Monitoring dehydration and condensation processes of Na2HPO4 · 12H2O using thermo-Raman spectroscopy

Article abstract of DOI:10.1016/S1386-1425(02)00395-5

Thermal dehydration and condensation processes of disodium hydrogen phosphate dodecahydrate (Na₂HPO₄ · 12H₂O) were monitored by thermo-Raman spectroscopy (TRS). Various hydrated forms Na₂HPO₄ · 12H₂O, Na₂HPO₄ · 8H₂O, Na₂HPO₄ · 7H₂O, Na₂HPO₄ · 2H₂O, Na₂HPO₄ · H₂O and Na₂HPO₄ were observed, followed by condensation of Na₂HPO₄ to sodium pyrophosphate (Na₄P₂O₇) in a dynamic thermal process. Representative Raman spectra of all the hydrated forms Na₂HPO₄ · 12H₂O, Na₂HPO₄ · 8H₂O, Na₂HPO₄ · 7H₂O, Na₂HPO₄ · 2H₂O, Na₂HPO₄ · H₂O and Na₂HPO₄ were detected in both H₂O and PO₄^3- regions are reported. The thermo-Raman intensity (TRI) thermogram also showed systematic loss of water in five steps of dehydration, with the differential TRI thermogram in agreement shows five dips corresponding to the five steps of dehydration, respectively. Thermogravimetry (TG) and differential thermogravimetry (DTG) are in harmony with the results of TRS, though, the two could not resolve the steps involved.

Full text of DOI:10.1016/S1386-1425(02)00395-5

Spectrochimica Acta Part A 59 (2003) 1529Á1539
/
www.elsevier.com/locate/saa
Monitoring dehydration and condensation processes of
Na2HPO4 × 12H2O using thermo-Raman spectroscopy
/
Anil Ghule, Chetan Bhongale, Hua Chang *
Department of Chemistry, National Tsing Hua University, Hsinchu 30043, Taiwan, ROC
Received 22 July 2002; accepted 24 September 2002
Abstract
Thermal dehydration and condensation processes of disodium hydrogen phosphate dodecahydrate (Na2HPO4 ×
12H2O) were monitored by thermo-Raman spectroscopy (TRS). Various hydrated forms Na2HPO4 × 12H2O,
Na2HPO4 × 8H2O, Na2HPO4 × 7H2O, Na2HPO4 × 2H2O, Na2HPO4 × H2O and Na2HPO4 were observed, followed by
condensation of Na2HPO4 to sodium pyrophosphate (Na4P2O7) in a dynamic thermal process. Representative Raman
spectra of all the hydrated forms Na2HPO4 × 12H2O, Na2HPO4 × 8H2O, Na2HPO4 × 7H2O, Na2HPO4 × 2H2O,
Na2HPO4 ×
H2O and Na2HPO4 were detected in both H2O and PO34ꢀ regions are reported. The thermo-Raman
/
/
/
/
/
/
/
/
/
/
/
intensity (TRI) thermogram also showed systematic loss of water in five steps of dehydration, with the differential TRI
thermogram in agreement shows five dips corresponding to the five steps of dehydration, respectively. Thermo-
gravimetry (TG) and differential thermogravimetry (DTG) are in harmony with the results of TRS, though, the two
could not resolve the steps involved.
# 2002 Elsevier Science B.V. All rights reserved.
Keywords: Thermal analysis; Thermo-Raman spectroscopy; Dehydration; Condensation; Disodium hydrogen phosphate
dodecahydrate (Na2HPO4 × 12H2O)
/
1. Introduction
dynamic process. The thermo-Raman spectro-
scopy (TRS) is a viable technique, in which,
successive collection of Raman spectra could yield
much detailed information in a dynamic thermal
process. The composition changes of all the
intermediates and phase transformations could
easily be monitored and identified on a time scale
of seconds or on a temperature scale of 18 by
Raman spectra. The intensity and its derivative of
the characteristic band are plotted as a function of
temperature and are the thermo-Raman intensity
(TRI) and differential thermo-Raman intensity
(DTRI) thermograms, respectively. These provide
Thermogravimetry (TG), differential thermal
analysis (DTA) and differential scanning calori-
metry (DSC) are techniques generally used to
study the thermal properties of solid sample in a
dynamic and static thermal processes [1] but, these
techniques fail to provide direct information about
the compositions and phases involved during a
* Corresponding author. Fax: ꢁ
E-mail address: hchang@inn.cc.nthu.edu.tw (H. Chang).
/886-35-711082.
1386-1425/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved.
doi:10.1016/S1386-1425(02)00395-5

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A. Ghule et al. / Spectrochimica Acta Part A 59 (2003) 1529Á1539
/
detailed quantitative information about the com-
position changes and phase transformations [2Á8]
and could readily be comparable to thermograms
of TG, DTA and DSC.
The thermal process of disodium hydrogen
Herein, the thermal variations of Na2HPO4 ×
/
/
12H2O as a starting material by TRS in the
temperature range from 25 to 600 8C are reported.
Spectral variation observed in H2O and PO4
regions could show five dehydrations from
phosphate dodecahydrate (Na2HPO4 ×
very interesting for its variety and had attracted
many researchers in the recent past [9Á16]. Two
forms of disodium hydrogen phosphate dodecahy-
drate (Na2HPO4 × 12H2O), a-form (stable at 30-
35 8C) and b-form (stable at 0Á30 8C), where, the
b-form can be converted to a-form by heating to
31Á32 8C, were reported by solubility studies by
Hammick et al. [9]. In their solubility studies, they
observed a-Na2HPO4 × 12H2O, b-Na2HPO4 ×
2H2O and
/
12H2O) is
Na2HPO4 ×
Na2HPO4 ×
2H2O, Na2HPO4 ×
/
12H2O, resulting in the formation of
8H2O, Na2HPO4 × 7H2O, Na2HPO4 ×
H2O and Na2HPO4, respec-
/
/
/
/
/
tively. Raman spectra representing all the inter-
mediates could be detected. The TRI thermogram
for H2O band revealed five steps of dehydration
with proportionate loss of water in each step. The
DTRI thermogram also substantiates the TRI
results by showing five dips at 38, 41, 52, 72 and
82 8C. The TG showed loss of 59.3% water for
entire dehydration process indicating loss of 12
H2O molecules, with differential thermogravimetry
(DTG) thermograms showing dips at 45, 60 and
66 8C, but could not resolve the five steps of
dehydration involved. The formation of Na4P2O7
was identified by Raman spectra in TRS in the
/
/
/
/
/
12H2O, Na2HPO4 ×
/
7H2O, Na2HPO4 ×
/
anhydrous Na2HPO4 species in the temperature
range from 0 to 95 8C. Dufet [10] in his optical
studies
Na2HPO4 ×
drous Na2HPO4.
Na2HPO4 × 7H2O from water was reported at
35Á48 8C, and also at 25 8C in presence of excess
also
7H2O, Na2HPO4 ×
The crystallization
reported
Na2HPO4 ×
2H2O and anhy-
of
/
12H2O,
/
/
/
temperature range 246Á345 8C in the condensa-
/
/
tion process from Na2HPO4. Further, temperature
induced phase transformations were also observed
in agreement with earlier report [17].
of phosphoric acid [11]. Duval [12] in thermo-
gravimetric studies reported two hydrated forms
Na2HPO4 ×
/
12H2O and Na2HPO4 ×
/
7H2O, leading
Based on the earlier experience with TRS and its
applicability to detect the dehydration steps in-
volved in the thermal process, it was possible to
detect all the dehydration steps involved with
characteristic Raman spectra for each hydrated
species. It is expected that this experimental setup
would lead to increased applications in the field of
analytical chemistry, where intermediates are to be
detected.
to Na2HPO4 at 180 8C at relatively higher tem-
perature than most of the reports. This was
followed by condensation in the temperature range
318Á
and Hazel [13] with thermogravimetric studies on
Na2HPO4 × 7H2O reported the completion of de-
hydration in the temperature range of 60Á140 8C,
and condensation in the temperature range of
330Á340 8C. Wendrow and Kobe [14] in their
solubility studies reported the presence of
Na2HPO4 × 12H2O, Na2HPO4 × 8H2O, Na2HPO4 ×
7H2O, 2H2O and anhydrous
Na2HPO4 ×
/
355 8C to form pyrophosphate. Steinbrecher
/
/
/
/
/
/
2. Experimental
/
Na2HPO4 species. These hydrated species were
reported to be stable in the temperature range
from 25 to 100 8C [15]. During their studies on
entropy and crystal structure, Templeton et al. [16]
Na2HPO4 ×
/
12H2O from Showa Chemicals was
re-crystallized at room temperature to ensure no
hydrated water was lost and sample retains all the
H2O molecules. The rhombohedral needle-shaped
crystals of size about 5 and 6 mm3 were used for
the experiments with slightly crushing to powders.
The Raman spectra were excited by a laser beam
at a wavelength of 514.5 nm (30 mW) from an
argon ion laser (Coherent, Innova 100-15). A filter
was used to remove the plasma lines. The scattered
reported the presence of Na2HPO4 ×
Na2HPO4 × 7H2O, Na2HPO4 × 2H2O, Na2HPO4 ×
H2O and Na2HPO4. However, no study on the
12H2O could
/
12H2O,
/
/
/
thermal properties of Na2HPO4 ×
/
prove the existence of all the hydrated species
and the phases in a single experiment.

A. Ghule et al. / Spectrochimica Acta Part A 59 (2003) 1529Á
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1531
light was collected at right angles, dispersed by a
single spectrometer (Spex, 0.5 m) with a resolution
of 5 cmꢀ1 and detected by a CCD camera
3.1. Dehydration
The crystal structure for Na2HPO4 ×
/
12H2O
(Princeton Instruments, 1024ꢂ/1024 pixels). A
reported at 23 8C, belongs to the space group
I2/c, aꢃ14.178, bꢃ9.021, cꢃ12.771, bꢃ
108.888, Zꢃ4 [16]. Two types of disorders in
Notch filter was set in front of the spectrometer
to remove the Rayleigh scattered light. The sample
was in a sample holder, mounted on the thermo-
couple in a homemade oven in stationary air
atmosphere. The thermal process was set from 25
to 600 8C at a rate of 2 8C minꢀ1 and controlled
by a temperature controller. Thermo-Raman spec-
tra were collected continuously without disturbing
the sample in the dynamic thermal process. The
exposure time of CCD was such that the tempera-
ture covered by each spectrum was 1 8C. Over all,
575 spectra were collected and checked for their
reproducibility.
/
/
/
/
/
terms of positions of protons in hydrogen bonds
and orientations of phosphate ions were observed,
and are in agreement as two forms of Na2HPO4 ×
12H2O exists. The structure reported was in
agreement with Catti et al. [20] for b-Na2HPO4 ×
/
/
12H2O with slight variation in the parameters.
The structure indicates that the Naꢁ ion coordi-
nates with six water molecules so as to form a
fairly regular octahedron. Stoichiometry and co-
ordination requirements are satisfied without the
sharing of O atoms between Na(H2O)6 and PO4
polyhedra, and without any interstitial water
molecules. The structure was built up by pairs of
centrosymmetric (001) layers of close-packed oc-
tahedral, one third of which are substituted by P
tetrahedral. Adjacent layers are tied only by
hydrogen bonding and, therefore, easy cleavage
was possible leading to quick loss of water from
Thermogravimetric analyzer (PerkinÁElmer
/
TGA6) was used to record the thermograms in
the temperature range from 30 to 600 8C with a
heating rate of 2 8C minꢀ1 in a flow of nitrogen
gas at 20 ml minꢀ1
.
3. Results and discussion
Na2HPO4 ×
/
12H2O. According to Waterfield and
Staveley, [21] the hydrogen bonding scheme seems
to be more complex due to large number of water
molecules and the nature of the HPO24ꢀ ions.
During the heating process two different species,
PO34ꢀ and P2O74ꢀ were observed. These could be
readily assigned based on their characteristic Ra-
man spectra and could be used as finger-print to
identify these species. The internal modes for
3.2. First dehydration
PO34ꢀ region, in the range 400Á
/
1200 cmꢀ1 provide
The spectral variations in the H2O region from
2700 to 3700 cmꢀ1 for the first dehydration in the
temperature range from 31 to 40 8C were observed
as shown in Fig. 1a. The water molecules being
weakly bounded were released easily in this
thermal process, hence re-crystallization of the
sample was necessary to ensure presence of
12H2O before the study of the thermal process.
The spectra at 31 8C showed three overlapped
bands 3474, 3408 and 3215 cmꢀ1. With the rise in
temperature, the intensity of the strong band at
3408 cmꢀ1 started to decrease and that band
shifted to 3412 cmꢀ1 as the temperature ap-
proached 40 8C. The band at 3474 cmꢀ1, which
appeared as shoulder at 31 8C disappeared gra-
dually with rise in temperature. The intensity of
the band at 3215 cmꢀ1 also decreased and showed
information about the environment around the
PO34ꢀ ion. Phosphate ion PO34ꢀ is tetrahedral with
point group Td symmetry [18]. But, in dynamic
thermal process in crystals, structural distortion
may occur, thereby lowering the symmetry, and
shifting and splitting the bands. The broad band
from 2800 to 3600 cmꢀ1 was due to H2O. Dipho-
sphate ion P2O47ꢀ having C2v symmetry are
composed of two PO3 groups and a non-linear
PÃ
/
OÃ
/
P bridge showing two strong bands at 1030
and 723 cmꢀ1 [19]. The band at 1030 cmꢀ1 was
the symmetric stretching mode of PO3. The other
distinct band at 723 cmꢀ1 was characteristic of
symmetric PÃ
/
OÃ
/
P stretching mode. Raman bands
for both PO34ꢀ and P2O47ꢀ, generally fall between
400 and 1200 cmꢀ1 with assignments as in Table 1.

Table 1
Raman bands for Na2HPO4 ×
/
12H2O, Na2HPO4 ×
/
8H2O, Na2HPO4 ×
/
7H2O, Na2HPO4 ×
/
2H2O, Na2HPO4 ×
/
H2O, Na2HPO4 and Na4P2O7
Na2HPO4 × 12H2O Na2HPO4 ×
/
/
8H2O
Na2HPO4 ×
44 8C
/
7H2O
Na2HPO4 ×
65 8C
/
2H2O
Na2HPO4 ×
76 8C
/
H2O
Na2HPO4
84 8C
assign.a
Na4P2O7
290 8C
assign.b
31 8C
40 8C
3474w
3408s
3412s
3412s
3391w
3320w
3391w
3320w
3215s
3238w
3238w
3080w
3028w
1149
3073w
3020w
1139
H2O (stre. vib.)
1139m
1122
PO3
PO3
1084w
1068w
1089
1084w
1064w
1072
1072
1072m
1001w
n3 PO34ꢀ
1030s
987s
869s
987s
869s
987s
869s
950
867
950
867
939s
855
n1 PO34ꢀ
O(H)
PÃ
/
723m
556w
PÃ
/
OÃP
/
583w
571w
556
583w
571w
556 m
567w
541w
513w
520m
518m
518m
n4 PO34ꢀ
PO3
501
461
501w
461
446
411
395
407
388
407w
388
404w
385w
n2 PO34ꢀ
395
397
OÃ
/
PÃO
/
s-strong; m-medium; w-weak and stre. vib., strectching vibrations.
a
Refs. [18,26].
Refs. [19,27].
b

A. Ghule et al. / Spectrochimica Acta Part A 59 (2003) 1529Á
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1533
However, the structure of Na2HPO4 ×
/
8H2O
12H2O,
should be similar to that of Na2HPO4 ×
/
because the spectra are similar.
3.3. Second dehydration
Fig. 2a shows the spectral variations in the H2O
region for the second dehydration in the tempera-
ture range from 40 to 44 8C. This temperature
interval being very small, not much variation was
observed in spectra. The band at 3412 cmꢀ1
showed sharpening with slight decrease in intensity
as the temperature rose to 44 8C. Overall broad-
ening was observed with increase in temperature.
The corresponding changes observed in the
PO34ꢀ region in the second dehydration in the
temperature range from 40 to 44 8C are as shown
in Fig. 2b. The band observed at 1089 cmꢀ1
showed split to form a band at 1084 cmꢀ1 and a
shoulder at 1064 cmꢀ1. Considerable decrease in
intensity of the strong band at 987 cmꢀ1 was
observed, with slight decrease observed in the
band at 869 cmꢀ1 as the temperature increased
Fig. 1. Thermo-Raman spectra of Na2HPO4 × 12H2O in the
/
temperature range from 31 to 40 8C for the first dehydration in
(a) H2O region from 2700 to 3700 cmꢀ1 and (b) PO43ꢀ region
from 350 to 1250 cmꢀ1
.
shift to 3238 cmꢀ1. The band at 3238 cmꢀ1
showed overlapping with the band at 3412 cmꢀ1
.
The corresponding changes observed in the
PO34ꢀ region from 350 to 1250 cmꢀ1 in the first
dehydration were distinct in the temperature range
from 31 to 40 8C. Sudden loss of water was
observed with no considerable change in symmetry
of PO34ꢀ. The two bands at 1084 and 1068 cmꢀ1
merged to form a single broad band at 1089 cmꢀ1
with the rise in temperature to 40 8C. The bands
at 987 and 869 cmꢀ1 showed considerable de-
crease in intensity and the band at 520 cmꢀ1
shifted to 518 cmꢀ1 as the temperature increased
to 40 8C. The doublet at 407 and 388 cmꢀ1 was
distinctly observed at 31 8C, but showed increased
overlap with increase in temperature. The spectral
changes were distinctly evident in the temperature
range from 31 to 40 8C as seen from the spectra as
shown in Fig. 1b. For the first dehydration, it was
the transformation of Na2HPO4 ×
Na2HPO4 × 8H2O. To our knowledge no crystal
data is available for Na2HPO4 × 8H2O, probably
because of the difficulty in isolation of the species.
/
12H2O to
Fig. 2. Thermo-Raman spectra of Na2HPO4 × 12H2O in the
/
/
temperature range from 40 to 44 8C for the second dehydration
in (a) H2O region from 2700 to 3700 cmꢀ1 and (b) PO43ꢀ region
/
from 350 to 1250 cmꢀ1
.

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A. Ghule et al. / Spectrochimica Acta Part A 59 (2003) 1529Á1539
/
to 44 8C. The bands at 407 and 388 cmꢀ1 shifted
to 404 and 385 cmꢀ1, respectively. The spectral
changes observed in the temperature range from
40 to 44 8C correspond to second dehydration
3080 and 3028 cmꢀ1 emerged at 53 8C and
became distinct with rise in temperature to 65 8C.
The spectral variations observed in the PO43ꢀ
region for the third dehydration step in the
temperature range from 45 to 65 8C are as shown
in Fig. 3b. The doublet at 1084 and 1064 cmꢀ1
merged to form a single sharp band at 1072 cmꢀ1
and a new band at 1149 cmꢀ1 started to appear at
47 8C, and became distinct as the temperature
increased to 65 8C. The strong band observed at
987 cmꢀ1 gradually decreased and disappeared
around 57 8C. A new band at 950 cmꢀ1 appeared
around 47 8C and showed a sharp increase in
intensity as the temperature rose to 65 8C. The
sharp band at 869 cmꢀ1 showed gradual decrease
in intensity with broadening. The broad band at
518 cmꢀ1 showed shift to band at 513 cmꢀ1 and
two new bands appearing around 47 8C, formed
bands at 567, 541 and 513 cmꢀ1 as the tempera-
ture reached 65 8C. The bands at 404 and 385
cmꢀ1 shifted to 411 and 395 cmꢀ1 and a new
weak band at 446 cmꢀ1 was observed with rise in
temperature. The major changes observed in the
spectral pattern indicate the drastic change in
symmetry of PO34ꢀ, as large amount of water
was lost in this step resulting into the transforma-
resulting in the formation of Na2HPO4 ×
from Na2HPO4 × 8H2O. Na2HPO4 × 7H2O crystal
structure with space group P21/n, aꢃ10.432, bꢃ
11.000, cꢃ9.252, bꢃ95.62 8, Zꢃ4 at 24 8C
/
7H2O
/
/
/
/
/
/
/
using X-ray diffraction (XRD) studies was re-
ported by Templeton et al. [16], was in agreement
with that reported by Baur and Kahn [22] except
for the minor shift in atomic positions.
3.4. Third dehydration
The spectral variations in the H2O region for the
third dehydration in the temperature range from
45 to 65 8C were observed as shown in Fig. 3a.
The spectra at 45 8C showed broad bands at 3412
and 3238 cmꢀ1. The band at 3412 cmꢀ1 shifted to
3391 cmꢀ1 and the band at 3238 cmꢀ1 disap-
peared as the temperature approached to 57 8C. A
new band at 3320 cmꢀ1 and a weak doublet at
tion from Na2HPO4 ×
in the third dehydration. The crystal structure of
Na2HPO4 × 2H2O with space group Pbca, aꢃ
10.343, bꢃ6.596, cꢃ16.83, Zꢃ8 was reported
/
7H2O to Na2HPO4 ×
/
2H2O
/
/
/
/
/
at 23 8C by Templeton et al. [16] is in agreement
with Catti et al. [23], who indicated the presence of
two distinct types of water molecules.
3.5. Fourth dehydration
The spectral variations in the H2O region for the
fourth dehydration in the temperature range from
68 to 76 8C were observed as shown in Fig. 4a. At
68 8C, two bands observed at 3391 and 3320
cmꢀ1 gradually became weak and broad as the
temperature reached 76 8C. Similarly, the bands
at 3080 and 3028 cmꢀ1 observed at 65 8C
appeared as a broad band at 3073 cmꢀ1 at
68 8C. The band at 3073 cmꢀ1 became broader
with a weak shoulder around 3020 cmꢀ1 and
further reduced in intensity as temperature rose to
76 8C.
Fig. 3. Thermo-Raman spectra of Na2HPO4 × 12H2O in the
/
temperature range from 45 to 65 8C for the third dehydration
in (a) H2O region from 2700 to 3700 cmꢀ1 and (b) PO43ꢀ region
from 350 to 1250 cmꢀ1
.

A. Ghule et al. / Spectrochimica Acta Part A 59 (2003) 1529Á
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1535
3.6. Fifth dehydration
The spectral variations in the H2O region for the
fifth dehydration, in the temperature range from
76 to 84 8C were observed as shown in Fig. 5a.
The weak broad bands observed at 3391 and 3320
cmꢀ1 gradually disappeared when the temperature
reached 84 8C. Similarly, the broad band at 3073
cmꢀ1 and that at 3020 cmꢀ1 also disappeared.
This indicated complete loss of water in the
dehydration to form anhydrous Na2HPO4. The
background signal appeared curved.
The corresponding changes clearly observed in
the PO34ꢀ region in the fifth dehydration and in
the temperature range from 76 to 84 8C are as
shown in Fig. 5b. The band at 1139 cmꢀ1 became
sharp and intense with increase in temperature.
The band at 1072 cmꢀ1 became broad and a new
weak peak at 1001 cmꢀ1 gradually developed with
rise in temperature to 84 8C. The strong sharp
band at 950 cmꢀ1 broadened considerably and
shifted to 939 cmꢀ1. Similarly, the band at 867
cmꢀ1 also showed broadening with shift to appear
Fig. 4. Thermo-Raman spectra of Na2HPO4 × 12H2O in the
/
temperature range from 68 to 76 8C for the fourth dehydration
in (a) H2O region from 2700 to 3700 cmꢀ1 and (b) PO43ꢀ region
from 350 to 1250 cmꢀ1
.
as shoulder at 855 cmꢀ1 to the band at 939 cmꢀ1
.
The corresponding changes clearly observed in
the PO34ꢀ region for the fourth dehydration in the
temperature range from 68 to 76 8C are as shown
in Fig. 4b. The broad band at 1149 cmꢀ1
gradually shifted to appear as a sharp band at
1139 cmꢀ1. The band at 1072, 950 and 867 cmꢀ1
became broad with rise in temperature to 76 8C.
The strong sharp band at 950 cmꢀ1 broadened
considerably. The bands at 567 cmꢀ1 resolved into
triplet at 583, 571 and 556 cmꢀ1 and the band at
541 cmꢀ1 disappeared gradually with the
temperature. The band at 513 cmꢀ1 showed
broadening and shifted to 501 cmꢀ1. The weak
band at 451 cmꢀ1 disappeared as the temperature
reached 76 8C. The spectral changes observed in
the temperature range from 68 to 76 8C are
the result of fourth dehydration where
Na2HPO4 ×
/
2H2O was transformed to Na2HPO4 ×
/
H2O. The crystal structure of Na2HPO4 ×
/
H2O,
well resolved by XRD and discussed in relation to
entropy data was reported to be triclinic with
¯
Fig. 5. Thermo-Raman spectra of Na2HPO4 × 12H2O in the
/
space group P
/
1, aꢃ
/
5.565, bꢃ
/
7.949, cꢃ
/5.735,
temperature range from 76 to 84 8C for the fifth dehydration in
(a) H2O region from 2700 to 3700 cmꢀ1 and (b) PO43ꢀ region
aꢃ
/
103.838, bꢃ
/
102.158, gꢃ
/
107.478 and Zꢃ/2 by
Templeton et al. [16].
from 350 to 1250 cmꢀ1
.

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A. Ghule et al. / Spectrochimica Acta Part A 59 (2003) 1529Á1539
/
The triplet at 583, 571 and 556 cmꢀ1 further
resolved and became intense. A new weak band at
461 cmꢀ1 also gradually appeared with the rise in
temperature to 84 8C. The spectral changes ob-
served in the temperature range from 76 to 84 8C
are the result of fifth dehydration, where
observed in Figs. 4 and 5 indicates random
orientations of the PO34ꢀ ion in Na2HPO4 ×
/
2H2O. It leads to the formation of distorted
crystal structures and hence the argument on the
possibility of different space groups of anhydrous
Na2HPO4 reported by earlier researchers. More-
over, the broadening arises due to the vibrations of
the ionic species with varying frequencies aligned
in different orientations spread in the short range.
Na2HPO4 ×
/
H2O was transformed to Na2HPO4.
Anhydrous Na2HPO4 has been reported to crystal-
lize in a monoclinic structure of space group P21/
m [24] with possibility of space group with lower
symmetry P21/a, P21c or P21/n [16]. Later,
Baldus et al. [25] with combination of NMR and
powder XRD gave convincing evidence of anhy-
drous Na2HPO4 crystallizes in space group P21/c,
3.8. Intensity variation
The intensity of the water bands in the range
2800Á
3600 cmꢀ1 from 30 to 100 8C was measured
/
aꢃ
4.
/
5.450, bꢃ
/
6.845, cꢃ
/
10.944, bꢃ
/
116.358, Zꢃ
/
and is as shown in Fig. 6a. It is the TRI
thermogram. The entire intensity was assigned to
be equivalent to 12 molecules of water. The steep
slope in the TRI thermogram indicated loss of
water molecule and the stationary stage implied
intermediate formed. The TRI thermogram
showed five steep slopes and four stationary stages
corresponding to five steps of dehydration. A steep
slope for the first dehydration was observed in the
3.7. Raman spectra and crystal structure
Raman
Na2HPO4 ×
similar reflecting close similarity in their structure.
H2O
spectra
of
Na2HPO4 ×
/
12H2O,
/
8H2O and Na2HPO4 ×
/
7H2O appear
Similarly, the Raman spectra of Na2HPO4 ×
/
and Na2HPO4 are also similar indicating close
identity in the environment around PO34ꢀ ion.
temperature range 34Á40 8C. It revealed a loss of
/
33.6% of H2O or equivalent to 3.85 molecule of
water. This resulted in the formation of
However, Raman spectra of Na2HPO4 ×
that of Na2HPO4 × 2H2O showed considerable
/
7H2O and
/
Na2HPO4 ×
/
8H2O from Na2HPO4 ×
/
12H2O. The
difference, and can be attributed to the drastic
structural changes. In accordance with the crystal
structures, notable difference in the Raman spectra
second steep decrease was in the temperature
range from 40 to 43 8C. It showed a loss of 8.6%
of H2O or equivalent to 1.1 molecule of water
of Na2HPO4 ×
/
2H2O and Na2HPO4 was also ob-
resulting in the formation of Na2HPO4 ×
/
7H2O.
served.
The third prolonged gradual slope representing the
third dehydration was observed in the temperature
range from 46 to 65 8C, which revealed a loss of
39.7% of H2O or equivalent to 4.8 molecule of
It is interesting to note a direct correlation of the
band shape and position of the Raman spectra
with the crystal structures. The bands in PO43ꢀ
region for Na2HPO4 ×
/
12H2O and Na2HPO4 ×
/
water with transformation from Na2HPO4 ×
/
7H2O
8H2O appeared very sharp and intense as in
Figs. 1 and 2, and indicate ordered crystal
structure with uniform environment around the
PO34ꢀ ion. Similar trend was observed during
second dehydration as the bands appeared sharp
and intense, as only water molecules were lost. The
shift in band from 987 to 950 cmꢀ1 and band
broadening during third dehydration as in Fig. 3
depicts drastic change in the crystal structure
leading to change in environment around PO43ꢀ
ion and collapse of the network of H2O molecules
around PO34ꢀ ion. Further, broadening of bands
to form Na2HPO4 × 2H2O. The fourth slope in the
/
extended temperature range from 68 to 76 8C was
observed for fourth dehydration, contributing for
loss of 10.3% of H2O or equivalent to 1.2 molecule
of water. The fifth slope was observed in the
temperature range from 76 to 84 8C was observed
for fifth dehydration contributing for loss of 7.8%
of H2O or equivalent to 0.9 molecule of water.
This resulted in the formation of anhydrous
Na2HPO4. After 84 8C, no decrease in intensity
was observed as temperature reached 100 8C. This
indicates completion of the dehydration process.

A. Ghule et al. / Spectrochimica Acta Part A 59 (2003) 1529Á
/1539
1537
band at 1030 cmꢀ1 characteristic of pyropho-
sphate species appeared and became strong band.
In addition to that two more bands at 1122 and
723 cmꢀ1 emerged and became distinct with the
increase in temperature to 290 8C. The spectral
changes observed in the temperature range from
240 to 290 8C resulted in the transformation from
Na2HPO4 to Na4P2O7.
Further studies at higher temperature showed
several phase transformations of the sodium
pyrophosphate (Na4P2O7) formed in this conden-
sation process similar to that reported earlier [17].
3.10. Thermograms
As no further weight loss was observed after
condensation, the TG and DTG thermograms of
Na2HPO4 ×
/
12H2O in the temperature range from
30 to 400 8C are shown as in Fig. 8a, though it
was recorded from 30 to 600 8C. TG thermogram
showed a loss of 59.3% weight in the entire
dehydration process contributing to the loss of
Fig. 6. (a) TRI thermogram and (b) DTRI thermogram for the
stretching bands of H2O during dehydration of Na2HPO4 ×
/
12H2O in the temperature range from 30 to 100 8C.
The derivative of the TRI thermogram was also
plotted in the temperature range from 30 to
100 8C and is shown in Fig. 6b. It is the DTRI
thermogram. It showed dips at 38, 41, 52, 72 and
82 8C indicating maximum rate of dehydration in
correspondence to the five dehydration steps,
respectively.
3.9. Condensation
Fig. 7 shows the spectral variations observed in
the PO34ꢀ region from 350 to 1250 cmꢀ1 for the
condensation process in the temperature range
from 240 to 290 8C. The bands observed at 84 8C
(Fig. 4b) gradually decreased in intensity as the
temperature rose to 240 8C (not shown). With
increase in temperature from 240 to 290 8C, a new
Fig. 7. Thermo-Raman spectra of Na2HPO4 ×
temperature range from 240 to 290 8C for the condensation to
Na4P2O7 in the range from 350 to 1250 cmꢀ1
/
12H2O in the
.

1538
A. Ghule et al. / Spectrochimica Acta Part A 59 (2003) 1529Á1539
/
12 water molecules against the calculated weight
loss of 60.3%. The TG thermogram showed
sudden loss in weight starting from 30 to 84 8C
and the dehydration steps involved could not be
isolated. The entire dehydration process for
losses, but could not clearly resolve the dehydra-
tion steps. The slight shoulder at 38 8C to broad
dip at 45 8C might be due to the first and second
dehydration, respectively. The dips corresponding
to 60 and 66 8C might be as a result of third and
fourth steps of dehydration, respectively. A broad
dip at 293 8C was observed due to the condensa-
tion process. No further information could be
extracted at higher temperature.
Na2HPO4 × 12H2O extended up to 84 8C. After
/
84 8C, there was no considerable weight loss till
246 8C. After 246 8C, loss in weight was observed
again corresponding to the condensation. This
condensation extended in a considerable large
temperature range from 246 to 345 8C resulting
in the Na4P2O7 formation. The observed weight
loss (3.3%) matched with the expected loss (3.2%),
accounting for the loss of 0.5 molecule of water for
the formation of the Na4P2O7. This can be
attributed to water of constitution. Above
345 8C, no weight loss was observed till 600 8C
indicating the completion of the composition
changes.
4. Conclusion
Systematic study using TRS in the temperature
range from 25 to 600 8C was carried out on
Na2HPO4 ×
/
12H2O as a starting material for the
dehydration and condensation processes involved
in a dynamic thermal process. Based on the
spectral variation at the temperature interval of
one degree, observed in H2O and PO34ꢀ regions,
five different Raman spectra for five dehydrations
The DTG thermogram, which is the derivative
of TG thermogram is also shown in Fig. 8b. It
showed dips at 45, 60 and 66 8C indicating weight
from Na2HPO4 ×
formation of intermediates Na2HPO4 ×
Na2HPO4 × 7H2O, Na2HPO4 × 2H2O, Na2HPO4 ×
/
12H2O could be identified for the
/
8H2O,
/
/
/
H2O and Na2HPO4. The TRI thermogram for
H2O band supported the results revealing five
steps of dehydration with proportionate loss of
water in each step. The DTRI thermogram also
showed five dips at 38, 41, 52, 72 and 82 8C for the
maximum rate of dehydration. The TG showed
loss of 59.3% water for entire dehydration process
indicating loss of 12 H2O molecules, with DTG
thermograms showing dips at 45, 60 and 66 8C
but unable to resolve the five steps of dehydration
involved. The formation of Na4P2O7 in the
temperature range 246 to 345 8C in the condensa-
tion process from Na2HPO4 was also observed
indicating the loss of 0.5 H2O molecule in TG
thermogram, which is in agreement with the
expected loss of 0.5 molecule of water. Corre-
sponding dip for condensation at 293 8C was
observed in DTG thermogram. The fact that the
TRS setup could detect all the dehydration steps
involved and a condensation process followed by
the phase transformations indicates its sensitivity
and potential future applications if a commercia-
lized instrument is developed.
Fig. 8. (a) TG and (b) DTG thermograms from 30 to 400 8C
started with Na2HPO4 × 12H2O.
/

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/1539
1539
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Acknowledgements
[14] B. Wendrow, K.A. Kobe, Chem. Rev. 54 (1954) 891.
[15] A.D.F. Toy, in: A.F. Trotman-Dickenson (Ed.), Phospho-
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This work was supported by the National
Science Council of Republic of China (NSC-90-
2113-M007-032).
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