Russian Journal of Applied Chemistry, 2009, Vol. 82, No. 1, pp. 42−46.
Pleiades Publishing, Ltd., 2009.
Original Russian Text
O.V. Surov, 2009, published in Zhurnal Prikladnoi Khimii, 2009, Vol. 82, No. 1, pp. 44−47.
OF SYSTEMS AND PROCESSES
Thermogravimetric Method Used To Determine the Saturated
Vapor Pressure in a Wide Range of Values
O. V. Surov
Institute of Solution Chemistry, Russian Academy of Sciences, Ivanovo, Russia
Received April 8, 2008
Abstract—Thermogravimetric method was used to study the evaporation of a number of solvents. The evaporation
coefficients in the Langmuir equation were calculated in relation to the conditions of a thermogravimetric
experiment (atmospheric pressure, carrier-gas) and nature of a substance under study.
There exist a large number of various methods for
measuring the saturated vapor pressure. They include
direct measurements with a gage, use of mass spectrometry
for recording the concentration of evaporating particles
in the gas phase, evaporation of a sample into a vacuum
from a Knudsen cell, application of chromatography,
and determination of the boiling point at a lowered
Recently, papers describing application of thermo-
gravimetry for determining the saturated vapor pressure of
organic compounds have been appearing in the scientiﬁ c
literature increasingly frequently [1–3]. Probably, use
of thermogravimetry for studying the evaporation
was first demonstrated by Gueckel and coworkers
, who measured the evaporation rates of pesticides
under atmospheric pressure by means of isothermal
thermogravimetry. The basic principle underlying the
experiment is that both evaporation and sublimation are
zeroth-order kinetic processes. Consequently, the rate of
mass loss by a substance under isothermal conditions as
a result of evaporation must be constant, provided that the
free surface area of evaporation remains unchanged.
Among methods used for thermal analysis, thermo-
gravimetry has shown good performance as a fast and
reliable technique for analyzing the sublimation and
evaporation of substances. Although the conventional
methods for measuring the vapor pressure are rather
accurate, they involve gross expenditure of time.
Thermogravimetry has the following main advantages: it
uses standard factory equipment, requires comparatively
small amounts of a sample, and takes a short time for
With the use of programmed temperature and
a thermogravimetric analyzer, the loss of mass by a sample
is recorded from a constant surface area in a controlled
atmosphere. The data obtained can be represented as a TG
curve, in which the loss of mass is recorded as a function
of temperature or time. The Langmuir equation for free
evaporation in a vacuum relates the rate of mass loss to
the saturated vapor pressure:
where –dm/dt is the rate of mass loss from unit surface
area; P, vapor pressure; M, molecular mass of the
vapor; R, gas constant; T, absolute temperature; and α,
evaporation coefﬁ cient (commonly taken to be unity in
Samples are placed in a crucible with parallel walls in
such a way that a clearly deﬁ ned surface area is obtained.
Measurements can be made in isothermal or dynamic
conditions of linearly increasing temperature in a pure
inert atmosphere under atmospheric pressure.
When a material is evaporated in a gas ﬂ ow under
atmospheric pressure, the coefficient α cannot be
considered to be unity, as it can be done in a vacuum.
Transformation of Eq. (1) gives