34
N.T. Madhu et al. / Thermochimica Acta 400 (2003) 29–36
presence of GA, indicating only a partial removal of
the ligand. The DTG peak of this stage is observed at
372 ◦C. The second stage of decomposition starts at
450 ◦C and is a continuous one. The steady mass loss
observed in this stage may be due to the expulsion
of the remaining half of the molecule of GA together
with volatilization of the residue of anhydrous cobalt
bromide [8] above 678 ◦C.
The iodide complex, [Co(GA)I2], undergoes a two
stage decomposition. The first stage begins at 179 ◦C
and ends at 461 ◦C with the DTG peak at 326 ◦C. The
corresponding mass loss (59.56%) is due to the de-
composition of the GA molecule. The second stage
starts at 461 ◦C and ends at 539 ◦C with the DTG peak
at 483 ◦C. The corresponding mass loss (35.71%) is
due to the decomposition of two iodide ions. The de-
composition complete at 539 ◦C and the final residue
is anhydrous cobalt oxide as confirmed by qualitative
analysis.
stability of the complexes is in the order bromide >
perchlorate > iodide > chloride > nitrate.
3.3. Kinetic aspects
study of the kinetics of decomposition of the com-
plexes. The kinetic parameters (the activation energy
E and the pre-exponential factor A) were calculated
using the Coats-Redfern equation [9].
ꢀ
ꢁ
ꢀ
ꢁ
g(α)
T2
AR
2RT
E
log
= log
1 −
−
φE
E
2.303RT
In the present case, a plot of L.H.S of this equation
against 1/T gives straight line (Figs. 7 and 8) whose
slope and intercept are used for calculate the kinetic
parameters by the least-square method. The goodness
of fit was checked by calculating the correlation coef-
ficient.
The entropy of activation ꢀS can be calculated us-
ing the equation
3.2. Structure and thermal stability correlation
kTs
eꢀS/R
The TG and DTG data reveal that although the
molecular composition of the complexes is the same,
viz., Co(GA)X2 ( X = ClO4−, NO3−, Cl−, Br− or
I−), the nature of the decomposition pattern is dif-
ferent. The perchlorate and nitrate complexes exhibit
a three-stage decomposition whereas a two-stage
decomposition is observed in the case of chloride,
bromide and iodide complexes. Within the series the
nitrate complex is the least stable and the bromide
complex is the most stable. The difference in thermal
A =
h
where, k is the Boltzmann constant, h the Planck’s
constant and Ts the peak temperature.
The various kinetic parameters calculated are given
in Table 2. The activation energy (E) in the different
stages are in the range 30.16–306.99 kJ mol−1. The re-
spective values of the pre-exponential factor (A) vary
from 2.72 × 10−2 to 7.84 × 107 s−1. The correspond-
ing values of the entropy of activation (ꢀS) are in the
Table 2
Kinetic parameters for the thermal decomposition of cobalt(II) complexes
Complex
Stage
E (kJ mol−1
)
A (s−1
)
ꢀS (J mol−1
)
[Co(GA)(ClO4)]ClO4
I
97.94
174.90
232.48
66.49
114.15
306.99
30.16
187.53
49.88
35.66
45.79
1.01
−218.61
−251.88
−173.89
−238.94
−218.07
−100.89
−281.02
−200.45
−266.63
−275.78
−162.08
II
III
I
II
III
I
II
I
I
1.46 × 103
[Co(GA)(NO3)]NO3
[Co(GA)Cl2]
3.73
55.96
7.84 × 107
2.72 × 10−2
6.33 × 102
0.15
[Co(GA)Br2]
[Co(GA)I2]
4.84 × 10−2
II
221.64
5.30 × 104