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been determined. In addition, possible mechanisms
of thermal decompositions of the two complexes are
proposed on the basis of thermogravimetric (TG)
analysis.
The heater wires were wound on the out wall of the
cell. The lid of the cell with a copper capillary was
sealed to the sample cell with cycleweld after the sam-
ple was loaded in it. The air in the cell was pumped out
and a small amount of helium gas (0.1 MPa) was in-
troduced into it to enhance the heat transfer in the cell.
The capillary was pinched off and the resultant fracture
was soldered with a little amount of solder to ensure
the cell for sealing. The evacuated can was kept within
ca. 10−3 Pa during the heat-capacity measurements so
as to eliminate the heat loss due to gas convection. Liq-
uid nitrogen was used as the cooling medium. One set
of chromel–copper thermocouples were used to detect
the temperature difference between the sample cell and
the inner shield. Likewise, another set of thermocou-
ples was installed between the inner and outer shields.
The temperature difference between them was kept to
be 0.5 mK during the whole experimental process. The
sample cell was heated by the standard discrete heating
method. The temperature of the cell was alternatively
measured. The temperature increment for a heating pe-
riod was 2–4 K, and temperature drift was maintained
at about 10−3 K min−1 in equilibrium period. All the
data were automatically picked up through a Data Ac-
quisition/Switch Unit (Model: 34970A, Aglient, USA)
and processed by a computer.
2. Experimental
2.1. Sample synthesis and characterization
According to [8], rare-earth oxides (Pr2O3, Nd2O3,
99.9%), hydrochloric acid and glycine were used to
prepare the experimental samples. First, the rare-earth
oxides were dissolved in hydrochloric acid to get the
aqueous solutions of the rare-earth chlordates, then
the aqueous solutions were mixed with glycine at the
mole ratio 1:3 at about pH = 3, which was regulated
by adding a suitable amount of NaOH. The mixed so-
lution was concentrated by evaporation, cooled and
filtered. The filtrate placed into a desiccator with P2O5
until crystalline products isolate from the solutions.
The crystals were filtered out and washed with anhy-
drous alcohol for three times. After this procedure, the
colour changed to green and pink needle-like crystals.
Finally, these crystals were desiccated in a dryer until
their mass became constant.
The purity of the crystals was proved to be more
than 99.90% by EDTA titrimetric analysis, good
enough to meet the requirements of the present calori-
metric study.
The mass of the [Pr(Gly)3(H2O)2]Cl3·2H2O and
[Nd(Gly)3(H2O)2]Cl3·2H2O used for heat-capacity
measurements was 1.3802 and 1.6322 g, which are
equivalent to 0.002535 and 0.002980 mol, based
on their corresponding molar mass of 544.467 and
547.76 g mol−1, respectively.
2.2. Adiabatic calorimeter
To verify the reliability of the adiabatic calorimeter,
the molar heat capacities for the reference standard
material ␣-Al2O3 were measured. The mass of ␣-
Al2O3 used for the measurement was 1.6382 g, which
was equivalent to 0.01607 mol based on its molar
mass M(Al2O3) = 101.9613 g mol−1. The deviations
of our experimental results from the recommended
values of the National Bureau of Standards [10] were
within 0.2% in the temperature range of 80–400 K.
Heat-capacity measurements were carried out in
a high-precision automatic adiabatic calorimeter de-
scribed in detail elsewhere [9]. The principle of the
calorimeter is based on the Nernst stepwise heating
method. The calorimeter mainly consists of a sam-
ple cell, an adiabatic (or inner) shield, a guard (outer)
shield, a platinum resistance thermometer, an electric
heater, two sets of chromel–copel (Ni 55%, Cu 45%)
thermocouples and a high vacuum can. The sample
cell was made of gold-plated copper and had an inner
volume of 6 cm3. Four gold-plated copper vanes of
0.2 mm thickness were put into the cell to promote heat
conduction from the cell to sample. The platinum re-
sistance thermometer was inserted into a copper sheath
which was soldered at the bottom of the sample cell.
2.3. Thermal analysis
A thermogravimetric analyzer (Model TGA/SDTA
851 e, METTLER TOLEDO, Switzerland) was
used for TG measurements of the solid complexes
under high purity (99.999%) nitrogen atmosphere
Liu, Beiping
