1070-4272/03/7612-1909 $25.00 C 2003 MAIK [Nauka/Interperiodica]
Russian Journal of Applied Chemistry, Vol. 76, No. 12, 2003, pp. 1909!1913. Translated from Zhurnal Prikladnoi Khimii, Vol. 76, No. 12, 2003,
Original Russian Text Copyright + 2003 by Sladkov.
OF SYSTEMS AND PROCESSES
A Simple Criterion of Thermodynamic Similarity
for Molecular Inorganic Compounds
I. B. Sladkov
St. Petersburg State Technical University, St. Petersburg, Russia
Received November 12, 2002; in final form, October 2003
Abstract-The new determining criterion of thermodynamic similarity was suggested and used to obtain
correlations that ensure reliable prediction of the physicochemical properties of molecular inorganic com-
pounds on the basis of a single experimental characteristic of a substance, its boiling temperature.
The accumulation of reference information lags far
behind the needs of science and technology for data
on the physicochemical properties of substances. Be-
cause the experimental determination of properties is
frequently a complicated task, it becomes even more
important to determine the lacking data by calculation.
Presently, such methods use the thermodynamic sim-
ilarity theory as a theoretical basis .
The similarity theory originates from the reduced
Van der Waals equation, from which the law of cor-
responding states follows. The concept of the corre-
sponding states, which appeared as a consequence of
the reduced Van der Waals equation, further became
separated from the particular form of the equation
of state and transformed into a method for prediction
of the physicochemical properties of substances in
terms of the similarity theory .
The similarity theory is based on the generalized
form of the reduced equation of state
= f (T
is an invariant that represents the dimen-
sionless form of the property to be determined using
the law of the corresponding states. Such an invariant
may be a dimensionless combination of any physico-
Equation (1) means that, in terms of the dimension-
less parameters, the physicochemical behavior of sim-
ilar substances must be identical. This gives reason
to believe that Eq. (1) is the criterion equation of
thermodynamic similarity. The criterion equation in
the form (1) is referred to as the two-parameter form
of the law of corresponding states.
As is known from the similarity theory, criterion
equations can be applied solely to identical objects.
Correspondingly, Eq. (1) is only applicable to ther-
modynamically similar substances.
To judge whether or not the thermodynamic sim-
ilarity is the case, one commonly uses the dimension-
less parameters named the determining criteria of ther-
modynamic similarity. In the absence of such criteria,
Eq. (1) has a very narrow applicability range, being
solely suitable for small groups of substances exhibit-
ing rather rigorous thermodynamic similarity. To ex-
tend the applicability range of Eq. (1), a determining
criterion A is introduced as its additional argument to
give a three-parameter form of the law of correspond-
= f ( T
, A ).
Establishing the determining criteria of thermo-
dynamic similarity is a rather complicated task. It is
this circumstance that explains why more than half
a century passed between the formulation of the law
of corresponding states and its practical application
(in the form of methods for approximate calculation of
physicochemical properties). In the mid-1950s, sev-
eral criteria of the same kind were suggested, among
which the Pitzer acentric factor gained the widest ac-
ceptance . The correlations developed on the basis
of the acentric factor have found wide application for
predicting properties of organic substances.
The use of the acentric factor in the case of inor-
ganic molecular compounds meets certain difficulties,
which are caused both by the specific features of the
prediction method  and by the fact that the correla-
tions based on the acentric factor cannot be applied to
strongly polar substances and associated liquids .