Phase-Diagram Determination 12-2 General principles

The general principles of phase-diagram determination are described by Taylor [G.19], with special emphasis on x-ray methods. 

Although in principle phase diagrams can be calculated from thermodynamic data, in complex systems the relationships are generally determined experimentally. The diagrams must nevertheless obey rules established by equilibrium thermodynamics. Therefore an understanding of the material in all the previous chapters is a prerequisite to a real understanding of the simple points, lines, and surfaces found in phase diagrams. 

Surface Pourbaix Diagram for 0 /Pt(l 11) from First Principles Phase diagrams, from a general perspective, show the thermodynamically stable phase of some combination of materials as a function of some set of environmental conditions. Input from DFT can greatly aid in the calculation of such diagrams, but as shown previously, thermodynamic models are needed to fully incorporate effects such as concentrations, pH, and electrostatic potential. In particular, the use of electrostatic reference state models aids in determining stable phases under a variety of electrochemical environmental conditions. 

The general principles and techniques used to establish phase equilibrium are common for both equilibrium and nonequilibrium diagrams. These can be divided into three categories. One pertains to the evaluation of the chemical aspects for the system, for example, composition and structure. The second concerns the determination of the physical conditions within the system, for example, temperature, total pressure, and time. The result of such studies is a description of the variations in... 

Herriott (1957) has reviewed the use of the solubility diagram as a tool for detecting the presence of impurities in soluble substances, particularly proteins. In these days of advanced types of instrumentation and sophisticated methods, it is worth recalling the use of a method that depends on solubility principles and is not difficult to apply. The method is based on application of the phase rule, but one need not apply the phase rule or even understand it to determine whether a preparation contains one or more components. Herriott has discussed the general procedure, the interpretation of results, the quantitative limitations of the method, the conditions and details of technique, and the separation of protein components by methods based on the solubility diagram. 

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