Ebulliometric Methods

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... 

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. 

In spite of the apparently successful performance of the ebulliometric system in routine use, a number of variables exist in connection with the operation of the system as it has been described. Many of these variables, as many as possible, are overcome by standardization in routine opera tion. However, they must be recognized if the ebulliometric method is to be understood and applied to special problems. 

This paper has attempted to show that the ebulliometric method can be used successfully for the routine determination of molecular... 

C, ebulliometric method, measured range 310.17-356.7 K, Lubomska Malanowski 2004)... 

Vapour pressures can be measured directly using a differential ebulliometric method [81,82], or by static measurements with different kinds of manometers [83-85]. 

Ebulliometric method is used for a simultaneous measurement of the boiling and condensation temperature that is required for evaluation of purity of a substanee and its molecular mass. 

Manometric and ebulliometric methods may be used to measure vapor pressiue. Quasistatic methods have been developed based on the molecular kinetic theory of gases. The Langmuir method is based on the evaporation of a substance from a free surface into a vacuum and the Knudsen method is based on the evaluation of the outflow rate of a vapor jet from a mesh. 

In ebulliometric methods a second component is always present the gas by which the pressure in the system is controlled. It is assumed (and the assumption is borne out in practice) that at low pressures this gas does not affect the behaviour of the substance under study, but at higher pressures the basis for the assumption is less secure. If the gas dissolves in the liquid, the boiling and condensation temperatures of the latter could be affected, and for this reason helium is in principle to be preferred as the gas because its solubility in all liquids is smaller than that of other gases. Equally it can be demonstrated, both theoretically and practically, that pressure applied to a liquid or solid will raise its vapour pressure for example, the concentration of water in a compressed gas saturated with water vapour is greater... 

Application of the ebulliometric method at elevated pressures has already been discussed. The method has been used only occasionally, and most vapour-pressure measurements in the range now considered have been made by the static method in apparatus which is, in principle, the same as that used by Andrews for his work on the isotherms of carbon dioxide, and subsequently developed by Hannay and by Ramsay and Young. The sample is confined over mercury in a thick-walled glass tube sealed at its upper end, and the tube is attached to a steel U-tube containing mercury the pressure is transmitted through the mercury to the pressure gauge and a piston on a screw allows the level of the mercury, and hence the volume occupied by the sample, to be adjusted. The experimental tube is heated by means of a vapour jacket. (Alternatively, the glass tube itself may be bent into the form of a U, often then known as a Cailletet tube, and the apparatus is heated by immersion in a normal thermostat bath. )... 

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