued decomposition of the salt will result in a solution containing dissolved oxide and the partial pressures will depend on the melt composition, and will therefore change as the decomposition proceeds. Because of the form of Kt, the partial pressure calculation will be worst for small oxide concentrations. An examination of the various tables shows that 02 and NO are the major products of nitrate decomposition, the concentration of N02 being rather minor. This results from the fact that the equilibrium 2 N02 = 2 NO + 02 lies to the right for low pressures. 

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