Differential ebulliometry

Although calorimetric methods are usually regarded as yielding the most accurate enthalpies of vaporization [39], the measurement of the saturation vapor pressures of a liquid as a function of temperature is also widely used for the same purpose and may afford good quality data. Among these so-called vapor pressure methods [35], differential ebulliometry is probably one of the most reliable. Briefly, the ebulliometric method consists in measuring the boiling temperatures of a liquid at different pressures. In the differential set-up, the pressure over the... 

Measurements of binary vapor-liquid equilibria can be expressed in terms of activity coefficients, and then correlated by the Wilson or other suitable equation. Data on all possible pairs of components can be combined to represent the vapor-liquid behavior of the complete mixture. For exploratory purposes, several rapid experimental techniques are applicable. For example, differential ebulliometry can obtain data for several systems in one laboratory day, from which infinite dilution activity coefficients can be calculated and then used to evaluate the parameters of correlating equations. Chromatography also is a well-developed rapid technique for vapor-liquid equilibrium measurement of extractive distillation systems. The low-boiling solvent is deposited on an inert carrier to serve as the adsorbent. The mathematics is known from which the relative volatility of a pair of substances can be calculated from the effluent trace of the elutriated stream. Some of the literature of these two techniques is cited by Walas (1985, pp. 216-217). 

Direct measurement of y would confirm whether or not the solution is infinitely dilute at saturation. Lobien and Prausnitz (23) have attempted to measure this effect in a few systems by comparing the solubility limit with measurements of y from differential ebulliometry. The systems they studied all had solubilities of a few percent, and for these systems they found significant deviations from yi = 1/xi. It would be useful to have measurements for more dilute solubilities, but in this case the limiting activity coefficient becomes very large, and ebulliometry is inapplicable for high relative volatilities. Perhaps such data could be taken by ebulliometry for systems where the solute is much less volatile than water, or by chromatographic methods. 

A dynamic technique known as differential ebulliometry can measure the effect of small amounts of solute on a solvent. The difference in temperature is measured between a pure fluid and a solution, both of which are boiling at the same pressure. Differential ebulliometry is a conunon technique for determining infinite-dilution activity coefficients, particularly when the solute is less volatile than the solvent. 

There are many methods which enable determination of activity coefficients in infinite dilution. They are mostly based on differential ebulliometry or on gas chromatographic measurement of retention time, subsequently retention volume. The headspace chromatographic analysis is another popular technique which enables measurement of equilibrium compositions at given temperature. Some similarity with static methods may be found, however, degassing is not required since the pressure is not measured. The data may be obtained rather quickly, nevertheless their accuracy is not very high. Methods for measurement of activity coefficients in infinitely diluted solutions are not described here in detail because such data are not included in this volume. 

W. De Oliveira, Differential Ebulliometry, Ph. D. thesis, Clarkson College of Technology, Potsdam, N. Y, 1975. 

C, ebulliometry-differential thermal analysis, measured range 22.6-117.8°C, Kemme Kreps... 

As pointed out above, dynamic vapor-hquid equilibrium measurement mediods are not very suitable for concentrated polymer solutions, especially due to their heavy foaming behavior. For dilute polymer solutions, however, diere is continuing apphcation of ebulhomelry as an absolute method for the direct determination of die number-average molecular mass M . Dedicated differential ebulliometers allow the determination of values up to an order of 100,000 g/mol. Ebulliometry as a mediod for molar mass determination was recently reviewed by Cooper, Glover, and Mays and Hadjichristidis. ... 

Solvent activities of polymer solutions with polymer concentrations of up to about 30 wt% can be measured by osmometry (membrane as well as vapor-pressure osmometry), light scattering, ultracentrifuge (of course, all these methods can also be applied for polymer characterization and can be extrapolated to zero polymer concentration to obtain the second virial coefficient), and differential vapor pressure techniques. Ciyoscopy and ebulliometry can also be used to measure solvent activities in dilute and semidilute polymer solutions, but with limited success only. 

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