ISSN 1070-4272, Russian Journal of Applied Chemistry, 2006, Vol. 79, No. 12, pp. 1919!1923. + Pleiades Publishing, Inc., 2006.
Original Russian Text + E.V. Shkol’nikov, 2006, published in Zhurnal Prikladnoi Khimii, 2006, Vol. 79, No. 12, pp. 1942!1946.
OF SYSTEMS AND PROCESSES
Thermodynamic Characterization of the Amphoterism
of Solid Oxides MO in Aqueous Media
E. V. Shkol’nikov
St. Petersburg State Forestry Academy, St. Petersburg, Russia
Received October 2, 2006
Abstract-Thermodynamic method was used to calculate, with account of the formation of hydroxo
complexes, the molar solubilities of crystalline and amphoteric oxides MO as functions of the pH value
of an aqueous-alkaline nonoxidizing medium at 25oC and constants of heterogeneous equilibria in water
and alkaline and acid media for amphoteric metal MO, where M is Be, Mn3Zn, Cd, Hg, Sn, or Pb.
Previously, the thermodynamic method has been
used to calculate the effect of the pH value on
the molar solubility of solid hydroxides M(OH)
their amphoterism in aqueous media has been char-
acterized. Of interest is a thermodynamic calculation
of the solubility of solid oxides MO, where M is
Be, Sn, Pb, Cd, and 3d-elements from Mn to Zn, in
an aqueous alkaline medium.
Crystalline and amorphous oxides are widely used
in manufacture of ceramics and glasses, mineral paints
and enamels, oxide electrodes and catalysts, in metal-
lurgy, semiconductor electronics, and medicine .
As products of real dehydration of hydroxides, solid
oxides can be obtained by heating of hydroxides in
an inert medium. Anhydrous oxides are produced by
burning of metals in oxygen or by pyrolysis of car-
bonates, oxalates, and nitrates [43 6].
Ionic-covalent oxides MO are frequently formed
with defective crystal structure and, depending on
the preparation method and conditions, have notice-
able homogeneity regions and show differences in
coloration and chemical activity [5, 6].
The amphoteric nature of the oxides MO is poorly
understood, and quantitative data on their solubility
in aqueous media are rather scarce and frequently
contradictory . For example, according to the
reference book , the metal oxides under considera-
tion are poorly soluble (PbO, HgO) or virtually in-
soluble in water, all react with acids, and only BeO
(in fusion), ZnO, and PbO react with alkalis. Accord-
ing to , a white precipitate of Sn(OH)
, soluble in
acids and alkalis, is readily transformed to dark
amphoteric oxide SnO upon heating. Dehydration of
the white precipitate of Pb(OH)
yields red a-PbO at
100oC and yellow oxide b-PbO at lower temperatures.
Both modifications are dissolved in acids and con-
centrated alkalis . According to , the BeO oxide
reacts under heating with acids and alkalis, CoO is
amphoteric with predominance of basic properties,
and MnO reacts with alkalis under strong prolonged
heating. According to , black oxide a-SnO and red
a-PbO with a tetragonal structure are amphoteric with
predominance of basic properties. Brown cubic mod-
ification of CdO, black monoclinic CuO, and gray-
green cubic modification a-CoO form, when fused
with alkalis, cadmate, cuprates, and cobaltites(II).
Black iron(II) oxide reacts with acids and alkaline
solutions, but upon calcination, its chemical and
pyrophoric activity decreases. Uncalcined hexagonal
oxide a-BeO is hygroscopic, and that calcined below
500oC readily reacts with acids, and more difficultly,
with alkali solutions .
The procedure described in  was used to per-
form a thermodynamic calculation of the solubility of
crystalline and amorphous oxides MO in pure water
and aqueous alkaline medium (with addition of NaOH)
at pH 7315 and 25oC. The standard Gibbs energies
of formation of ions in aqueous solutions, DG
necessary for calculation of thermodynamic solubility
(SP = a
) were taken from ,
and the Gibbs energies of formation of solid oxides
at 298 K, from reference books [7, 9, 10]. The molar
solubility of MO in pure water and aqueous solutions