Determining Phase Border Curves

The supercritical fluid of interest is charged at ambient temperature to a suitable gas/liquid compressor, where it is compressed and delivered to a high-pressure equilibrium view cell. The constant-volume high-pressure view cell is immersed in a water bath, which can easily be controlled to within 0.rC. The pressure of the system is measured with an appropriate Bourdon tube Heise gage. The cell contents are mixed when the cell is rocked approximately 180°, causing a small stirring bar previously inserted into the cell to move through the cell contents. 

If a liquid and a vapor are initially present and if, upon isobaric cooling, a solid falls out of solution, an SLG point is obtained. But if all the liquid in the 

The critical mixture curve is measured in the following manner (Occhio-grosso et al., 1986). At a temperature slightly higher than the UCEP temperature, a vapor-liquid mixture at a fixed overall concentration is compressed to a single phase. The pressure is then isothermally decreased very slowly until the system becomes turbid and a second phase just begins to precipitate. A critical mixture point is obtained if critical opalescence is observed during the transition process and if two phases of equal volume are present when the mixture phase-separates. 

A number of researchers have developed techniques to determine partial molar volume. Eckert and coworkers (Eckert et al., 1986) provide a very good description of the method for obtaining partial molar volumes. They facilitated this difficult measurement by using a vibrating-tube densitometer (Mettler-Paar DMA 512). The major uncertainties with this technique are associated with the temperature control, which becomes crucial if experiments are performed at infinite dilution near the solvent s critical point. Shim and Johnston (1991) also note that it is possible to determine partial molar volume information using supercritical fluid chromatography if the mobile and stationary phases can be thermodynamically characterized. 

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Most of the studies reported in this chapter fail to include the phase behavior of the reacting mixture. Since multiple phases can occur in the mixture critical region, reaction studies need to be complemented with phase behavior studies so that we may gain an understanding of the fundamentals of the thermodynamics and kinetics of chemical reactions in solution. Chapter 5 describes how a simple cubic equation of state can be used to extend and complement the phase behavior studies. An equation of state can be used to determine the location of phase-border curves in P-T space and, with transition-state theory, to correlate the pressure dependence of the reaction rate constant when the pressure effect is large (i.e., at relatively high pressures). 

Phase border curves, methods for determining, 94-98 Phase rule, 28... 

Three analytes Si, S2, and Sj, each form a discrete pH zone behind the sharp retainer border in the order of their p Ta s and hydrophobicities. The proton transfer takes place at each zone boundary according to the difference in pH between the neighboring zones, causing solute exchange between the two phases, as indicated by curved arrows. Once the equilibrium is established, all solute zones move at a same rate determined by that of the sharp retainer border. Charged impurities present in each zone are... 

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