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LIU et al.
Laboratory of Dalian Institute of Chemical Physics,
Chinese Academy of Sciences, The structure and
operation principle of the calorimeter have been
described in detail elsewhere [11–13].
The adiabatic calorimeter mainly consisted of a
sample cell with electric heater, a miniature platinum
resistance thermometer, an inner and outer adiabatic
shields with electric heater; two sets of six-junctions
chromel-constantan thermopiles and a high vacuum
can [14–16].
The heating rate was 0.1 to 0.4 K min–1; the tempera-
ture increments of each experimental point were be-
tween 1 and 4 K; the heating duration was 10 min and
the temperature drift rates of the sample cell, which
was measured in an equilibrium period, were kept
within 10–3 to 10–4 K min–1.
Prior to the heat capacity measurement of the
sample, the molar heat capacities of a-Al2O3 (NIST,
Standard Reference Material 720) were measured from
78 to 400 K to verify the reliability of the adiabatic cal-
orimeter. The results showed that the deviation of our
calibration data from those recommended by
NIST [14] was within ±0.3%.
The sample cell was made of god-plated copper
with an inner volume of 6 cm3. The heater wire was bi-
filarly wound on the outside wall of the cell, in which
there were four gold-plated copper vanes of 0.2 mm in
thickness with X-shape to promote heat distribution.
The thermometer (IPRT No. 2, fabricated by Shanghai
Institute of Industrial Automatic Meters, 16 mm in
length, 1.6 mm in diameter and a nominal resistance
of 100 W, at 275.15 K) was inserted into a horizontal
copper sheath soldered on the bottom of the sample
cell. The thermometer was calibrated in terms of
ITS-90 by the Station of Low-temperature Metrology
and Measurements, Academia Sinica. The resistance
of the thermometer was measured by a 7 1/2 Digit
NanoVolt/Micro Ohm Meter, (Model 34420, Agilent,
USA). The two sets of thermopiles were installed be-
tween the sample cell and the inner shield and between
the inner and the outer shields, respectively. After the
sample cell was loaded, sealed and evacuated, a little
helium gas (0.1 MPa) was introduced into the cell
through a capillary on the lid of the cell to enhance the
heat transfer. Then, the capillary was pinched off and
soldered from the end.
First, the vacuum can was evacuated to about
10–3 Pa, then the sample cell was heated and the tem-
perature difference between the cell and the inner
shield was monitored by a set of differential
thermocouples. The signal from the thermocouples
was used to control the heating current through the
heating wires distributed on the walls of the inner
shields. The temperature difference between inner and
outer shield was monitored by another set of
thermocouples. Both shields were heated under the
control of a Temperature Controller (Model 340, Lake-
shore, USA) and kept at the same temperatures as that
of the sample cell. The electrical energy introduced
into the sample cell and the subsequent equilibrium
temperature of the cell were automatically measured
by a Data Acquisition/Switch Unit (Model 34970A,
Agilent, USA) and the 7 1/2 Digit Nano Volt/Micro
Ohm Meter, respectively. Both energy and temperature
data were processed on line by a computer.
In the present paper, the mass of Ho(Asp)Cl2·6H2O
used for the heat capacity measurement was 0.7826 g,
which was equivalent to 0.0016 mol on the base of its
molar mass M=477.06 g mol–1.
Thermal analysis
Thermal analysis of Ho(Asp)Cl2·6H2Owas performed
using a Differential Scanning Calorimeter (DSC-141,
Setaram, France) and a thermogravimetric analyzer
(Model: DT-20B, Shimadzu, Japan).
The DSC test was carried out at the heating rate
of 10 K min–1 under high purity nitrogen with flowing
rate of 50 mL min–1. The mass of the sample used in
the experiment was 2.9 mg. The calibrations for the
temperature and heat flux of the calorimeter were
performed prior to the experiment. The temperature
scale was calibrated by measuring the melting points
of Hg, In, Sn, Pb and Zn, at different heating rates,
and the heat flux was calibrated by the Joule effect.
Measurement of the melting temperature and the
enthalpy of fusion of benzoic acid (NIST, Standard
Reference Material 39i) were made in our laboratory
to check the accuracy of the instrument.
The TG measurement of the complex was con-
ducted at the heating rate of 10 K min–1 under high pu-
rity nitrogen with flow rate of 30 mL min–1. The mass
of the sample used in the experiment was 9.9 mg. The
reference crucible was filled with a-Al2O3. The
TG-DTG equipment was calibrated by the SRM in the
thermal analysis, CaC2O4·H2O(s).
Results and discussion
Heat capacity and thermodynamic functions
The experimental molar heat capacities, Cp,m, and
temperaure, T of Ho(Asp)Cl2·6H2O, from 80 to 400 K
were listed in Table 1 and plotted in Fig. 1.
The heat capacity measurements were performed
by the standard procedure of intermittently heating
the sample and alternately measuring the temperature.
From the figure it can be seen that the Cp,m–T
curve is smooth and continuous; no any thermal anom-
aly occurred in the entire experimental temperature
284
J. Therm. Anal. Cal., 89, 2007
Liu
