Ebulliometric constant

This paper describes an ebulliometric system for routine and special determinations of molecular weights. The system uses a simple ebulliometer, an immersion heater, and a Cottrell-type pump. Temperature sensing is by differential thermopile. Precision varies from about 1 to 6%, and values compare well with those from other laboratories and those from other methods. Values as high as 170,000 have been successfully measured. Some problems encountered in using the ebulliometric method are selection and effect of reference temperature, limitations of the vapor lift pump and a possible substitute for it, measurement of equilibrium concentrations within the operating ebulliometer, and the experimentally determined ebulliometric constant and some factors which influence its value. 

The last variable to be discussed is that of the experimentally determined value for the ebulliometric constant, K6. Equation 1 indicates... 

The ratio MiRTVA p H is called the ebulliometric constant. For the determination of solvent activities from ebulliometric data, tabulated ebulliometric constants should not be used, however. On the other side, it is sometimes recommended to use reference solutes to establish an experimental relationship for the equipment in use, i.e., unprecise data for the enthalpy of vaporization or perhaps some non-equilibrium effects cancel out of the calculation. Enthalpies of vaporization are provided by several data collections, e.g., by Maj er and Svoboda, or through the DIPPR database. 

The continuous curve shown in Fig. 2 was calculated from Eq. (5) by use of the following values for the two parameters K = 11.2 mole-1 liters, k — 41.9 ml. (at 515 mm.) min.-1 mole-1. This dimerization constant is in agreement with the approximate value established by separate ebulliometric experiments. It is clear that within experimental error the data can be fit by an expression of the form of Eq. (5) i.e., the data are consistent with the hypothesis that the active catalyst is a dimer of cuprous acetate. 

Thus, from the ebulliometric experiment, one obtains the infinite-dilution activity coefficient directly. Now repeating the experiment by starting with pure component 2 and adding an infinitesimal amount of component 1, y x —> 0) = can be obtained. These two data points can then be used to determine the parameters in a two-constant activity coefficient model. For example, from the van Laar model of Eqs. 

In comparison to the usual ebulliometric equipment where the polymer solution is placed into the evaporator, only pure solvent is evaporated. The vapor flows through the cell and is condensed at its head-condenser to flow back into the reservoir at the bottom. The vapor pressure is kept constant using a manostat and is measured additionally outside the apparatus after the condenser. Equilibrium times decrease somewhat, degassing of the solvent is not necessary, air leakage does not play any role. 

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