DEHYDRATION STUDIES OF Co(II), Cu(II) AND Zn(II) METHANESULFONATES
Tm2 of DTG under four heating rates (5, 10, 15 and
20ºC min–1) are 381, 383, 396, 404 K, respectively.
An increase in the heating rate causes an increase in
the peak temperature, which means the apparent de-
hydration temperature becomes higher with the in-
creasing heating rate.
Differential scanning calorimetry
The dehydration process of Co, Cu and Zn methane-
sulfonates is also monitored by DSC in the temperature
range of 40–220°C. Comparing experimental results,
we find the endothermic peaks of DSC cannot corre-
spond to the mass loss stages of TG, though TG and
DSC measurements are conducted under the same ex-
perimental conditions. The same problems also exist in
the dehydration of gadolinium and lutetium methane-
sulfonates [12] and the dehydration of hydrated rare
earth p-toluenesulfonates [13]. In order to clarify this
question, the procedural variables that can affect the
experimental results have been investigated. The vari-
ables include heating rates, Al pan states (unsealed and
sealed), sample mass and sample granularity, etc. In
our experiments, methanesulfonates sample are the
same size as they are ground in agate mortar before
measurements. Therefore, the sample granularity has
little effect on the experimental results.
In order to investigate the effect of Al pan state on
dehydration, the DSC curves of Zn(CH3SO3)2·4H2O
with different sample mass in unsealed and sealed Al
pan at the same heating rate are investigated in Fig. 5I
and II. As can be observed, the peak temperature of
sealed DSC curve is higher than that of unsealed. Mean-
while, the DSC curves of different sample mass under
sealed condition do not correspond to the TG/DTG
curve, so do Co and Cu methanesulfonates. However,
the DSC curves (a and b) of unsealed state seem to be
reasonable. Such differences could be explained as fol-
lows: in the case of sealed Al pan, the evolved water is
not easy to gasify, which may lead to the higher peak
temperature of DSC curve. Thus the DSC measure-
ments of Co, Cu and Zn methanesulfonates should be
conducted under unsealed condition in the experiments.
The DSC curves of Co(CH3SO3)2·4H2O and
Cu(CH3SO3)2·4H2O with different sample mass under
unsealed condition are shown in Figs 6 and 7. All
DSC curves show two endothermic peaks due to de-
hydration in two distinct steps, and the second dehy-
dration step needs more heat than the first one (Ta-
ble 1). Noticeably, the peak temperatures of DSC
curves become higher with the increasing sample
mass. Such fact suggests a correlation between peak
temperature and sample mass. From Table 1, it is evi-
dent that the DSC curves of 0.5–1.0 mg samples are in
good agreement with the TG/DTG curves, while DSC
Fig. 5 TG/DTG/DSC curves of Zn(CH3SO3)2·4H2O with differ-
ent sample mass in I – unsealed and II – sealed Al pan
curves of more than 1.0 mg samples lag behind
TG/DTG curves. With the sample mass increasing,
the decrease of sensitivity probably causes this fact.
On the basis of that, we conclude the sample mass of
DSC should be in certain proportion to that of TG. In
our experiment, the DSC curve of less than 1.0 mg
sample mass show good accordance with TG, and the
sample mass of TG is about 2.5 mg.
The effect of Al pan state and different sample
mass on the dehydration at the same heating rate has
been discussed above. As can be observed from
Fig. 5, the dehydration takes place in two steps using
both unsealed and sealed Al pan. But there is a little
difference in the first stage. For unsealed one, the
DSC curve contains two endothermic peaks, which
Table 1 Summary for DTG and DSC data of the dehydration pro-
cess of Co(CH3SO3)2·4H2O and Cu(CH3SO3)2·4H2O
Tm1
°C
/
Tm2
°C
/
mi/
mg
T1/
°C
T2/
°C
DH1/ DH2/
J g–1 J g–1
M
0.368
0.761
1.305
1.847
85.3 156.2 326.9 421.3
85.4 163.9 326.4 396.3
87.3 171.2 316.6 417.5
88.3 180.6 307.0 438.8
Co 83.0 164.0
Cu 95.3 122.2
0.489
93.6 121.3 279.4 314.6
1.101 102.3 134.4 297.0 334.0
2.261 107.6 142.4 316.6 350.8
3.244 112.6 148.1 318.8 351.4
mi – sample mass used for DSC measurement;
m1, Tm2 – peak temperature of DTG; T1, T2 – peak
temperature of DSC
T
J. Therm. Anal. Cal., 85, 2006
753