1070-4272/05/7803-0409C2005 Pleiades Publishing, Inc.
Russian Journal of Applied Chemistry, Vol. 78, No. 3, 2005, pp. 409!413. Translated from Zhurnal Prikladnoi Khimii, Vol. 78, No. 3,
2005, pp. 413! 417.
Original Russian Text Copyright + 2005 by Pestova, Myund, Khripun, Prigaro.
OF SYSTEMS AND PROCESSES
Polythermal Study of the Systems M(ClO
O. N. Pestova, L. A. Myund, M. K. Khripun, and A. V. Prigaro
St. Petersburg State University, St. Petersburg, Russia
Received January 28, 2005
Abstract-The solubility polytherms in the systems for which the cryoscopic data are lacking were measured.
The structural models of the electrolyte solutions were discussed on the basis of the solubility polytherms.
Unique properties of eutectic compositions of solu-
tions and salt melts make them important for metal-
lurgical applications. They are widely used in modern
technology  to improve the quality of metal produc-
tion and obtain materials with prescribed properties.
The overwhelming majority of physicochemical
studies concerning formation of natural salts is based
on the thermodynamic principles and, as theoretical
background, uses the phase diagrams of water3salt
systems corresponding in the composition to natural
brines [2, 3].
The solubility polytherm is the dependence of the
freezing point of a solution on the solute concentra-
tion. In the certain range of the concentrations (dilute
solution) it is described by the Raoult’s law, which is
valid for any binary system. However, as the solute
concentration is increased, a deviation from the linear-
ity followed by a change in the behavior of the curve
above the eutectic concentrations is observed, i.e.,
the freezing point of a solution increases with the con-
centration. In addition, the supereutectic branch can
have additional singularities, depending on how many
(two or more) crystal hydrates of the salt are formed.
The solubility isotherm represents in the graphical
form the Raoult’s law and the deviations from it and
reflects the dynamics of structural processes in a solu-
tion as its concentration increases. The developed
structural models of aqueous electrolyte solutions are
based on comparison of the character of changes in
physicochemical properties with the solubility poly-
therm of the salt. With this approach, regions of the
predominance of different structures corresponding to
the structure of the solid phase crystallizing from the
solution with dereasing temperature can be revealed
over the entire concentration range of existence of
the solution .
In the region of subeutectic concentrations, water
as a major component determining the solution struc-
ture crystallizes from the solution upon cooling. On
cooling of supereutectic solutions, the solute in the
form of a crystal hydrate or anhydrous salt precipitates
as the bottom phase. This means that in the super-
eutectic region a certain structural type of the asso-
ciates termed cybotactic group [4, 5] dominates in
a solution. Water becomes a solute and a component
of this group. For some salts, the solubility polytherm,
apart from the eutectic, contains one or several sin-
gularities (peritectics). They separate the region of
the concentrated solution into the regions of pre-
dominance of the corresponding crystal hydrates.
The way of formation and the origin of the eutectic
remain still an open question stimulating much in-
terest. The whole set of the data obtained by many
researchers shows the existence of two principal
concepts. According to one of them, the eutectic is
a mechanical mixture of solid components (in the case
of an aqueous solution, these are crystals of salt and
ice) having constant composition and melting point.
According to this traditional concept, the eutectic is
formed owing to the short-term local deviations from
the average composition of the solution. According
to the other concept, the eutectic is a chemical com-
As known, the solution of eutectic composition
(at 25oC) is characterized by the following specific
(1) The isotherms of the viscosity and proton mag-
netic relaxation rate (which is the measure of the
water mobility), taken for solutions of salts forming
crystal hydrates, undergo an inversion in the region of
eutectic concentrations, which is not the case for solu-
tions of anhydrous salts [6, 7].