Cent. Eur. J. Chem. • 10(5) • 2012 • 1584-1589
DOI: 10.2478/s11532-012-0076-4
Central European Journal of Chemistry
Thermodynamic study of binary mixture
of citric acid and tartaric acid
Research Article
Viorica Meltzer, Elena Pincu*
Department of Physical Chemistry, Faculty of Chemistry,
University of Bucharest, Bucharest 030018, Romania
Received 1 February 2012; Accepted 7 May 2012
Abstract: The solid – liquid phase equilibria for binary mixture of citric acid with tartaric acid were measured using differential scanning
calorimetry. The phase diagram showed the existence a simple eutectic behavior. The thermal properties of this system as heat
of mixing, entropy of fusion and excess thermodynamic functions were computed using enthalpy of fusion values. The composition
of eutectic was determined using a Tammann diagram.
Keywords: Phase diagram • DSC • Excess functions • Eutectic
© Versita Sp. z o.o.
1. Introduction
The study of solid-liquid phase equilibria is very important
in food and pharmaceutical chemistry because most
materials used are composite materials consisting of
two or more constituent materials with significantly
different physical or chemical properties. In the food
industry, citric acid and tartaric acid are known as the
food additives E330 and E334, respectively, while in the
pharmaceutical industry they are known as excipients.
Citric acid is a natural compound and is used as an
antioxidant and preserving agent in the food industry.
In the pharmaceutical industry, citric acid is used as a
stabilizer of active ingredients of preparations and as a
flavor additive in tablets [1-3]. Tartaric acid is mainly used
in the food, pharmaceutical and cosmetics industries
[4,5]. Tartaric acid, in either the dextrorotary or racemic
form, is used as a flavoring in foods and beverages.
In medicine, tartaric acid is rarely used alone and is a
constituent of many proprietary granular effervescent
preparations. In the pharmaceutical industry, a mixture
of citric acid and tartaric acid is used to prepare the
fast-disintegrating tablets of lorazepam [6]. For this
formulation, a ternary mixture of sodium bicarbonate
and citric acid and tartaric acid were preheated at a
temperature of 80oC. The aim of the present study was to
obtain a solid – liquid phase diagram for binary mixture of
citric acid and tartaric acid in order to determine thermal
behavior of this mixture at processing temperature used
in pharmaceutical and food industry.
2. Experimental procedure
2.1. Materials
Anhydrous citric acid (2 hydroxypropane-1,2,3tricarboxylic acid) (≥99.5%, Aldrich Ref. No. 251275)
and (+)-Tartaric acid (2,3-dihydroxybutanedioic acid)
(>99.5%, Aldrich Ref. No. 251380) were purchased from
Sigma-Aldrich. Their melting points and enthalpies of
fusion were measured using a DSC (Perkin Elmer). The
melting temperatures of pure components were in close
agreement with literature data and no further purification
was done.
Mixtures with a known concentration were prepared
by weighing and dissolving in water and slow evaporation
of the solvent at room temperature.
2.2. Methods
The melting temperature and enthalpy of fusion
of eutectic phases were determined by differential
scanning calorimetry (Perkin Elmer Diamond DSC)
under a heating rate of 10 degree/min over a temperature
range (293–450) K. The apparatus was calibrated for
* E-mail: [email protected]
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V. Meltzer, E. Pincu
Table 1. Melting points, enthalpy of fusion and entropy of fusion of pure components and eutectic mixture.
Compound
ΔfusH/(kJ mol-1)
Tfus /K
ΔfusS/(J mol-1K-1)
α
Exp.
Lit.
Exp.
Lit.
Exp.
Citric acid (CA)
427.80±0.09
428.75 [7]
40.32±0.58
40.34 [7]
40.15 [8]
94.26±1.34
Tartaric acid (TA)
445.04±0.25
443.15 [9]
36.31±0.17
-
81.59±0.43
9.81
Eutectic
409.00±0.05
-
36.13±0.12
-
88.34±0.29
10.62
Figure 1.
DSC curves at different composition for citric acid
(CA) + tartaric acid (TA) mixture.
temperature and enthalpy by melting high purity indium.
The instrument was flushed with helium. Sample of 9 to
13 mg were transferred into aluminium crucibles which
were sealed and weighed with the Partner XA balance
with a precision of 10 μg.
The microstructure of pure components and
eutectic was recorded by placing a drop of molten
compound on a hot glass slide. The melt was covered
with cover-slip and it was solidified unidirectionally. The
slide was put on the platform of an optical microscope
(BioLux Al, Bresser) and interesting regions were
photographed.
3. Results and discussion
3.1. Determination
diagram
of
solid-liquid
phase
Before constructing the phase diagram, DSC
measurements were performed for pure components
and for binary mixtures.
A typical DSC curves obtained for citric acid (CA),
tartaric acid (TA) and binary mixtures are shown
in Fig. 1.
For the pure components only one melting DSC
peak is found, while for the binary mixtures CA/TA
two melting DSC peak were observed. The first peak
11.34
which appears at lower temperature was attributed to
the melting of eutectic composition and second one, at
higher temperature, correspond to the melting of major
component.
The melting points of the pure substances are
in good agreement with the value published in the
literature [7-9]. Physical properties of pure compounds
and eutectic mixture are given in Table 1.
The mixing process of two components is regarded
as the ideal solution model and transition temperature
is lower that of the pure components. An ideal eutectic
mixture implies the existence of complete insolubility
between the two components at all concentrations.
Thus, according to the Schroder-van Laar equation [10]
eutectic transition temperature of mixture of CA and TA
can be calculated:
(1)
In this equation xi is the mole fraction of the components
at the temperature T, R the gas constant,
the
molar enthalpy of fusion of component i and Tfus,i is the
melting temperature of the pure component.
The melting temperature of eutectic and liquid
transition obtained from the DSC curves is presented
in Table 2.
The ideal phase diagram was compared with real
phase diagram obtained from DSC curves and is shown
in Fig. 2 as temperature – composition plot.
The experimental and theoretical solidus – liquidus
curves do not coincide with each other, confirming its
deviation from ideality and the occurrence of molecular
interactions between condensed phases.
The phase diagram show that the binary mixture
of CA and TA exhibits a simple eutectic behavior with
a eutectic point at 409 K. More information can be
obtained from the enthalpy of fusion of the eutectic
composition. Thus, after deconvolution of the melting
endotherm DSC peaks using the multiple nonlinear
regression method, we constructed the Tamman
diagram and obtain a characteristic triangle shape
(Fig. 2b). It is known that the Tamman’s diagram
provides a “thermodynamic” stoichiometry that may
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Thermodynamic study of binary mixture
of citric acid and tartaric acid
differ from the structural stoichiometry obtained from
crystalline lattice [11]. At eutectic temperature the value
of enthalpy of fusion clearly establish that the eutectic is
Table 2. Melting
temperature and enthalpy of the binary system
citric acid / tartaric acid.
xTA
1st DSC peak
T/K
2nd DSC peak
ΔH/(kJ mol-1)
T/K
0.00
-
-
427.80
0.10
409.92
11.08
425.59
0.20
408.50
21.93
420.50
0.30
408.94
29.18
416.00
0.40
409.00
36.13
409.00
0.50
408.83
28.56
413.15
0.60
410.02
22.15
418.02
0.70
408.32
17.43
424.16
0.80
408.98
12.63
430.15
0.90
407.87
5.45
439.12
1.00
-
-
445.04
Figure 2.
a thermodynamically preferred phase, whose formation
is limited by the system stoichiometry [12]. From
the Tamman plot the temperature and composition
of eutectic system were accurately determined:
Teu = 408.9K and xe =0.38.
3.2. Thermochemistry
For the simple eutectic system heat of fusion can be
obtained from DSC endotermic peak, (Δ fusH)exp, or from
mixture law, (ΔfusH) calc, [13] if the eutectic is a simple
mechanical mixture of the two components without any
type of association in the melt:
(2)
were x1 and x2 are the mole fraction and
and
are the experimental value of heat of fusion for
citric acid and tartaric acid, respectively.
The value of enthalpy of mixing, ∆ mix H , of the
eutectic is given by the following equation [14] and are
presented in Table 1:
(3)
a) solid – liquid phase diagram for binary mixture citric acid - tartaric acid. (I) Ideal curve; (II) liquidus curve; (III) solidus
temperature curve. b) Tamman plot of binary system citric acid - tartaric acid.
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Table 3. Activity coefficients and excess thermodynamic function data for binary mixture.
xTA
T (K)
GE (J mol-1)
SE (J mol-1K-1)
µ 1E
(J mol-1)
µ E2
(J mol-1)
0.1
425.59
0.0465
1.8541
804.07
-1.89
164.48
6560.37
0.2
420.50
0.0263
1.0367
798.47
-1.90
92.00
3624.35
0.3
416.00
0.0351
0.5189
623.30
-1.50
121.31
1794.61
0.4
409.00
-0.0103
0.0515
48.99
-0.12
-35.11
175.15
0.5
413.15
0.2911
-0.0644
389.43
-0.94
999.96
-221.11
0.6
418.02
0.6510
-0.1235
647.45
-1.55
2262.60
-429.33
0.7
424.16
1.1067
-0.1264
858.69
-2.02
3902.65
-445.87
0.8
430.15
1.6714
-0.1166
861.93
-2.00
5977.30
-416.91
0.9
439.12
2.5949
-0.0269
858.80
-1.96
9473.43
-98.38
is the heat of fusion determined
where
experimentally from DSC curves and
is the
corresponding calculated value.
The negative value of heat of mixing (-2.67 kJ mol-1),
suggests the formation of clusters in the eutectic melt.
This is possible because the molecules of TA and CA
contain hydroxyl group and can associate by hydrogen
bonds. Exothermic mixing indicates a tendency to
ordering between the two components.
In order to know the nature of interaction between
the components forming the eutectic the excess
thermodynamic function were calculated [15]:
(4)
(5)
(6)
were xi,
and
are the mole fraction, activity
coefficient in the liquid state and the variation of
logarithm of activity coefficient in the liquid state as a
function of temperature of the component i.
The activity coefficient of a component i present
in the eutectic melt, neglecting the difference for
heat capacity of the liquid and solid phases, can be
calculated from equation [16]:
(7)
where xi, ,
and Tfus,i are the mole fraction,
activity coefficient, heat of fusion and the melting
temperature of the component i, R is the gas constant
and T is the liquidus temperature.
The solid-liquid phase diagram obtained from
experimental data do not coincide with theoretical one
and confirms the deviation of the system from ideality
and the occurrence of molecular interaction between
condensed phases. The activity coefficients determine
the mixing behaviour of the system in the solid state
and in the liquid state. The values of activity coefficients
and excess function are given in Table 3 and graphically
drawn in Fig. 3.
The positive values of excess free energy, GE,
predict that molecular association between like
molecules is stronger than between unlike molecules
[17]. The negative values of excess entropy for the
entire composition range indicates that in the case of
citric acid + tartaric acid system there is a negative
deviation from ideal behavior [18].
3.3. Microstructure
The interactions that can occur between the molecules
affect the values of enthalpy and entropy of fusion of
pure components and eutectic mixture. The entropy of
fusion were calculated using the following relation:
(8)
The growth morphology from melt depending to the
nature of solid – liquid interface and can be predicted
from the entropy of fusion value. The predicted structure
is related to roughness factor (α) which is closely related
to entropy of fusion by the equation [15,19]:
(9)
The crystallographic factor î is usually less than or equal
to one and represents the fraction of total number of
neighbors situated in the newly formed layer, while
is the entropy of fusion in dimensionless units.
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Thermodynamic study of binary mixture
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Figure 3. Excess functions GE and SE for binary system citric acid - tartaric acid.
Figure 4. Microstructure of a) tartaric acid; b) citric acid and c) eutectic.
The values of entropy of fusion and Jackson’s
roughness parameter are presented in Table 1.
The value of α being higher than 2 suggests a faceted
morphology for the eutectic system.
The microstructure of the eutectic phase, recorded
on a microscope, is presented in Fig. 4 to compare with
microstructure of the pure components.
The microstructure of the eutectic system shows a
feather morphology where the two phases are grown
almost together.
4. Conclusions
The phase diagram study of the binary system of citric
acid – tartaric acid shown the existence a simple eutectic
with 0.38 mole fraction of tartaric acid. The negative
value of the heat of mixing, suggests the formation of
cluster of molecules. On the other hand, the positive
value of excess free energy indicate that molecular
interactions between like molecules is stronger that
between unlike molecules.
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V. Meltzer, E. Pincu
The value of Jackson’s roughness parameter,
suggest that phases grow with faceted morphology.
The microstructural studies of the eutectic system show
a feather morphology.
References
[1] S. Schilling, C. Bruce, N. Shah, A. Waseem Malick,
J. McGinity, Int. J. Pharm. 361, 158 (2008)
[2] R.G. Strickley, Pharm. Res. 21(2), 201 (2004)
[3] Q. Lu, G. Zografi, J. Pharm. Sci. 86(12), 1374
(1997)
[4] P. Thorey, P. Bombicz, I.M. Szilágyi, P. Molnár,
G. Bánsághi, E. Székely, B. Simándi, L. Párkányi,
G. Pokol, J. Madarász, Thermochim. Acta 497, 129
(2010)
[5] L. Ribeiro, T. Loftsson, D. Ferreira, F. Veiga, Chem.
Pharm. Bull. 51(8), 914 (2003)
[6] S.B. Shirsand, S. Suresh, L.S. Jodhana,
P.V. Swamy, Indian J. Pharm. Sci. 72(4), 431
(2010)
[7] A. Forster, J. Hempenstall, I. Tucker, T. Rades, Int.
J. Pharm. 226, 147 (2001)
[8] D.
Wyrzykowski,
E.
Hebanowska,
G. Nowak-Wiczk, M. Makowski, L. Chmurzyński,
J. Therm. Anal. Calorim. 104(2), 731 (2010)
[9] F.Q. Meng, M.K. Lu, Z.H. Yang, H. Zeng, J. Therm.
Anal. Calorim. 52(2), 609 (1998)
[10] Z. Sedláková, I. Malijevská, Fluid Phase Equilib.
261, 129 (2007)
[11] J.M. Guenet, Macromol. Symp. 258, 179 (2007)
[12] J.W. Rice, E.M. Suuberg, J. Chem. Thermodyn. 42,
1356 (2012)
[13] U.S. Rai, K.D. Mandal, Can. J. Chem. 67, 239
(1989)
[14] U.S. Rai, R.N. Rai, J. Cryst. Growth 191, 234
(1998)
[15] R.S.B. Reddi, V.S.A. Kumar Satuluri, U.S. Rai,
R.N. Rai, J. Therm. Anal. Calorim. 107, 377 (2012)
[16] B.L. Sharma, R. Kant, R. Sharma, S. Tandon,
Mater. Chem. Phys. 82, 216 (2003)
[17] U.S. Rai, R.N. Rai, J. Therm. Anal. Calorim. 53,
883 (1998)
[18] P. Gupta, T. Agrawal, S. Saran Das, N.B. Singh,
J. Chem. Thermodyn. 48, 291 (2012)
[19] R.S.B. Reddi, V.S.A. Kumar Satuluri, R.N. Rai,
J. Therm. Anal. Calorim. 107, 183 (2012)
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