numerous sophisticated home-made devices described in earlier compilations [11-13] are of
historical interest. Since samples are often corrosive or are taken to temperatures outside the
operating range of the pressure gauge, it is usual practice to isolate the sample from the pressure
gauge by using a differential pressure indicator (DPI) such as a U-tube manometer filled
with mercury or a metal diaphragm. When the DPI indicates that pressures of an inert reference
fluid and the sample are equal, the reference fluid pressure is measured using an appropriate
pressure gauge.
Direct vapour pressure measurements tabulated in the literature have ranged over an
astonishingly wide temperature range of over 200 K [ 14-17]. The advantage of the static
technique is that it allows for an absolute calibration and accurate temperature determination
(___0.002 K). With a suitably selected pressure sensor, the static method can be applied
over a wide range of pressures from 10 -3 Pa to 10 MPa at any temperature. Disadvantages
are its crucial sensitivity to volatile impurities and the time required for a set of measurements,
often 2-3 weeks for a sample.
2.2 Ebulliometry
Ebulliometry is the most frequently employed dynamic method. If the boiling is carried out
under reflux, and the pressure in the apparatus can be varied, measurement of the boiling temperatures
at differing pressures is a convenient way of determining the vapour pressure of the
liquid. An ebulliometer is the main part of a vapour pressure measuring apparatus. In principle,
it is similar to an ordinary open reflux condenser/boiler that operates at ambient pressure.
Ebulliometry is a relatively old technique [18], and as a result, many different boiler designs
have evolved. The goal of any design is the promotion of smooth, even boiling and the minimization
of superheating. There are two main boiler designs that can be distinguished. The
first one is a still with circulation of both the liquid and vapour phases [19-21]. This method
is based on the application of a so-called Cottrell pump, which was originally used for the
determination ofvapour pressures and boiling points of pure components. The second method
is similar to the open reflux condenser/boiler and is designed without a Cottrell pump, but has
the capability to reduce overheating and bumping of the boiling liquid [22]. Ebulliometers are
often designed to measure both boiling and condensation temperatures. The difference
between these two temperatures should lie within 0.005 K if the substance is thoroughly purified
and decomposition does not occur.
Recent trends in ebulliometry show that this method is preferably used for investigation
of VLE for mixtures of organic compounds and the vapour pressure measurements of pure
components are merely a starting part for such studies. There are several comprehensive
reviews of experimental procedures and equipment for vapour pressure and VLE measurements
[ 19,21,23,24]. Of particular importance is the remarkable retrospection of ebulliometric
techniques published by Malanowski [21]. Ambrose [ 1] has provided detailed insight into
the basics of ebulliometric measurements. Analysis of the recent literature reveals that the
dynamic method with circulation of both the liquid and vapour phases is applied more frequently
than other modifications. A typical ebulliometer, equipped with the Cottrell pump,
was designed by Rogalski and Malanowski [20,21]. Two versions of the apparatus were