Oxide components standard states

But that is not all. For dilute solutions, the solvent concentration is high (55 mol kg ) for pure water, and does not vary significantly unless the solute is fairly concentrated. It is therefore common practice and fully justified to use unit mole fraction as the standard state for the solvent. The standard state of a close up pure solid in an electrochemical reaction is similarly treated as unit mole fraction (sometimes referred to as the pure component) this includes metals, solid oxides etc. 

Although in crystalline phases determination of the enthalpy of formation from the constituent elements (or from constituent oxides) may be carried out directly through calorimetric measurements, this is not possible for molten components. If we adopt as standard state the condition of pure component at T = 298.15 K and P = bar , it is obvious that this condition is purely hypothetical and not directly measurable. If we adopt the standard state of pure component at P and T of interest , the measurement is equally difficult, because of the high melting temperature of silicates. 

In these formulae / and Tin indicate the standard electrode potentials which are established, when all the half cell components arc in their standard state and have activities equalling unity. The relation between the standard oxidation and standard reduction potentials of the same element is exactly the same as those formed with other potentials, i. e. for example s) = — n. ... 

A particular half-cell reaction, such as Equation 6.15, can accept or donate electrons. We quantitatively describe this by the redox potential for that reaction, as expressed by Equation 6.9 [Ej = E u — (RT/qF) In (reduced))/(oxidized))]. We will use (NADPH) to represent the activity of all of the various ionization states and complexed forms of the reduced nicotinamide adenine dinucleotide phosphate, and (NADP+) has an analogous meaning for the oxidized component of the NADP+-NADPH couple. For redox reactions of biological interest, the midpoint (standard) redox potential is usually determined at pH 7. By using Equation 6.9, in which the number q of electrons transferred per molecule reduced is 2, we can... 

In general, only the reduction half-reaction potentials are listed in tables, as in Table 9.1. The potential of an oxidation half-reaction is the negative of the value of the reduction half-reaction. Moreover, it is convenient to standardise the concentrations of the components of the cells. If the ceU components are in their standard states, standard electrode potentials, E°, are recorded ... 

The origin of this dilemma appears to lie in the standard states assumed by both the Toop - Samis and Masson models for the "basic" oxide component. These mixing models assume 100 dissociation of the basic oxide constituent of the binary. Thus in the pure liquid end-member oxide melt the ion fraction of free oxide Xq2- is assiuned to be 1.0. In oxide melts containing strongly polarizing cations this is unlikely to be correct since the dissociation reactions e.g. 

The Masson and Toop and Samis mixing models assume complete dissociation of the basic oxide components. Thus, while these models may allow the calculation of oxide activities within any binary, activities in different binary systems cannot be compared since they relate to different standard states. The magnitude of the polymerization constant for a system is a measure of the shape of the titration curve not its absolute position. Thus with decreasing K the curves become steeper, or more sharply inflected, reflecting strong interactions between 02- and 0° or Si-O-Si. As pointed out by Hess (l97l) values of K decrease as Z/r decreases. So the activity curves for Ca2+ systems should be more steeply inflected than in the case of the equivalent Mg2+ compositions and these effects are shown in Fig. 11. 

In attempting to use the polymer or quasi-chemical models to calculate the mixing properties of ternary and more complex melts, the differences in the standard state values of X 2- assumed for the end-member oxide components may be very important. In the absence of any additional cross-interactions the polymerization constants in a multicomponent system containing a mols of AO, b mols BO etc. will be given by... 

To draw these diagrams, one needs the E[J values. These are listed in data books or may be determined by the standard potentials for each of the components of the reactions using Eq. 7.16. The dissolution and chemical reaction equations are then written for aU possible oxidation states with suitable values of x as a function of pH. Using dilferent values of (M +(aq))/ MO t), one can then calculate Ejj and plot the results vs. pH. 

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