Non-isothermal studies for the decomposition course of CdC 2O4-ZnC2O4 mixture in air

Article abstract of DOI:10.1016/j.tca.2003.08.025

The thermal decomposition of cadmium-zinc oxalate (1:1mol ratios) mixture in air was investigated by non-isothermal thermogravimetric analysis. Intermediates and the final products were characterized by X-ray diffraction and scanning electron microscopy techniques. TG showed that the mixture dehydrated in two steps and decomposed exothermically to CdO-ZnO mixture at 390°C in a further two steps. The observed increase in the intensity of the X-ray diffraction lines by raising the calcination temperature from 400 to 800°C is attributed to the grain growth of CdO-ZnO mixture as revealed from SEM experiments. The kinetics of the oxalate decomposition steps was performed using the various reaction interface models and differential techniques of computational analysis of non-isothermal data. The activation parameters, calculated using the integral composite method, showed that the reactions were best described by the random nucleation model characteristic of solid state nucleation growth mechanism. The results of activation parameters for the different decomposition steps are compared and discussed.

Full text of DOI:10.1016/j.tca.2003.08.025

Thermochimica Acta 412 (2004) 55–62
Non-isothermal studies for the decomposition course
of CdC2O4–ZnC2O4 mixture in air
M.A. Gabal∗
Chemistry Department, Faculty of Science, Benha University, Benha, Egypt
Received 28 March 2003; received in revised form 27 August 2003; accepted 30 August 2003
Abstract
The thermal decomposition of cadmium–zinc oxalate (1:1 mol ratios) mixture in air was investigated by non-isothermal thermogravimetric
analysis. Intermediates and the final products were characterized by X-ray diffraction and scanning electron microscopy techniques. TG showed
that the mixture dehydrated in two steps and decomposed exothermically to CdO–ZnO mixture at 390 ◦C in a further two steps. The observed
increase in the intensity of the X-ray diffraction lines by raising the calcination temperature from 400 to 800 ◦C is attributed to the grain growth
of CdO–ZnO mixture as revealed from SEM experiments. The kinetics of the oxalate decomposition steps was performed using the various
reaction interface models and differential techniques of computational analysis of non-isothermal data. The activation parameters, calculated
using the integral composite method, showed that the reactions were best described by the random nucleation model characteristic of solid
state nucleation growth mechanism. The results of activation parameters for the different decomposition steps are compared and discussed.
© 2003 Elsevier B.V. All rights reserved.
Keywords: CdC2O4; ZnC2O4; DTA-TG; SEM; Kinetics; Oxalates
1. Introduction
sic metal carbonate precipitates have been investigated by
means of various techniques [5]. Mucka and Pavek [6] tested
the catalytic activity of the two-component oxide catalyst
CdO–ZnO prepared from the thermal decomposition of ni-
trate precursors by means of hydrogen peroxide decompo-
sition in a water solution. Giullermo et al. [7] studied the
electrical and optical properties of a mixed thin film oxides
of cadmium and zinc with different compositions prepared
by spray pyrolysis deposition on a glass substrates.
The thermal decomposition of metal oxalates is occasion-
ally adopted as a means of preparing metals or metal oxides
possessing pores, lattice imperfections and other characteris-
tics necessary for their function as reactive solids [1]. Much
researches [2] has been done on the structure, properties
and thermal decomposition behaviors of metal oxalates and
a considerable amount of information on the kinetics of the
thermal dehydration and decomposition of them have been
reported [3]. The combination of transition metal oxides may
results in the modification of their thermal behavior, geomet-
ric structure and electronic properties that lead to changes
in their catalytic functions [4]. Thus, the different synthesis
methods of the mixed metal oxides and their characteriza-
tion have been the subject of many numerous investigations.
The physico-chemical properties as well as the reactiv-
ity towards the reduction with hydrogen of CuO–Cr2O3
mixed oxides prepared by the thermal decomposition of ba-
The mixed metal oxalates [8]; FeCu(OX)2·3H2O,
−
CoCu(OX)2·3H2O and NiCu(OX)2·3.5H2O (OX = C2O4
)
have been prepared by coprecipitation technique and their
thermal behaviors in nitrogen and oxygen have been exam-
ined using thermogravimetry (TG), thermomagnetometry
(TM), differential scanning calorimetry (DSC) and evolved
gas analysis (EGA). In a previous paper [9], the thermal de-
composition processes taking place in the solid state mixed
oxalates of (CuC2O4–ZnC2O4·2H2O, 1:1 mol ratios) was
studied in air using differential thermal analysis thermo-
gravimetry (DTA-TG), X-ray diffraction (XRD) and scan-
ning electron microscopy (SEM) techniques. The kinetics
of the oxalate decomposition steps were performed under
non-isothermal conditions using different integral methods
of analysis of dynamic data.
∗
Present address: Institute of Mechanical Engineering and Mechanics
(IMVM), University of Karlsruhe, Geb. 30.70, Kaiser Street 12, Karlsruhe
76128, Germany. Tel.: +49-201-01048600; fax: +49-201-3222578.
E-mail address: mgabalabdo@yahoo.com (M.A. Gabal).
0040-6031/$ – see front matter © 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.tca.2003.08.025

56
M.A. Gabal / Thermochimica Acta 412 (2004) 55–62
The dynamic thermogravimetric analysis has been widely
SEM is performed using Jeol T 300 (Japan) scanning elec-
tron microscope operated at 15 keV. Mixtures were mounted
separately on aluminum substrates evacuated to 10−3 Torr
and precoated (20 and 5 min for each side of the four sides)
in a sputter-coater with a thin uniform gold/palladium film
to minimize charging in the electron beam. The applied volt-
age is 1.2–1.6 kV.
used as a tool to investigate the sample composition, the ther-
mal stability as well as the kinetic data relating the chemical
changes occur on heating [10]. In the present study, the ther-
mogravimetry was used to study the kinetics of the thermal
decomposition of CdC2O4·3H2O–ZnC2O4·2H2O (1:1 mol
ratios) mixture in air. The kinetic analysis of data were
performed and considered with reference to the various the-
oretical model equations and integral methods of dynamic
data analysis. The thermal decomposition course of the
mixture was characterized using DTA-TG, XRD and SEM
techniques.
2.3. Data analysis
Kinetics parameters such as the activation energy (E)
and the frequency factor (A) were calculated from the dy-
namic TG curves assuming the Diefallah’s composite in-
tegral method based either on the modified Coats–Redfern
equation (composite method I) or on Doyle’s equation (com-
posite method II) [9,12,13]. In the composite method I, the
modified Coats–Redfern equation was rewritten in the form:
2. Experimental
2.1. Materials
ꢀ
ꢁ
ꢂ
ꢃ
βg(α)
T2
AR
E
Pure metal oxalates were prepared by precipitation from
aqueous solutions of analytical reagents 3CdSO4·8H2O
or ZnCl2·2H2O on the addition of hot oxalic acid solu-
tion. The physical mixture CdC2O4·3H2O–ZnC2O4·2H2O
(1:1 mol ratios) was prepared by the impregnation method.
The appropriate mole ratios of the individual metal oxalates
were grounded together in a porcelain mortar. During the
vigorous stirring, drops of bi-distilled water were added to
assure complete homogeneity. The wet oxalate mixture was
dried at 60 ◦C for about 2 h.
On the basis of the thermal analysis results, the calcina-
tion products of the mixture were obtained by heating the
mixture in static air at 340 ◦C for 15 min, at 400 ◦C for 1 h
or at 800 ◦C for 2 h. Prior to the analysis, the calcinations
products were kept dry over anhydrous calcium chloride in
a desiccator.
ln
= ln
−
(1)
E
RT
where A is the frequency factor, E the energy of activation
and g(α) the kinetic model function calculated for the frac-
tion reacted (α) at temperature T and heating rate β. Table 1
shows the different kinetic model functions used in this work
[9]. In the composite method II, Doyle’s equation has been
rewritten in the form:
ꢀ
ꢁ
AE
E
log g(α)β = log
− 2.315 − 0.4567
(2)
R
RT
hence, the dependence of ln[βg(α)/T2] or log[g(α)β], calcu-
lated for the different α-values at their respective β-values
on 1/T must give rise to a single master straight line for
the correct form of g(α). Generally, the results always show
that [9,12,13] both the composite methods of analysis gave
equivalent curves and nearly identical values for the cal-
culated activation parameters. The composite method also
2.2. Apparatus
Thermal analysis experiments including thermogravime-
try (TG), differential thermogravimetry (DTG) and dif-
ferential thermal analysis (DTA) were carried out using a
Shimadzu DT-40 thermal analyzer. All the experiments were
carried out in a dynamic (30 ml min−1) atmosphere of air in
order to prevent the accumulation of the gaseous products.
Sample weights in the Pt-cell of 8–10 mg, were used in these
experiments, to ensure linear heating rates and accurate tem-
perature measurements. Highly sintered ␣-Al2O3 powder
(Shimadzu) was used as the reference material for the DTA
measurements. In the dynamic studies, the temperature was
Table 1
Kinetic equations examined in this work
Reaction model
g(α)
Symbol
One-dimensional diffusion
Two-dimensional diffusion
α2
D1
D2
D3
α + (1 − α) ln(1 − α)
Jander equation, three-dimensional [1 − (1 − α)1/3
]
2
diffusion
Ginsling–Braunshtein equation,
three-dimensional diffusion
Two-dimensional phase boundary
reaction
(1 − 2/3α) − (1 − α)2/3 D4
[1 − (1 − α)1/2
]
]
R2
R3
F1
Three-dimensional phase boundary [1 − (1 − α)1/3
raised up to 400 ◦C at heating rates of 1, 2, 3 and 5 ◦C min−1
.
reaction
X-ray powder diffraction analysis of the solid decompo-
sition products was carried out using a Philips PW 1710
diffractometer at ambient temperature using Ni-filtered Cu
K␣ radiation (λ = 1.5406 Å). For the identification purpose,
the relative intensities (I/I0) and the d-spacing (Å) were com-
pared with standard diffraction patterns of the ASTM pow-
der diffraction files [11].
First order kinetic, Mampel
unimolecular law
Random nucleation: Avrami
equation
Random nucleation: Erofeev
equation
Exponential law
[−ln(1 − α)]
[−ln(1 − α)]1/2
[−ln(1 − α)]1/3
ln α
A2
A3
E1

M.A. Gabal / Thermochimica Acta 412 (2004) 55–62
57
involves a model-fitting kinetic approach, since it does not
assume a particular reaction model, but it allows the choice
of the kinetic mechanism which best fits the data and gives
the highest correlation coefficient.
The activation parameters were then calculated using two
other integral methods: the Coats–Redfern method [14] and
the Ozawa method [15] assuming the best fit kinetic model
obtained from the composite method. The obtained activa-
tion parameters were then compared with those obtained ac-
cording to the composite method and discussed.
These steps were associated by 16.5 and 16.7% mass loss
due to the decomposition of CdC2O4 and ZnC2O4 in their
mixture to CdO and ZnO (theoretical mass loss in both
the steps is 16.2%), respectively. The total mass loss after
complete decomposition was calculated to be 42.4% of the
molecular mass of the reactants. From the above data, the
temperatures equivalent to the theoretical mass losses can
then be considered as the minimum temperatures which can
be selected for the calcination process.
The DTG curve displays four broad peaks, which are
closely correspond to the weight changes observed on the
TG curves, with maximum rates of loss at about 92, 136,
326, and 366 ◦C, respectively.
3. Results and discussion
The DTA curve reveals also four broad peaks maximized
at 90 ◦C (endo), 132 ◦C (endo), 330 ◦C (exo) and 365 ◦C
(exo), respectively. The two endothermic peaks are attributed
to dehydration. Dollimore et al. [18,19] studied the thermal
decomposition of ZnC2O4 and CdC2O4 in air and nitro-
gen atmospheres and found that the end product in case of
ZnC2O4 to be ZnO in both atmospheres whereas in case of
CdC2O4 the end product in nitrogen is Cd metal and in air
is CdO. Thus the exothermic nature of the DTA peaks ac-
companying the first oxalate decomposition process (i.e. the
decomposition process of CdC2O4) is attributed to the air
oxidation of Cd metal to CdO, whereas the exothermic be-
havior of the second oxalate decomposition process (i.e. the
decomposition process of ZnC2O4) is due to the catalytic
oxidation reaction of CO as it forms to CO2 on the surface
of ZnO product.
The above thermal analysis results indicate that,
CdC2O4·3H2O and ZnC2O4·2H2O decomposed in their
mixture similarly to their individual metal oxalates [18,19].
This behavior is ascribed to the absence of any solid so-
lution between the two metal oxalates and that each one
behaves as it is present alone in the mixture. A similar be-
havior was obtained for the thermal decomposition reaction
of CuC2O4–ZnC2O4·2H2O mixture [9].
3.1. Thermal analysis
Fig. 1 shows the characteristic features of heating
CdC2O4·3H2O–ZnC2O4·2H2O (1:1 mol ratio) mixture in
air atmosphere from the ambient temperature to 400 ◦C. It
shows the DTA, TG and DTG curves that were measured at
5 ◦C min−1. The TG curve shows four mass loss steps. The
first two steps occurring at 65–140 ◦C and are accompanied
by 11.5 and 8.5% mass loss, i.e. a total mass loss of 20.0%
which is attributed to the complete dehydration of the mix-
ture with the formation of anhydrous mixture (theoretical
mass loss, 20.3%). From the theoretical mass loss, the first
step can be attributed to the dehydration of CdC2O4·3H2O
and the second one is attributed to the dehydration of
ZnC2O4·2H2O. Nagase et al. [16] showed that the dehydra-
tion temperature is a function of the size of metal ion and
that it increases with the decrease in the radius of the metal
ion. The ionic radius [17] of Cd(II) and Zn(II) ions are
0.97 and 0.76 Å, respectively. Thus, CdC2O4·3H2O must
be dehydrated at lower temperature than ZnC2O4·2H2O.
The anhydrous mixture is thermally stable up to 290 ◦C
after which a two consecutive mass loss steps were oc-
curred in the temperature ranges 290–340 and 340–380 ◦C.
Fig. 1. DTA-TG curves of CdC2O4·3H2O–ZnC2O4·2H2O (1:1 mol ratios) mixture in air at heating rate of 5 ◦C min−1
.

58
M.A. Gabal / Thermochimica Acta 412 (2004) 55–62
Fig. 2. Characteristic parts of XRD patterns of CdC2O4·3H2O–ZnC2O4·2H2O (1:1 mol ratios) mixture calcined at different temperatures. Mixture calcined
at (a) 400 ◦C and (b) 800 ◦C.
3.2. X-ray diffraction
havior is ascribed to the sintering process. (ZnO)x(CdO)1−x
thin films of (ZnO)x(CdO)1−x prepared by spray pyroly-
sis on a glass substrates [7] showed the presence of CdO
XRD pattern for low zinc concentrations and a mixture of
CdO (cubic) and ZnO (hexagonal) phases for the higher
concentrations. The XRD pattern of this system shows also
an increase in the crystallite sizes with annealing. A simi-
lar trend was also obtained for CuO–ZnO mixture prepared
from CuC2O4–ZnC2O4·2H2O (1:1 mol ratios) mixture [9].
The XRD diffraction pattern of the initial mixture
matched the standard data compiled in the JCPDS data
[11] no. 14–750 and 25–1029 for individual CdC2O4·3H2O
and ZnC2O4·2H2O, respectively. The XRD pattern of the
decomposition products at 340 ◦C reveals the characteristic
XRD lines for CdO (JCPDS no. 5–640). The presence of
some XRD lines characteristic of hydrated zinc oxalate may
be attributed to the hydration of the anhydrous zinc oxalate
[20] formed at this decomposition stage.
3.3. Electron microscopy
Fig. 2 shows the characteristic X-ray diffraction patterns at
ambient temperature for mixture calcined at 400 and 800 ◦C
for the mixture calcined at 400 ◦C, the presence of the indi-
vidual characteristic XRD lines (Fig. 2a) of both CdO and
ZnO (JCPDS no. 5–640 and 21–1486, respectively) indicate
the complete decomposition of the mixture without forma-
tion of any solid solution. The broadening of the XRD lines
is attributed to the weak crystallinity of the decomposition
products. Generally no solid solution was detected between
the two metal oxides even by raising the calcination temper-
ature to 800 ◦C. This was achieved by comparing the XRD
patterns of the mixture calcined at 400 ◦C with that cal-
cined at 800 ◦C (Fig. 2a and b), where the only difference
is the increase in the intensities of the XRD lines. This be-
Samples of the initial mixture as well as intermediate
decomposition products formed at different temperatures
were examined using scanning electron microscopy to
characterize the textural changes that occurring during the
decomposition reaction of the mixture. The parent mix-
ture has very small individual crystallites (<0.5 ␮m) which
make the textural features resolution difficult. The change
in the particle size and shape throughout the decomposition
process is shown in Fig. 3. The scanning electron micro-
graph of the mixture calcined at 340 ◦C (Fig. 3a) shows,
in addition to the large irregularly shaped crystals of differ-
ent sizes, aggregates of approximately spherical particles
showing no regular or characteristic crystalline structures.

M.A. Gabal / Thermochimica Acta 412 (2004) 55–62
59
roughening of the crystal edges, which provide evidence
that the oxides formed at this stage are microcrystalline as
previously suggested by XRD experiments. In the mixture
calcined at 800 ◦C, these fine granules were coalesced into
large aggregates of cubic crystals of different sizes (Fig. 3c).
3.4. Kinetic studies
The kinetics of the thermal decomposition in air of
CdC2O4 to CdO (step III) and of ZnC2O4 to ZnO (step IV)
in CdC2O4·3H2O–ZnC2O4·2H2O (1:1 mol ratio) mixture
were studied under non-isothermal conditions. Typical plots
of weight changes from dynamic measurements at different
constant heating rates versus decomposition temperature
for these two steps are shown in Fig. 4. Kinetic analysis of
data according to the composite method showed that the de-
composition steps were best described by Avrami–Erofeev
random nucleation equations (A2 and A3) characteristic of
solid state nucleation growth mechanism, consistent with
the textural changes that accompanying the decomposition
as revealed by SEM experiments. The phase boundary con-
trolled reaction equations (R2 and R3), in which an interface
moves at constant velocity and nucleation occurs virtually
instantaneously, gave also a satisfactory results. The other
model equations give a less satisfactory fit of the data and
the exponential law equation (E1) gave the least fit. Table 2
lists the values of activation parameters, calculated accord-
ing to the composite method based on the Doyle’s equation
(composite II method) assuming different heterogeneous
solid state models, for the non-isothermal decomposition
of CdC2O4 and ZnC2O4 in their mixture. Fig. 5 shows the
typical results calculated by the composite method on the
basis of the Doyle’s equation assuming A2 and E1 models.
The activation parameters were also calculated on the
basis of the Coats–Redfern and Ozawa methods assuming
A2 model (as revealed by the composite method). Then
the calculation results according to the three computational
methods were summarized in Table 3. The listed values for
the activation parameters in case of the Coats–Redfern and
the Ozawa methods are the average ones calculated at dif-
ferent heating rates (β) and fractional reaction (α) values,
respectively.
Approximately constant E with α was obtained on plotting
the calculated activation energy (E), estimated by the Ozawa
method, versus the fractional reacted vales (α) for the two
oxalate decomposition reactions (Fig. 6). As a result, this
will enhance that the rate-limiting step is a single reaction
step [9] in accordance with the single master straight line
for all (α, β, T) values obtained at different heating rates
estimated by the composite method (Fig. 5).
The results of data analysis using the different compu-
tational methods (Table 3) show a good agreement within
experimental errors between the values of the calculated ac-
tivation parameters. The data also shows that, the integral
composite method of analysis gave the least standard devi-
ation in the calculated experimental parameters. Generally,
Fig. 3. Scanning electron micrograph showing the changes in tex-
ture and morphology that accompany the thermal decomposition of
CdC2O4·3H2O–ZnC2O4·2H2O (1:1 mol ratios) mixture in air. Mixture
calcined at (a) 340 ◦C, (b) 400 ◦C and (c) 800 ◦C (scale bar 10 ␮m).
At this calcinations temperature the DTA-TG results show
only the decomposition of CdC2O4 content into CdO while
ZnC2O4 content remains undecomposed. The large crystals
can be assigned to the undecomposed zinc oxalate while the
particles aggregates can be ascribed to the decomposition
of cadmium oxalate content into small and fine granules
with roughening of the crystal surfaces. At 400 ◦C, the zinc
oxalate content was decomposed, so, the micrograph ob-
tained for the mixture at this temperature (Fig. 3b) shows
the complete breaking into fine granules, with superficial

60
M.A. Gabal / Thermochimica Acta 412 (2004) 55–62
Fig. 4. Dynamic measurements for the thermal decomposition of CdC2O4·3H2O–ZnC2O4·2H2O (1:1 mol ratios) mixture in air. (a) Decomposition of
CdC2O4 to CdO and (b) ZnC2O4 to ZnO. Heating rate: curve A, 1 ◦C min−1; curve B, 2 ◦C min−1; curve C, 3 ◦C min−1 and curve D, 5 ◦C min−1
.
Table 2
Activation parameters of the non-isothermal decomposition in air of CdC2O4 and ZnC2O4 in their mixture, calculated according to the composite method
II, assuming different kinetic models
Model
Decomposition of CdC2O4 (step III)
E (kJ mol−1 log A (min−1
Decomposition of ZnC2O4 (step IV)
E (kJ mol−1 log A (min−1
)
)
r
)
)
r
D1
D2
D3
D4
R2
R3
F1
A2
A3
E1
355
378
407
387
218
225
241
142
109
133
21
23
25
24
8
8
9
2
4
30.8
32.8
35.0
33.1
18.3
18.8
20.9
11.9
8.9
2.1
2.2
2.4
2.3
0.7
0.8
0.9
0.2
0.4
2.3
0.872
0.873
0.872
0.873
0.952
0.951
0.946
0.994
0.855
0.467
529
562
604
575
318
327
357
210
163
168
35
44
47
45
15
16
18
3
30.0
30.0
30.0
30.0
25.4
26.1
28.5
16.6
12.7
15.2
3.4
3.7
3.9
3.8
1.3
1.4
1.6
0.2
0.3
3.2
0.817
0.816
0.816
0.816
0.961
0.913
0.907
0.995
0.982
0.402
3
38
25
13.3
Fig. 5. Composite analysis of TG data for the non-isothermal decomposition in air of (A) CdC2O4 and (B) ZnC2O4 in their mixture based on Doyle’s
equation assuming: (1) A2 model; (2) E1 model.

M.A. Gabal / Thermochimica Acta 412 (2004) 55–62
61
Table 3
critically on the size and charge of the metal ion. Whereas in
these decompositions where the oxide is produced in nitro-
gen the decomposition temperatures represents the energy
required to break the C–OI bond and this will depend less
critically on the nature of cation. If the reaction proceeds
by the rupture of the C–OI bond, this would be followed by
the rupture of the second M–OI bond because the inability
of the metal to accommodate two oxygen atoms.
The thermal decomposition of ZnC2O4 in air or nitrogen
atmospheres results in the production of ZnO, whereas the
thermal decomposition of CdC2O4 in nitrogen produces Cd
metal, which in air is oxidized to CdO. Since the ionic radius
[17] of Cd(II) is much higher than that of Zn(II), thus, based
on the coulombic attraction the zinc will have a stronger
M–OI bond than that of cadmium. This would increase the
energy needed to break the Zn–O bond, and thus increases
the activation energy for the reaction. Dollimore and Griffths
[18] also showed that, there is a decrease in the decompo-
sition temperature with increase in the ionic radius of the
metal ion. A decrease in the M–OI bond strength would thus
lead to a decrease in the decomposition temperature. Thus,
the CdC2O4 will posses a lower decomposition temperature
than ZnC2O4 as can be observed from the DTA-TG curves
(Fig. 1).
Activation parameters of the non-isothermal decomposition in air of
CdC2O4 and ZnC2O4 in their mixture calculated according to A2 model
Method of
analysis
Decomposition of
CdC2O4 (step III)
Decomposition of
ZnC2O4 (step IV)
E
log A
(min−1
E
log A
(min−1
)
(kJ mol−1
)
)
(kJ mol−1
)
Composite II
Coats–Redfern
Ozawa
142
136
153
2
10
4
11.9
11.3
12.9
0.2
0.7
0.3
210
216
198
3
16
5
16.6
17.2
15.6
0.2
1.4
0.5
on comparing the activation energies obtained in Table 3
for the thermal decomposition of CdC2O4 and ZnC2O4 in
their mixture it is obvious that the activation energy for the
thermal decomposition of zinc oxalate is higher (about 30%
more) than that needed for the thermal decomposition of
cadmium oxalate. In addition, the DTA-TG results (Fig. 1)
shows that the thermal stability of CdC2O4 is less than that
of ZnC2O4.
In the oxalates of bivalent metals:
In addition, the thermal stability of the metal oxalates de-
crease with the increase in the electron affinity of the metal
ion. The oxidation–reduction potential measures the relative
electron affinities of the metal ions forming oxalates, since
the extent to which the M–OI bond is covalent depend on
the electronegativity of the metal. Decomposition will oc-
cur when a temperature is reached at which the rupture of
the M–OI link is possible or at which the rupture of C–OI
bond occurs. For these oxalates which produce the metal in
nitrogen the decomposition temperature represents the tem-
perature at which the M–OI link ruptured and will depend
−
Dq values of H2O is near to that of C2O4 [21]. The reduc-
tion potential of cadmium (EC0 d2+
= −0.403 V) is higher
/Cd
than that of zinc (EZ0n2+
= −0.763 V), therefore, the
/Zn
Fig. 6. Plots of the activation energies versus α calculated by Ozawa method for the thermal decomposition of CdC2O4 and ZnC2O4 in their mixture.

62
M.A. Gabal / Thermochimica Acta 412 (2004) 55–62
thermal stability of CdC2O4 is lower than that of ZnC2O4.
The same effect was observed for central metal ions in potas-
sium salts of metal oxalato complexes [21].
Acknowledgements
The author would like to thank Prof. Dr. A.A. El-Bellihi,
Chemical Department, King Abdul-Aziz University, Jedah
(KSA) for doing SEM experiments.
Moreover, the effect of the enthalpy of the reaction on the
sample temperature and activation energy of the decompo-
sition cannot be ignored [22,23]. The exothermicity of the
CdC2O4 decomposition is more than the ZnC2O4 decompo-
sition as evident from DTA curve (Fig. 1). This is due to the
fact that the CdC2O4 decomposition in air is accompanied
by the exothermic oxidation reaction of cadmium metal to
CdO. The excessive heat transfer would allow ease rupture
of the M–OI bond in the CdC2O4 than in the ZnC2O4.
References
[1] V.V. Boldyrev, M. Bulens, B. Delmon, The Control of the Reactivity
of Solids, Elseveir, Amsterdam, 1979.
[2] D. Dollimore, Thermochim. Acta 117 (1987) 331.
[3] A. Coetzee, M.E. Brown, D.J. Eve, C.A. Strydom, J. Therm. Anal.
41 (1994) 357.
[4] W.M. Shahheen, M.M. Selim, Thermochim. Acta 322 (1998) 117.
[5] Pospisil, Thermochim. Acta 119 (1987) 261.
4. Conclusions
[6] Mucka, J. Pavek, Z. Phys. Chem. 131 (1982) 217.
[7] S. Giullermo, M. Arturo, V. Osvaldo, C. Francisco, C. Gerardo, Thin
Solid Films 373 (2000) 235.
[8] A. Coetzee, D.J. Eve, M.E. Brown, J. Therm. Anal. 39 (1993)
947.
[9] M.A. Gabal, Thermochim. Acta 402 (2003) 199.
[10] L.H. Tagle, F.R. Diaz, L. Rivera, Thermochim. Acta 118 (1987) 111.
[11] International Center for Diffraction Data, JCPDS, PDF2 Data Base
Swarthmore, PA, USA, 1996.
[12] El-H.M. Diefallah, Thermochim. Acta 202 (1992) 1.
[13] El-H.M. Diefallah, A.Y. Obaid, A.H. Qusti, A.A. El-Bellihi, M.A.
Abdel Wahab, M.M. Moustafa, Thermochim. Acta 274 (1996) 165.
[14] A.W. Coats, J.P. Redfern, Nature 20 (1964) 68.
[15] T. Ozawa, Bull. Chem. Soc. Jpn. 38 (1965) 1881.
[16] K. Nagase, K. Sato, N. Tanaka, Bull. Chem. Soc. Jpn. 48 (1975) 439.
[17] J.A. Dean, Lange’s Hand Book of Chemistry, 12th ed., McGraw-Hill,
New York, 1979.
[18] D. Dollimore, D.L. Griffths, J. Therm. Anal. 2 (1970) 229.
[19] D. Dollimore, D.L. Griffths, D. Nicholson, J. Chem. Soc. (1963)
2617.
[20] J.D. Danforth, J. Dix, J. Am. Chem. Soc. 93 (1971) 6843.
[21] K. Nagase, Bull. Chem. Soc. Jpn. 45 (1972) 2166.
[22] El-H.M. Diefallah, M.A. Gabal, A.A. El-Bellihi, N.A. Eissa, Ther-
mochim. Acta 376 (2001) 43.
DTA-TG experiments of CdC2O4·3H2O–ZnC2O4·2H2O
(1:1 mol ratio) mixture in air show that the mixture was
completely decomposed to CdO–ZnO mixture through four
well-defined steps. Intermediate decomposition products
as well as the final products were characterized by X-ray
diffraction and SEM techniques. XRD experiments show
the absence of any formed solid solution even by raising
the calcinations temperature to 800 ◦C. The increase in the
intensity of the diffraction lines by raising the calcinations
temperature from 400 to 800 ◦C is attributed to the grain
growth of the decomposition products. SEM experiments
confirmed this result as the fine granules obtained at 400 ◦C
were coalesced into large aggregates of large crystals when
calcined at 800 ◦C. Kinetic analysis of the data for the
oxalate decomposition steps according to the integral com-
posite method showed that the oxalate decomposition steps
are best described by the Avrami–Erofeev equation charac-
teristic of the solid state nucleation growth mechanism. The
activation parameters were then calculated and discussed
in view of the ionic radius and the electronegativity of the
metal ions.
[23] El-H.M. Diefallah, M.A. Mousa, A.A. El-Bellihi, E.H. El-Mossalamy,
G.A. El-Sayed, M.A. Gabal, J. Anal. Appl. Pyrl. 62 (2002) 205.