Distillation apparatus, isothermal

A simple isothermal distillation apparatus is illustrated in Fig. 9.9. While not specifically designed for air-sensitive compounds, the apparatus may be loaded in a dry box. After introduction of samples and solvent, it is sealed off under vacuum and allowed to equilibrate, with the apparatus arranged so the liquid is in the large bulbs in order to afford a maximum area of exposed solvent. The apparatus is periodically tipped so the two solutions flow into the calibrated legs. When no change in volume is observed from one time to the next, the relative volumes of the solutions are, to a good approximation, proportional to the moles of solute in the two legs. 

Since equilibration sometimes takes days or weeks, the isopiestic method is not suitable for unstable compounds or compounds with appreciable volatilities. Also, long equilibration times require leak-free and grease-free apparatus. Another design for an isothermal distillation apparatus is given in Fig. 9.10. 

A new area of microtechnique was opened up when Conway and Byrne92,93 introduced a simple apparatus for diffusion-distillation or isothermal distillation. 

The isopiestic (isothermal distillation) method for the determination of molecular weights is closely related to the vapor pressure depression method.10 A weighed amount of standard is introduced into one leg of an apparatus and a weighed portion of the unknown is placed in the other leg. Solvent is introduced into the apparatus, which is then evacuated and thermostated. The solvent will distill from one solution to the other until the vapor pressures (and therefore mole fractions) of the two have equalized. If the solutions are ideal, or if the deviations from ideality are similar, equilibrium will occur when the mole fraction of the known equals that of the unknown. 

Fig. 9.10. Apparatus for the determination of molecular weights of air-sensitive compounds by the isothermal distillation technique. Temperature fluctuations of the two solvents are minimized in this illustration by a Dewar filled with water. This apparatus is used in the following manner. In a dry box the sample is placed in a tared tube through sidearm A, the sidearm is then sealed off, the tube and remnant of the sidearm are weighed, and the tube is attached to the apparatus by glassblowing at B. A weighed portion of a standard is introduced into the other bulb, and the filling tube is sealed off. After evacuation, opening of the break-seal, and reevacuation, a measured portion of solvent is distilled into both arms of the apparatus. The process of equilibration is followed by periodic removal and measurement of the solvent from one arm. The solvent may be measured volumetricly in the liquid or gas stales, or by weight.

Pure aqueous HF solutions (up to 25M) can be prepared by isothermal distillation in polyethylene, polypropylene or platinum apparatus [Kwestroo Wissev Analyst 90 297 1965]. It attacks glass and is used for etching glass. HIGHLY TOXIC. 

A second apparatus originally developed by Abribat, Rosano, and Vaillet (5) for studying the transfer of film (distillation isotherme super-ficielle) was used to determine if the subphase was also being dragged with the surface film. A cathetometer was used to measure the change in height of the substrate. 

The silicagel supports were obtained from Merck and Grace. Some were treated with 2n H(31 (3h, 25 C), then washed 15 times with a large excess of distilled water and finally dried under vacuum, first at 80 C (2h) and then at 350 C (12 h) (Table 1). The BET surface areas were determined from adsorption isotherms [181 and the pore size distribution [191 by standard methods from the desorption isotherm of N2 at 77K (apparatus Omicron, Omnisorp joO). The mean pore size calculated according to [201. The number of surface... 

Isothermal measurements of hydraulic permeability at various temperatures were performed with the simple apparatus schematically represented in fig. 2. It essentially consists of a permeability cell (P.C.), a reservoir (R) containing distilled water, both kept at the same constant temperature T by the bath (TB), from which the thermostating fluid is circulated through circulation pump (C.P.). 

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