Thermodynamics and phase equilibrium studies of silver halide-cobalt(II) halide systems

Article abstract of DOI:10.1007/s10973-008-9135-7

Phase diagrams for the systems AgCl-CoCl₂and AgBr-CoBr ₂were determined by differential scanning calorimetry. The systems are of the eutectic type. Eutectic points are at 271±2 K, 19.5 mol% CoCl₂and 653±2 K, 17.0 mol% CoBr

Full text of DOI:10.1007/s10973-008-9135-7

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Journal of Thermal Analysis and Calorimetry, Vol. 93 (2008) 3, 721–726
THERMODYNAMICS AND PHASE EQUILIBRIUM STUDIES OF SILVER
HALIDE–COBALT(II) HALIDE SYSTEMS
E. Krzyüak* and Alina Wojakowska
WrocÓaw Medical University, Department of Inorganic Chemistry, Laboratory of Thermal Analysis, ul. Szewska 38
50 139 WrocÓaw, Poland
Phase diagrams for the systems AgCl–CoCl2 and AgBr–CoBr2 were determined by differential scanning calorimetry. The systems
are of the eutectic type. Eutectic points are at 271±2 K, 19.5 mol% CoCl2 and 653±2 K, 17.0 mol% CoBr2, respectively. The solid
solubility does not exceed 2 and 4 mol% in the systems AgCl–CoCl2 and AgBr–CoBr2, respectively. Thermodynamic activities of
components in molten mixtures and molar free enthalpies of mixing were determined for both systems on the basis of liquidus
curves. Deviations from ideality were not found to be considerable.
Keywords: AgBr, AgCl, CoBr2, CoCl2, DSC, phase diagram
Introduction
hexahydrates: CoCl2·6H2O (98%, Aldrich) and
CoBr2·6H2O (99%, Aldrich). Silver chloride and sil-
ver bromide were precipitated from a dilute aqueous
solution of AgNO3 (reagent grade) with a solution of
HCl or HBr, respectively. Then the product obtained
was carefully washed, dried under vacuum and
melted. Details of the preparation technique were re-
ported in our previous works [10, 11].
Studies of thermodynamics and phase equilibria are
of primary importance in the search for new materials
having interesting properties [1–6]. Knowledge of
phase diagrams is necessary for many applications.
For instance, in preparation of cobalt dichloride
graphite compounds by intercalation of CoCl2 from
molten salts [7], the phase diagrams played a decisive
role in the choice between the CoCl2–NaCl and the
CoCl2–KCl systems. It is evident that the melt compo-
sitions and temperatures of the intercalation process
were imposed by the phase diagram. Chemical reac-
tions performed in molten salt mixtures, especially in
low melting eutectics, allow to prepare some impor-
tant materials like CoAl2O4 at a lower temperature
than in solid-state reactions methods [8]. The study of
the appropriate phase diagrams of the eutectic type is
thus appreciable.
Phase equilibria in the AgCl–CoCl2 and
AgBr–CoBr2 systems were investigated by differen-
tial scanning calorimetry performed with a Mettler
Toledo DSC 25 measuring cell with TC15 TA Con-
troller. Basic software STARe Version 6.0 comple-
mented by a few optional Mettler Toledo programs
were used to carry out the measurements and to read
the results. The calorimeter was calibrated with melt-
ing points of indium, tin, lead, zinc and aluminium.
Appropriate quantities of the components for
DSC experiments were weighed on Mettler To-
ledo AT 261 balance (0.01 mg). The composition of
samples was taken at intervals of around 2–3 mol% or
less. Mixtures of salts were prepared under argon and
red light, directly in small silica ampoules (6 mm in
outer diameter and 14 mm in height), next used for
DSC measurements. The total mass of a mixture was
between about 20 and 145 mg. Ampoules were sealed
under vacuum and heated in an electric furnace above
the melting point of the respective cobalt dihalide.
Then they were cooled slowly to 400 K and annealed
during 24 h. The equilibrated samples were then
cooled to room temperature.
In this work, the phase diagrams for
AgCl–CoCl2 and AgBr–CoBr2 systems have been
completed and thermodynamic properties, based on
the measured liquidus temperatures have been calcu-
lated. Literature data on these systems are very lim-
ited. Admittedly, both systems were supposed to be of
the eutectic type [9] but no data besides the values of
temperature and composition of the eutectic points
were given.
Experimental
DSC runs for all the samples were carried out
with a heating rate of 2 and 5 K min–1 from room tem-
Anhydrous cobalt(II) chloride and anhydrous co-
balt(II) bromide were obtained from the respective
*
Author for correspondence: edward.krzyzak@chnorg.am.wroc.pl
1388–6150/$20.00
Akadémiai Kiadó, Budapest, Hungary
Springer, Dordrecht, The Netherlands
© 2008 Akadémiai Kiadó, Budapest

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KRZY¨AK, WOJAKOWSKA
perature to the liquidus temperature and with the
same rate in the opposite direction. In some cases,
heating and cooling curves were also taken with the
rate of 20, 15, 1 or 0.4 K min–1.
of CoCl2 (999 K [11]). The latter is quite close to the
eutectic temperature in the AgBr–CoBr2 system. That
is why the thermal effects corresponding to the
eutectic reaction and these involving the polymorphic
transition of CoBr2 are superimposed. This last was
observed for samples containing more than
70 mol% CoBr2. A few examples of DSC runs for
mixtures rich in cobalt dibromide are shown in Fig. 3.
Similar invariant equilibria: eutectic and metaeu-
tectic, one close to another, were observed for in-
stance in the GeSe2–SnSe system [14].
Results and discussion
Phase diagrams
The phase diagrams for the AgCl–CoCl2 and
AgBr–CoBr2 systems determined in this study are
presented in Figs 1 and 2.
In the range of composition between
x(CoBr2)=0.715 and x(CoBr2)=0.985, the onset of
overlapping peaks gives a rather constant value of
637.1±1.1 K, which has been ascribed to the invariant
equilibrium involving polymorphic transition of
CoBr2. This effect being negligible below
70 mol% CoBr2, the range of composition between
x(CoBr2)=0.043 and x(CoBr2)=0.703 was considered
for determining the eutectic temperature 652.8±0.3 K.
The compositions of the eutectic points were de-
termined by two methods: extrapolation of two
liquidus curves in each system to the appropriate
eutectic invariance and plotting values of thermal ef-
fects of the eutectic reaction vs. mole fraction of one
salt (the Tamman method [15, 16]). The maximal
value of the thermal effect should appear at the
Both systems are of the eutectic type. The tem-
perature of the eutectic reaction was found to be
671.0±0.5 and 652.8±0.3 K in the AgCl–CoCl2 and
AgBr–CoBr2 systems, respectively. The mole fraction
of the cobalt dihalide corresponding to the eutectic
point was found to be x(CoCl2)=0.195 and x(CoBr2)=
0.170 for AgCl–CoCl2 and AgBr–CoBr2, respec-
tively.
It should be noted that additional invariant equi-
libria appear on the phase diagrams: at 979 K
(AgCl–CoCl2) and at 637 K (AgBr–CoBr2). They re-
sult from the polymorphic transitions of the pure com-
ponents: CoCl2 at 979 K [11] and CoBr2 at 650 K
[11–13]. The former is not far from the melting point
Fig. 1 Phase diagram of the AgCl–CoCl2 system
Fig. 2 Phase diagram of the AgBr–CoBr2 system
722
J. Therm. Anal. Cal., 93, 2008

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SILVER HALIDE–COBALT(II) HALIDE SYSTEMS
solutions in the AgBr–CoBr2 system seem to be a lit-
tle larger, but not exceeding 4 mol% (Fig. 2). These
limits of solid solutions are also in agreement with
values of intersection points of the respective edges of
Tamman triangles with the abscissa (Fig. 4).
Thermodynamics of molten salt solutions
On the basis of liquidus curves, the thermodynamic
activities of components in molten solutions of
AgCl–CoCl2 and AgBr–CoBr2 systems have been
calculated in the whole range of compositions. They
are presented in Figs 5 and 6, respectively. The inte-
gral molar free enthalpies of mixing and the integral
molar excess free enthalpies for both systems are
shown in Fig. 7. Equations applied for the purposes of
calculation of the thermodynamic properties were
given in our previous papers [17–19]. Calculations
were made assuming that limiting solid solutions are
negligible.
Fig. 3 Examples of DSC curves on the CoBr2 rich side (heat-
ing rate of 5 K min–1), showing overlapping peaks cor-
responding to the eutectic and solid-state transition
Additional thermochemical data used in the cal-
culations, i.e. values of the enthalpies of fusion and
polymorphic transitions [11, 20, 21] as well as avail-
able heat capacity data for the respective phases as a
function of temperature [20, 22, 23] are gathered in
Table 1.
The knowledge of enthalpies of transition is
more important than that of variation in heat capacity
since the former have a relatively greater effect on the
value of the free enthalpy of a substance. If nothing is
known about the heat capacity of one or two phases
involved in a liquid–solid or solid–solid phase transi-
tion, then the difference between respective molar
heat capacities may be neglected without affecting
most calculations too gravely [23].
Fig. 4 Tamman triangles for the eutectic reactions:
a – AgCl–CoCl2 system; b – AgBr–CoBr2 system
eutectic point. Tamman triangles obtained are pre-
sented in Fig. 4. Thermal effects used for construction
of the Tamman triangle in the AgBr–CoBr2 system
(Fig. 4b) were considered as resulting merely from
the eutectic event.
Limiting solid solutions in the AgCl–CoCl2 and
AgBr–CoBr2 systems are not large. The thermal ef-
fect, corresponding to the eutectic reaction in the
AgCl–CoCl2 system, has been observed on DSC
scans practically in the whole range of compositions,
between x(CoCl2)=0.029 and x(CoCl2)=0.985
(Fig. 1). Hence limiting solid solution in this system
do not exceed 2 mol% on either side. Limiting solid
Fig. 5 Thermodynamic activities in the AgCl+CoCl2 molten
solutions
J. Therm. Anal. Cal., 93, 2008
723

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KRZY¨AK, WOJAKOWSKA
Table 1 Thermochemical data of the pure components
Cp=A+BT+CT–1/J mol–1 K–1
Enthalpy of fusion or
solid-state transition/kJ mol–1
Component
Ref.
Ref.
Phase
Liquid
A
B
C
57.30
DfusH=13.22
AgCl
AgBr
[20]
[21]
[20]
[22]
Solid
41.30
0.03323
0.21402
Liquid
Solid
67.722
DfusH=9.1365
–63.839
5896762
–498000
Liquid
Solid
Solid
99.1610
82.09
DfusH=36.4
DtrH=9.6
CoCl2
CoBr2
[11]
[23]
[23]
0.00674
0.02092
Liquid
Solid
Solid
DfusH=27.2
DtrH=0.18
[11,13]
92.05
73.39
Fig. 6 Thermodynamic activities in the AgBr+CoBr2 molten
solutions
When the AgCl–CoCl2 and AgBr–CoBr2 sys-
tems are concerned, information on the heat capacity
of the high-temperature solid form of CoCl2 as well as
that of liquid CoBr2 is lacking (Table 1). Accordingly,
in this work, the following assumptions have been
made:
Fig. 7 Molar free enthalpy of mixing GM for molten solutions:
p – AgCl+CoCl2; l – AgBr+CoBr2. Excess molar free
enthalpy GE for molten solutions: D – AgCl+CoCl2;
¡ – AgBr+CoBr2
• the difference between the heat capacity of liquid
CoCl2 and that of the high-temperature solid form
of CoCl2 was assumed to be zero
gram (Fig. 2), the low-temperature modification of
CoBr2 has no influence on liquid-solid equilibria in
the AgBr–CoBr2 system.
• below the transition temperature of 979 K, the dif-
ference between the heat capacity of supercooled
liquid CoCl2 and that of the low-temperature solid
form of CoCl2 was used
Thermodynamic activities (Figs 5 and 6) and ex-
cess free enthalpies in the AgCl+CoCl2 and
AgBr+CoBr2 molten mixtures (Fig. 7), calculated on
the basis of the respective phase diagrams, do not in-
dicate considerable deviations from ideality. The
chloride system may be compared for instance with
the alkali chloride – CoCl2 systems [24, 25], where
the above quantities reveal more pronounced negative
• the difference between the heat capacity of liquid
CoBr2 and that of the high-temperature solid form
of CoBr2 was assumed to be zero
The polymorphic transition in CoBr2 was not
considered in the thermodynamic calculations for
molten mixtures because, according to the phase dia-
724
J. Therm. Anal. Cal., 93, 2008