Low-temperature heat capacity and thermodynamic properties of crystalline [RE (Gly)3(H2O)2]Cl3 ·2H2O (RE = Pr, Nd, Gly = Glycine)

Article abstract of DOI:10.1016/S0040-6031(02)00293-9

Heat capacities of two solid complexes of rare-earth elements with glycine [RE(Gly)₃(H₂O)₂]Cl₃ ·2H₂O (RE = Pr, Nd, Gly = Glycine) have been measured with a high-precision automatic adiabatic calorimeter over the temperature range from 78 to 380 K. The melting point, molar enthalpy and entropy of fusion for the two complexes were determined on the basis of the heat capacity measurements. Thermal decompositions of the two complexes were studied by thermogravimetric (TG) techniques and a possible mechanism for the decompositions is suggested.

Full text of DOI:10.1016/S0040-6031(02)00293-9

Thermochimica Acta 397 (2003) 67–73
Low-temperature heat capacity and thermodynamic properties of
crystalline [RE (Gly)3(H2O)2]Cl3·2H2O
(RE = Pr, Nd, Gly = Glycine)
Beiping Liua,b, Zhi-Cheng Tana,∗, Jilin Lub, Xiao-Zheng Lana, Lixian Suna,
Fen Xua, Ping Yua, Jun Xinga
a
Thermochemistry Laboratory, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 110623, China
b
Department of Chemistry, Changde Normal College, Changde 415000, China
Received 25 March 2002; accepted 3 May 2002
Abstract
Heat capacities of two solid complexes of rare-earth elements with glycine [RE(Gly)3(H2O)2]Cl3·2H2O (RE = Pr, Nd,
Gly = Glycine) have been measured with a high-precision automatic adiabatic calorimeter over the temperature range from
78 to 380 K. The melting point, molar enthalpy and entropy of fusion for the two complexes were determined on the basis
of the heat capacity measurements. Thermal decompositions of the two complexes were studied by thermogravimetric (TG)
techniques and a possible mechanism for the decompositions is suggested.
© 2002 Elsevier Science B.V. All rights reserved.
Keywords: [Pr(Gly)3(H2O)2]Cl3·2H2O; [Nd(Gly)3(H2O)2]Cl3·2H2O; Heat capacity; TG analysis; Rare-earth; Glycine
1. Introduction
metabolism in living systems [3–7]. We must grasp
many thermodynamic properties of these complexes
before exploring this new research field. Heat capac-
ity is one of the basic thermodynamic properties of
substances and related to the structure and energetics
of materials. However, until now, the low-temperature
heat capacities and thermodynamic properties of
rare-earth complexes with amino acids were scarcely
reported in literature. Jin et al. [8] synthesized two
single crystals of rare-earth chlorides (RE = Pr, Nd)
with glycine, and measured their crystal structures.
In the present work, low-temperature heat capac-
ities of two complexes [Pr(Gly)3(H2O)2]Cl3·2H2O
and [Nd(Gly)3(H2O)2]Cl3·2H2O were measured over
the temperature range from 78 to 380 K. At the same
time, the molar enthalpies and entropies of fusion and
the melting temperatures of the two complexes have
Solid complexes of rare-earth elements with
amino acids are important chemical substances,
which are widely used in the wool dying industry
as dying promoter and in agriculture as additive
[1]. Since in 1975, Anghileri firstly reported that
[La(Gly)3(H2O)]Cl3·3H2O has anti-tumor effect [2],
rare-earth complexes with amino acids have been
extensively studied because of their physiological
and biochemical effects. Studying the interaction of
rare-earth metal ions with amino acids can provide
a large amount of information on biology effect and
∗
Corresponding author. Tel.: +86-411-4379215;
fax: +86-411-4691570.
E-mail address: tzc@dicp.ac.cn (Z.-C. Tan).
0040-6031/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved.
PII: S0040-6031(02)00293-9

68
B. Liu et al. / Thermochimica Acta 397 (2003) 67–73
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

B. Liu et al. / Thermochimica Acta 397 (2003) 67–73
69
with a flow rate of 60 ml min−1. The masses of
the samples used for TG analysis were 1.3180
and 1.7220 mg for [Pr(Gly)3(H2O)2]Cl3·2H2O and
[Nd(Gly)3(H2O)2]Cl3·2H2O, respectively. The heat-
where X = (T(K)−229)/149, T is the absolute temper-
ature. The correlation coefficient of the fitting, R2 =
0.9928.
For the solid complex [Nd(Gly)3(H2O)2]Cl3·2H2O,
ing rate was 10 K min−1
.
over the temperature ranges from 78 to 297 K,
C
p,m (J K−1 mol−1
)
= 327.5839 + 181.2793X + 209.2867X2
−171.6420X3 − 485.3327X4 + 143.2519X2
+312.8039X6
3. Results and discussion
3.1. Heat capacity
(2)
The low-temperature experimental molar heat ca-
pacities of the solid complexes are shown in Fig. 1
and listed in Tables 1 and 2, respectively. The molar
heat capacities of the samples are fitted to the follow-
ing polynomials in reduced temperature (X) by means
of the least square fitting.
For the solid complex [Pr(Gly)3(H2O)2]Cl3·2H2O,
over the temperature ranges from 80 to 353 K and from
364 to 377 K,
where X = (T(K)−187.5)/109.5, the correlation coef-
ficient, R2 = 0.9960.
From Fig. 1, it can be seen that the heat ca-
pacities of the two complexes increase with in-
creasing temperature in a smooth and continuous
manner in the temperature range from 78 to 297 K
for [Nd(Gly)3(H2O)2]Cl3·2H2O, from 78 to 350 K
for [Pr(Gly)3(H2O)2]Cl3·2H2O, which indicate that
the two complexes are stable in the above tem-
perature ranges. The melting peaks were observed
at 323.72 K for [Nd(Gly)3(H2O)2]Cl3·2H2O, at
359.35 K for [Pr(Gly)3(H2O)2]Cl3·2H2O, respec-
tively.
C
p,m (J K−1 mol−1
)
= 372.2648 + 322.5773X − 162.2029X2
−435.236X3+222.7372X4 + 381.9981X5 (1)
Fig. 1. Experimental heat-capacity of [RE(Gly)3(H2O)2]Cl3·2H2O (RE = Pr, Nd) as a function of temperature.

70
B. Liu et al. / Thermochimica Acta 397 (2003) 67–73
Table 1
The experimental molar heat capacities of [Pr(Gly)3(H2O)2]Cl3·2H2O (molar mass: M = 544.47)
T (K)
Cp (J K−1 mol−1
)
T (K)
Cp (J K−1 mol−1
)
T (K)
Cp (J K−1 mol−1
)
80.276
81.910
84.486
86.983
89.403
91.776
94.091
96.358
98.588
170.47
172.00
175.97
178.93
181.44
182.31
184.54
185.15
186.02
186.72
187.37
188.79
191.67
192.47
193.45
194.55
196.89
197.47
200.07
203.40
207.36
212.40
216.99
219.86
221.39
225.54
230.72
233.38
237.09
239.29
240.19
243.93
166.071
168.263
171.702
175.060
178.392
181.693
184.967
188.214
191.437
194.975
198.819
202.625
206.394
210.135
216.964
222.448
226.861
231.231
235.552
239.773
244.016
248.215
252.369
256.478
260.548
264.587
268.594
272.570
276.509
280.389
284.231
287.987
245.19
249.22
254.53
260.45
266.11
270.43
275.25
280.15
282.41
285.33
293.83
299.41
307.60
313.05
331.02
345.95
365.10
385.91
401.42
410.64
416.64
421.38
422.33
424.17
425.03
429.95
432.77
437.21
441.06
445.73
446.03
447.70
291.721
295.455
299.198
302.918
306.615
310.298
313.905
317.264
320.887
324.511
328.046
331.670
335.146
338.799
342.501
346.094
349.675
353.247
356.512
358.675
359.347
359.349
359.470
360.104
361.419
363.858
366.495
368.994
371.266
373.377
375.162
376.705
447.97
454.23
454.45
456.69
458.99
461.68
466.13
467.46
470.14
471.47
480.50
483.50
484.00
486.49
488.38
490.69
492.75
497.73
535.22
1081.15
3033.39
3697.93
3362.09
2449.63
1547.70
597.61
609.74
633.34
658.23
665.69
678.13
690.57
100.776
102.922
105.042
107.143
109.172
111.211
113.214
115.190
117.150
119.397
121.910
125.169
129.135
133.017
136.829
140.585
144.285
147.925
151.533
155.097
158.622
162.906
165.179
ꢀHm
Tm
3.2. The melting point, molar enthalpy and molar
entropy of fusion
ꢀSm
=
(4)
The melting temperature of the samples was ob-
tained from the Cp(T) curves. The peak temperature
of the heat-capacity curves (Fig. 1) of the samples was
selected as the melting point Tm. The molar enthalpies
ꢀHm, and molar entropies ꢀSm, of the melting tran-
sition can be calculated based on the following equa-
tions:
where T is the temperature a few degrees lower than
the initial melting temperature, Tf is a temperature
slightly higher than the final melting temperature,
Q is the total energy introduced into the sample
cell from Ti to Tf, H0 is the heat capacity of the
sample cell from Ti to Tf, Cp1 the heat capacity of
the sample in solid phase from Ti to Tm, Cp2 the
heat capacity of the sample in liquid phase from Tm
to Tf, and n is molar amount of the sample. The
melting point, molar enthalpy and molar entropy of
fusion were determined to be 359.35 K, 6.60 and
18.37 J K−1 mol−1 for [Pr(Gly)3(H2O)2]Cl3·2H2O,
ꢀHm
ꢀ T
Ti
ꢀ T
ꢀ T
f
Ti
m
f
Tm
Q − n
Cp,1 dT − n Cp,2 dT − n H0 dT
=
n
(3)

B. Liu et al. / Thermochimica Acta 397 (2003) 67–73
71
Table 2
The experimental molar heat capacities of [Nd(Gly)3(H2O)2]Cl3·2H2O (molar mass: M = 547.76)
T (K)
Cp (J K−1 mol−1
)
T (K)
Cp (J K−1 mol−1
)
T (K)
Cp (J K−1 mol−1
)
78.364
81.131
84.987
88.676
92.236
202.18
208.90
211.80
218.20
224.98
231.28
237.55
243.24
249.42
255.08
261.62
266.88
271.83
276.57
279.56
280.00
280.03
280.18
281.36
281.79
281.93
283.73
285.47
288.10
289.77
290.10
291.11
293.11
294.44
295.78
298.45
301.79
304.46
308.47
313.47
319.18
325.51
331.19
335.19
343.21
349.89
200.513
203.694
206.788
209.881
212.886
215.979
218.984
222.078
225.171
228.353
231.269
234.274
239.400
245.410
248.580
251.907
255.219
258.509
261.777
265.021
268.254
271.453
274.623
277.774
280.842
283.881
286.886
289.858
293.150
296.848
299.590
301.090
301.267
302.504
303.830
306.128
307.453
308.249
308.868
310.193
311.431
354.91
357.91
359.24
364.91
372.60
377.95
389.97
399.32
406.34
410.00
414.02
423.37
424.73
425.97
426.62
430.66
435.04
441.66
445.82
450.68
452.69
453.10
455.82
457.30
460.09
464.81
470.46
486.74
496.06
513.99
584.25
641.15
705.28
770.75
868.28
973.83
1060.67
1111.44
1186.26
1251.73
1310.52
311.607
311.696
313.021
313.110
313.287
313.375
313.552
314.789
314.966
315.143
315.496
315.850
316.026
316.292
316.468
316.645
316.910
317.175
317.440
317.706
317.971
318.148
318.413
318.589
318.855
319.120
319.385
319.562
319.031
319.120
319.650
320.004
320.711
321.506
321.948
322.655
323.716
328.753
334.410
348.020
364.840
379.772
1346.59
1418.74
1474.85
1533.64
1583.07
1640.52
1699.31
1772.79
1836.92
1902.39
1974.54
2042.68
2121.50
2186.97
2251.10
2292.52
2333.94
2363.33
2406.08
2452.85
2510.30
2543.70
2581.11
2623.86
2654.59
2657.26
2701.35
2733.42
2773.50
2778.84
2820.26
2883.06
2949.86
3040.71
3123.55
3187.68
3258.49
2637.22
1931.78
732.47
95.698
99.060
102.332
105.526
108.652
111.710
114.716
117.662
120.567
123.425
126.246
129.030
131.775
134.491
137.178
139.836
142.473
145.092
147.661
150.224
152.787
155.350
157.913
160.476
163.039
165.54
167.988
170.993
174.440
177.799
181.157
184.427
187.697
190.967
194.149
197.331
729.61
724.94
to be 323.72 K, 57.91 and 178.90 J K−1 mol−1 for
[Nd(Gly)3(H2O)2]Cl3·2H2O, respectively.
mass-loss curve that most of the activities occur in the
temperature range of 272–579 ◦C. The solid complex
was stable below 100 ◦C, and started decomposition
at 114 ◦C. The total mass-loss (%) is 40.88%. We
consider that the residue should be PrCl3 because the
mass loss (%) calculated theoretically is 41.37% if
the final residual is PrCl3.
3.3. TG results
The TG curve of Pr(Gly)3(H2O)2]Cl3·2H2O is
shown in Fig. 2. It can be seen clearly from the

72
B. Liu et al. / Thermochimica Acta 397 (2003) 67–73
Fig. 2. TG/DTG curve of [Pr(Gly)3(H2O)2]Cl3·2H2O under nitrogen atmosphere.
Fig. 3. TG/DTG curve of [Nd(Gly)3(H2O)2]Cl3·2H2O under nitrogen atmosphere.

B. Liu et al. / Thermochimica Acta 397 (2003) 67–73
73
Fig. 3 shows that the structure of [Nd(Gly)3(H2O)2]
Acknowledgements
Cl3·2H2O is stable below 100 ◦C. It starts mass-loss
at 105 ◦C. The experiment result of final mass-loss
was 42.28%, which suggested that the residual prod-
uct should be NdCl3, because the theoretical mass-
loss (%) of the decomposition is 41.11% when the
final residual is NdCl3. According to the mass-loss
in each step, a possible mechanisms of the thermal
decompositions may be as follows:
The authors gratefully acknowledge the National
Nature Science Foundation of China for financial sup-
port to this work under NSFC Grant no. 20073047.
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[Nd(Gly)3(H2O)2]Cl3 · 2H2O
105–167 ◦C
→
[Nd(Gly)3(H2O)2]Cl3
6.33%(6.57%)
180–255 ◦C
260–531 ◦C
→
[Nd(Gly)3]Cl342.28%(41.11%)
→
NdCl3
6.87%(6.57%)