Thermal behaviour of mandelic acid, sodium mandelate and its compounds with some bivalent transition metal ions

Article abstract of DOI:10.1016/j.tca.2012.01.012

Characterization, thermal stability and thermal decomposition of transition metal mandelates, M(C₆H₅CH(OH)CO₂)₂ (M = Mn(II), Fe(II), Co(II), Ni(II), Cu(II), Zn(II), as well as the thermal behaviour of mandelic acid C₆H₅CH(OH)CO₂H and its sodium salt were investigated employing simultaneous thermogravimetry and differential scanning calorimetry (TG-DSC), TG-DSC coupled to FTIR, infrared spectroscopy (FTIR), elemental analysis and complexometry. All the compounds were obtained in the anhydrous state and the thermal decomposition occurs in two or four consecutive steps. The final residue up to 320°C (Mn), 345°C (Fe), 400°C (Co), 405°C (Ni), 355°C (Cu) and 575°C (Zn) is Mn₃O₄, Fe₂O₃, Co₃O ₄, NiO, CuO and ZnO, respectively. The results also provided information concerning the ligand's denticity, thermal behaviour and identification of gaseous products evolved during the thermal decomposition of these compounds.

Full text of DOI:10.1016/j.tca.2012.01.012

Thermochimica Acta 533 (2012) 16–21
Contents lists available at SciVerse ScienceDirect
Thermochimica Acta
journal homepage: www.elsevier.com/locate/tca
Thermal behaviour of mandelic acid, sodium mandelate and its compounds with
some bivalent transition metal ions
D.J.C. Gomes, F.J. Caires∗, L.S. Lima, A.C. Gigante, M. Ionashiro
Instituto de Química, Universidade Estadual Paulista, CP 355, 14801-970 Araraquara, SP, Brazil
a r t i c l e i n f o
a b s t r a c t
Article history:
Characterization, thermal stability and thermal decomposition of transition metal mandelates,
M(C6H5CH(OH)CO2)2 (M = Mn(II), Fe(II), Co(II), Ni(II), Cu(II), Zn(II), as well as the thermal behaviour
of mandelic acid C6H5CH(OH)CO2H and its sodium salt were investigated employing simultaneous
thermogravimetry and differential scanning calorimetry (TG–DSC), TG–DSC coupled to FTIR, infrared
spectroscopy (FTIR), elemental analysis and complexometry. All the compounds were obtained in the
anhydrous state and the thermal decomposition occurs in two or four consecutive steps. The final residue
up to 320 ◦C (Mn), 345 ◦C (Fe), 400 ◦C (Co), 405 ◦C (Ni), 355 ◦C (Cu) and 575 ◦C (Zn) is Mn3O4, Fe2O3, Co3O4,
NiO, CuO and ZnO, respectively. The results also provided information concerning the ligand’s denticity,
thermal behaviour and identification of gaseous products evolved during the thermal decomposition of
these compounds.
Received 24 October 2011
Received in revised form 8 December 2011
Accepted 11 January 2012
Available online 25 January 2012
Keywords:
Bivalent transition metals
Mandelate
Thermal decomposition
TG–FTIR
© 2012 Elsevier B.V. All rights reserved.
Evolved gases
1. Introduction
to FTIR. The thermal studies were performed in dynamic air atmo-
sphere.
Mandelic acid is an aromatic alpha hydroxy acid with the molec-
ular formula C6H5CH(OH)CO2H. It has a long history of use in
medicine as an antibacterial, particularly in the treatment of uri-
nary tract infections [1]. A survey of literature shows that the
papers involving bivalent transition metal mandelates reported the
structure studies on the glycolato, lactato and mandelato solid com-
plexes of some bivalent iron series metal [2], infrared spectrum of
nickel (II) mandelate complex [3], mixed-ligand complexes of cop-
per (II) with ␣-hydroxycarboxylic acids and 1, 10-phenanthroline
[4], syntheses, structures and magnetic properties of layered metal
(II) mandelates [5] and mononuclear, dinuclear and hydroxo-
bridged tetranuclear complexes from reactions of Cu (II) ions,
mandelic acid diimine ligands [6]. However, few thermoanalyti-
cal studies with bivalent transition metal mandelates on solid state
were found in the literature.
In this paper, the object of the present research was to investi-
gate the thermal behaviour of mandelic acid and its sodium salt, as
well as to prepare solid-state compounds of some bivalent transi-
tion metal ions (i.e. Mn(II), Fe(II), Co(II), Ni(II), Cu(II), Zn(II)) with
mandelate and to characterize and to investigate by means of
complexometry, elemental analysis, X-ray powder diffractometry,
infrared spectroscopy (FTIR), simultaneous thermogravimetry and
differential scanning calorimetry (TG–DSC) and TG–DSC coupled
2. Experimental
The mandelic acid, C6H5CH(OH)CO2H with 99% purity was
obtained from Sigma and it was used as received. Aqueous solution
of sodium mandelate 0.1 mol L−1 was prepared by neutraliza-
tion of the aqueous solution of mandelic acid with 0.1 mol L−1
sodium hydroxide solution. Aqueous solutions of bivalent metal
ions 0.1 mol L−1 were prepared by dissolving the corresponding
chlorides (Mn(II), Co(II), Ni(II), Zn(II)) or sulphate (Fe(II), Cu(II)).
The solid-state compounds were obtained by adding 100.0 mL
of sodium mandelate solution 0.1 mol L−1 to 50.0 mL of the respec-
tive metal ions solutions 0.1 mol L−1. The resulting solutions were
heated at ebullition with continuous stirring for about 1 h, and the
precipitates were filtered off, washed with distilled water until
chloride or sulphate ions were eliminated, dried at ambient tem-
perature and kept in a desiccator over anhydrous calcium chloride.
To avoid oxidation of Mn(II) and Fe(II), all the solutions, the water
employed for washing the precipitate and the reactional system
were purged with nitrogen gas. In the solid-state compounds, metal
ions and mandelate contents were determined from TG curves. The
metal ions were also determined by complexometry with standard
EDTA solution after igniting the compounds to the respective oxides
and their dissolution in hydrochloric acid solution [7,8].
∗
Carbon and hydrogen contents were determined by calculations
based on the mass losses of the TG curves, since the ligand lost
Corresponding author. Tel.: +55 16 3301 9617.
E-mail address: caires.flavio@yahoo.com.br (F.J. Caires).
0040-6031/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.tca.2012.01.012

D.J.C. Gomes et al. / Thermochimica Acta 533 (2012) 16–21
17
Fig. 2. Simultaneous TG–DSC curves of the sodium mandelate (m = 10.1530 mg).
between 125 and 230 ◦C, 230 and 330 ◦C and 330 and 550 ◦C and
the corresponding DSC curve shows four thermal events. The first
endothermic peak at 118 ◦C is due to the melting of mandelic acid
and the endothermic peaks at 200 ◦C and 310 ◦C are attributed to
the thermal decomposition of mandelic acid, which occurs in two
steps with the formation of carbonaceous residue. The formation of
carbonaceous residues was confirmed based on the TG–DSC curves
of mandelic acid obtained up to 350 ◦C. The exothermic peak at
475 ◦C is attributed to the oxidation of the carbonaceous residue.
The gaseous products evolved during the thermal decomposition
were monitored and identified mostly on basis of their FTIR ref-
erence available on NICOLET libraries, as benzaldehyde and CO, as
shown in Fig. 1(b).
For the anhydrous sodium mandelate the simultaneous TG–DSC
curves, Fig. 2, show that the thermal decomposition occurs in two
steps between 240 and 320 ◦C and 320 and 570 ◦C with losses
of 26.99% and 31.62%, respectively corresponding to exothermic
peaks at 255 ◦C, 285 ◦C, 310 ◦C and 520 ◦C attributed to the oxidation
of the organic matter. The small thermal events observed between
350 and 450 ◦C are attributed to the initial thermal decomposition
corresponding to the second mass loss. The total mass loss up to
580 ◦C is in disagreement with the formation of sodium carbonate
(Calc. = 69.51%; TG = 58.60%), as observed in the thermal decompo-
sition of sodium succinate and sodium malonate [9,10]. Tests with
hydrochloric acid solution on sample heated up to the temperature
indicated by the TG–DSC curves (580 ◦C) confirmed the formation
of a black residue identified as a mixture of carbonaceous residue
and sodium carbonate. The mass loss between 680 and 800 ◦C, cor-
responding to an exothermic peak at 770 ◦C is attributed to the
oxidation of the carbonaceous residue.
The analytical and thermoanalytical (TG) data for the synthe-
sized compounds are shown in Table 1. These results establish the
stoichiometry of these compounds, which are in agreement with
general formula: M(L)2, where M represents Mn(II), Fe(II), Co(II),
Ni(II), Cu(II) and Zn(II), and L is mandelate (C6H5CH(OH)COO−).
The X-ray diffraction powder patterns (Fig. 3) show that
all the compounds have crystalline structure, without evidence
Fig. 1. (a) Simultaneous TG–DSC curves (m = 11.7730 mg) and (b) IR spectrum of the
gases released during the thermal decomposition of mandelic acid.
during the thermal decomposition occurs with the formation of
the respective oxides with stoichiometry known, as final residue.
X-ray powder patterns were obtained by using a Siemens D-
˚
5000 X-ray diffractometry, employing Cu K␣ radiation (ꢀ = 1.541 A)
and a setting of 40 kV and 20 mA.
The attenuate total reflectance infrared spectra for mandelic
acid and for its metal-ion compounds were run on a Nicolet iS10
FTIR spectrophotometer, using an ATR accessory with Ge window.
Simultaneous TG–DSC curves were obtained by using
a
TGA–DSC 1 Stare system, from Mettler Toledo. The purge gas was
an air flow of 50 mL min−1 and a heating rate of 10 ◦C min−1 was
adopted, with samples weighing about 10 mg. Alumina crucibles
were used for recording the TG–DSC curves.
The measurements of the gaseous products were carried out
using a TG–DSC Mettler Toledo coupled to a FTIR spectrophotome-
ter Nicolet with gas cell and DTGS KBr detector. The furnace and the
heated gas cell (250 ◦C) were coupled through a heated (T = 200 ◦C)
120 cm stainless steel line transfer with diameter of 3 mm, both
purged with dry air (50 mL min−1). The FTIR spectra were recorded
for formation of isomorphous compounds and the crystallinity
∼
of these compounds follow the order: Fe > Zn > Mn Cu > Co > Ni.
=
with 16 scans per spectrum at a resolution of 4 cm−1
.
The difference in the crystallinity of these compounds must
be due to the velocity of precipitation, since the solid com-
pounds are obtained heating the solution at ebullition temperature,
and the heating, as well as the time of ebullition were not
controlled.
3. Results and discussion
The TG–DSC curves and IR spectrum of the released products
during the thermal decomposition of mandelic acid are shown in
Fig. 1. The TG curve, Fig. 1(a), shows mass losses in three steps
The attenuate total reflectance spectroscopic data on mandelic
acid and its compounds with the metal ions considered in this work

18
D.J.C. Gomes et al. / Thermochimica Acta 533 (2012) 16–21
Table 1
Analytical and thermoanalytical (TG) data for the M(C6H5CHOHCOO)2 compounds.
Compounds
Metal oxide (%)
L (lost) (%)
Calc.
C (%)
Calc.
H (%)
Calc.
Final residue
Calc.
EDTA
TG
TG
TG
TG
MnL2
FeL2
CoL2
NiL2
CuL2
ZnL2
21.35
22.30
22.22
20.69
21.74
22.13
21.64
22.40
22.63
20.23
21.73
22.43
22.10
22.39
22.28
20.73
21.83
22.73
78.65
77.70
77.78
79.31
78.26
77.87
77.90
77.61
77.72
79.27
78.17
77.27
53.78
53.64
53.18
53.22
52.51
52.25
53.27
53.58
53.14
53.19
52.45
51.85
3.96
3.95
3.91
3.92
3.86
3.85
3.92
3.95
3.90
3.92
3.86
3.82
Mn3O4
Fe2O3
Co3O4
NiO
CuO
ZnO
L = mandelate.
Fig. 3. X-ray powder diffraction patterns of the compounds: (a) MnL2, (b) FeL2, (c) CoL2, (d) NiL2, (e) CuL2 and (f) ZnL2 (L = mandelate).
are shown in Table 2. These results show that in all the compounds
as(COO−), ␯(O H) and ␯(C OH) are shifted toward lower ener-
curves also show that all the compounds were obtained in the
anhydrous state.
␯
gies than that observed in the free acid. This behaviour indicates
that coordination is carried out through the hydroxyl and carboxyl
groups [11], in agreement with Ref. [2].
Simultaneous TG–DSC curves of the compounds are shown in
Fig. 4. These curves show mass losses in two or four steps, cor-
responding to endothermic and exothermic peaks attributed to
thermal decomposition and oxidation of organic matter. These
The thermal stability of the compounds (I), as well as the
final temperature of thermal decomposition (II) shown by TG–DSC
curves depend on the nature of the metal ion, and they follow the
order:
(I) Zn = Ni > Co > Cu > Mn > Fe
(II) Zn > Ni > Co > Cu > Fe > Mn
Table 2
Spectroscopic data for mandelic acid and its compounds with some bivalent transition metal ions.
Compounds
␯(OH) (cm−1
)
␯
as(COO−) (cm−1
)
␯s(COO−) (cm−1
)
␯(C OH−) (cm−1
)
HL
3395 w,br
3236 w,br
3249 w,br
3254 w,br
3246 w,br
3139 w,br
3246 w,br
1711 s
1563 s
1568 s
1571 s
1576 s
1631 s
1592 s/1584 s
–
1062 m
1018 m
1010 m
1001 m
999 m
997 m
1008 m/1002 m
MnL2
FeL2
CoL2
NiL2
CuL2
ZnL2
1413 m
1417 m
1411 m
1412 m
1350 m
1336 m/1406 m
L = mandelate; w, br = weak, broad; s = strong; m = medium;
␯(OH) = hydroxyl oxygen–hydrogen stretching frequency; ␯as(COO−) = antisymmetric carboxyl stretching frequency; ␯s(COO−) = symmetric carboxyl stretching frequency;
␯(C–OH) = carbon–hydroxyl stretching frequency.

D.J.C. Gomes et al. / Thermochimica Acta 533 (2012) 16–21
19
Fig. 4. Simultaneous TG–DSC curves of the compounds: (a) MnL2 (m = 10.060 mg), (b) FeL2 (10.019 mg), (c) CoL2 (10.044 mg), (d) NiL2 (10.089 mg), (e) CuL2 (10.064 mg) and
(f) ZnL2 (10.045 mg).
The thermal behaviour of the compounds is also heavily depen-
dent on the nature of the metal ion and so the features of each of
these compounds are discussed individually.
exothermic reaction must be occurring simultaneously. The total
mass loss up to 345 ◦C is in agreement with the formation of Fe2O3,
as final residue (Calc. = 77.70%; TG = 77.61).
3.1. Manganese compound
3.3. Cobalt compound
The simultaneous TG–DSC curves are shown in Fig. 4(a). The
thermal decomposition of the anhydrous compound occurs in two
steps, between 160 and 250 ◦C and 250 and 320 ◦C with losses
of 65.21% and 12.69%, respectively corresponding to exothermic
peak at 250 ◦C and 310 ◦C, which are attributed to oxidation of the
organic matter. The total mass loss up to 320 ◦C is in agreement
with the formation of Mn3O4 (Calc. = 78.65%; TG = 77.90%). The last
mass loss observed between 900 and 930 ◦C, corresponding to an
endothermic peak at 920 ◦C is attributed to the reduction of Mn3O4
to MnO in agreement with the literature [12].
The simultaneous TG–DSC curves are shown in Fig. 4(c). The
thermal decomposition of the anhydrous compound occurs in two
steps between 230 and 335 ◦C and 335 and 400 ◦C, with losses of
65.58% and 15.14% respectively corresponding to exothermic peaks
at 330 ◦C and 335 ◦C attributed to oxidation of the organic matter.
The total mass loss up to 400 ◦C is in agreement with the formation
of Co3O4 (Calc. = 77.78%; TG = 77.72%). The last mass loss between
895 and 915 ◦C is attributed to reduction reaction of Co3O4 to CoO
(Calc. = 1.48%; TG = 1.46%) [10,13].
3.2. Iron compound
3.4. Nickel compound
The simultaneous TG–DSC curves are shown in Fig. 4(b). The
thermal decomposition of the anhydrous compound occurs in four
steps, between 80 and 195 ◦C, 195 and 230 ◦C, 230 and 330 ◦C
and 330 and 345 ◦C, with losses of 29.32%, 18.40%, 17.79% and
12.10%, respectively corresponding to the exothermic peaks at
195 ◦C, 210 ◦C and 340 ◦C attributed to oxidation of Fe(II) to Fe(III)
and organic matter. Absence of thermal events corresponding to
the third mass loss suggests that in this step, endothermic and
The simultaneous TG–DSC curves are shown in Fig. 4(d). The
thermal decomposition of the anhydrous compound occurs in two
steps between 270 and 350 ◦C and 350 and 405 ◦C with losses
of 60.32% and 18.95%, respectively corresponding to exothermic
peaks at 340 ◦C and 380 ◦C attributed to oxidation of the organic
matter. The total mass loss up to 405 ◦C is in agreement with the
formation of NiO, as final residue (Calc. = 79.31%; TG = 79.27%).

20
D.J.C. Gomes et al. / Thermochimica Acta 533 (2012) 16–21
Fig. 5. IR spectra of gaseous products evolved during the decomposition of the compounds: (a) ZnL2 (b) NiL2. L = mandelate.
3.5. Copper compound
4. Conclusion
The simultaneous TG–DSC curves are shown in Fig. 4(e). The
thermal decomposition of the anhydrous compound also occurs in
two steps between 180 and 250 ◦C and 250 and 365 ◦C with losses of
73.25% and 4.92%, respectively corresponding to exothermic peaks
at 220 ◦C and 355 ◦C attributed to the oxidation of organic matter.
The total mass loss up to 355 ◦C is in agreement with the formation
of CuO, as final residue. (Calc. = 78.26%; TG = 78.17%.)
From TG, complexometry and elemental analysis results, a gen-
eral formula could be established for the synthesized compounds.
The X-ray powder patterns showed that all the compounds have
a crystalline structure, without evidence for formation of isomor-
phous compounds.
The infrared spectroscopic data suggests that both hydroxyl and
carboxyl groups act as coordination sites.
The simultaneous TG–DSC curves provided previously unre-
ported information about the thermal stability and thermal
decomposition of these compounds in dynamic air atmosphere.
The monitoring of evolved gases during the heating of mandelic
acid showed that the acid decomposes with the release of benzalde-
hyde, and CO. The thermal decomposition of metal mandelates
occurs with release of benzaldehyde, CO and CO2 for Mn(II), Fe(II),
Co(II), Cu(II) and Zn(II) compounds and benzaldehyde, benzoic acid,
CO and CO2 for Ni(II) compound.
3.6. Zinc compound
The simultaneous TG–DSC curves are shown in Fig. 4(f). The
thermal decomposition of the anhydrous compound occurs in four
steps between 270 and 320 ◦C, 320 and 350 ◦C, 350 and 440 ◦C and
440 and 575 ◦C with losses of 17.74%, 17.74%, 24.47% and 17.32%,
respectively corresponding to endothermic peak at 305 ◦C ascribed
to the thermal decomposition and exothermic peaks at 330 ◦C and
545 ◦C attributed to the oxidation of the organic matter.
No thermal event corresponding to the third mass loss is
observed in the DSC curve, probably because endothermic and
exothermic reactions must be occurring simultaneously and the
net heat is not sufficient to produce a thermal event. The total mass
loss up to 575 ◦C is in agreement with the formation of ZnO, as final
residue (Calc. = 77.87%; TG = 77.27%).
The gaseous products evolved during the thermal decomposi-
tion of the sodium and bivalent transition metal ion mandelates
studied in the present work were monitored by FTIR and it has
benzaldehyde, CO and CO2 (Na, Mn, Fe, Co, Cu, Zn) or benzalde-
hyde, benzoic acid, CO and CO2 (Ni). The IR spectra of the gaseous
products evolved during the thermal decomposition of these com-
pounds are shown in Fig. 5.
Acknowledgements
The authors thank FAPESP, CNPq and CAPES Foundations (Brazil)
for financial support.
References
[1] P.L. Putten, Antonie Van Leeuwenhoek 45 (1979) 622–623.
[2] A. Fischinger, A. Sarapu, A. Companion, Can. J. Chem. 47 (1969) 2629–2637.
[3] P.V. Khadikar, S.D. Chauhan, M.G. Kekre, R.L. Ameria, Sci. Cult. 37 (10) (1971)
491–492.
[4] R. Carballo, B. Covelo, S. Balboa, A. Castin˜eiras, J. Niclos, Z. Anorg. Allg. Chem.
627 (2001) 948–954.
[5] A. Beghidja, S. Hallynck, R. Welter, P. Rabu, Eur. J. Inorg. Chem. (2005) 662–669.

D.J.C. Gomes et al. / Thermochimica Acta 533 (2012) 16–21
21
[6] S. Balboa, R. Carballo, A. Castin˜eiras, J.M. Gonzales-Perez, J. Niclós-Gutiérrez,
Polyhedron 27 (2008) 2921–2930.
[10] F.J. Caires, L.S. Lima, C.T. Carvalho, R.J. Giagio, M. Ionashiro, Thermochim. Acta
497 (2010) 35–40.
[7] H.A. Flaschka, EDTA Titrations: An Introduction to Theory and Practice, 2nd ed.,
Pergamon Press, Oxford, 1964.
[8] C.N. de Oliveira, M. Ionashiro, C.A.F. Graner, Eclet. Quim. 10 (1985) 7–10.
[9] F.J. Caires, L.S. Lima, C.T. Carvalho, M. Ionashiro, Thermochim. Acta 500 (2010)
6–12.
[11] K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Com-
pounds, Part B, 5th ed., Wiley, New York, 1997, pp. 58–61.
[12] L. Biernack, S. Pokrzywnicki, J. Therm. Anal. Calorim. 55 (1999)
227–232.
[13] Z.P. Xu, H.C. Zeng, J. Mater. Chem. 8 (11) (1998) 2499–2506.