Apparatus simple pressure cell

Deaton and Frost (1946) suggested the same apparatus could be used for conditions below the ice point. In these experiments, gas was first bubbled through water above 273 K, to form a honeycomb mass of hydrate. Then free water was drained before the cell was cooled below the ice point. After the temperature was stabilized, gas was removed in small increments until a region of constant pressure was obtained, which indicated dissociation of the hydrate phase. Deaton and Frost used this procedure only for equilibria of simple hydrates, since the hydrated mass of guest mixtures was not constrained to be of uniform composition, and consequently would have decomposed at different pressures. 

The measurement of partial pressures over a liquid or solid mixture of two metals is not as simple. Mostly, it is restricted to higher temperatures or even to the molten phase. The direct measurement can be done, for example, in high or ultra high vacuum, using a Knudsen cell and a mass spectrometer for selective pressure determination. Dynamic measurements were developed, e.g., transportation methods. A steady stream of a reactive gas is passed over the sample transporting the reactive component to a cooled region of the apparatus. From the measured mass of the transported metal and the flow rate the vapor pressure can be calculated. Kubaschewski et alP have given a detailed description of the experimental possibilities. 

The instrumentation for SFE can be relatively simple as shown in Figure 29-10. Instrument components include a fluid source, most commonly a lank of carbon dioxide a syringe pump having a pressure rating of at least 400 atm and a flow rate for the pressurized fluid of at least 2 mL/min a valve to control the flow of the critical fluid into a heated extraction cell having a capacity of a few milliliters and an exit valve leading to a flow restrictor that depressurizes the fluid and transfers it into a collection device. In the simplest instruments, the flow restrictor is 10 to 50 cm of capillary tubing. In modern sophisticated commercial instruments, the restrictors are variable and controlled manually or automatically. Several iastrument manufacturers offer various types of SFE apparatus. ... 

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