Indian Journal of Chemistry
Vol. 48A, June 2009, pp. 806-811
Partial molar volumes of oxalic acid and
its salts in water-rich binary aqueous mixtures
of methanol
solvent. In an attempt to understand the interactions in
methanol + water systems, we report herein the partial
molar volumes of oxalic acid and oxalates in binary
aqueous mixtures of methanol
M L Parmar* & M K Guleria
Experimental
Oxalic acid, ammonium oxalate, sodium oxalate
and potassium oxalate (AR grade) were used after
drying over P2O5 in a desiccator. The reagents were
always placed in the desiccator over P2O5 to keep
them in dry atmosphere. Freshly distilled conductivity
water (sp. cond. ~ 10-6 Ω-1 cm-1) was used for
preparing aqueous mixtures of methanol (MeOH) as
well as standard liquid. Methanol (AR) was purified
by refluxing it over calcium oxide for several hours
and then distilling. Only the middle fraction was used
for the present study. The physical constants, density
and viscosity, of MeOH at 298.15 K are 0.7865 ×
10-3 kg m-3 and 0.5441 x 10-3 N s m-2, respectively,
which agree with reported values16 (0.7866 ×
10-3 kg m-3 and 0.5445 × 10-3 N s m-2).
Department of Chemistry, Himachal Pradesh University,
Summer Hill, Shimla 171 005, India
Email: [email protected]
Received 26 December 2008; revised and accepted 14 May 2009
Partial molar volumes of oxalic acid and its salts, viz., ammonium
oxalate, sodium oxalate and potassium oxalate, have been
determined in different compositions of water-rich binary aqueous
mixtures of methanol (3, 6, 9, 12, and 15 wt. percent of methanol)
from solution density measurements at 298.15 K and in 3% (w/w)
methanol + water at different temperatures. The data have been
analysed by using Masson equation and the obtained parameters
have been interpreted in terms of ion-solvent and ion-ion
interactions. The partial molar volumes vary with temperature as a
power series of temperature. Structure making/breaking capacities
of the electrolytes have been inferred from the sign of
([∂ 2φ v0 / ∂T 2 ) p , i.e. the second derivative of partial molar
volume with respect to temperature at constant pressure. Oxalic
acid and its salts act as structure breakers in binary aqueous
mixtures of methanol.
Keywords: Solution chemistry, Thermodynamics, Partial molar
volumes, Ion-solvent interactions, Solvent-solvent
interactions, Oxalates
Partial molar volumes of electrolytes provide valuable
information about ion-ion, ion-solvent and solventsolvent interactions.1-13 These interactions help in
better understanding of the nature of the solute and
solvent i.e. whether the added solute modifies or
distorts the structure of the solvent.
The addition of organic solvent to an aqueous
solution of electrolyte brings about a change in ion
solvation and often results in a large change in the
reactivity of the dissolved electrolyte.14,15 As partial
molar volume of a solute reflects the cumulative
effects
of
solute-solute
and
solute-solvent
interactions, it would be of interest to study partial
molar volumes of oxalic acid and its salts. Such data
are expected to highlight the role of cation and anion
of an electrolyte in influencing its partial molar
volume at infinite dilution in water-rich binary
aqueous mixtures of methanol, i.e., a protic organic
The aqueous mixtures of MeOH as well as the
solutions of oxalic acid and its salts were made by
weight and molalities, ‘m’, which were converted into
molarities ‘c’ using the standard expression:
c = 1000 d m / (1000 + m M2), where d is the solution
density and M2 molecular weight of the solute.
Density was measured by using Ward and Millero18
type apparatus described elsewhere17, 18.
The apparent molar volumes ( φ v ) were calculated
from the density data using the following standard
M2
10 3 ⎡ d − d 0 ⎤ , where d
expression: φ v =
−
0
⎥
⎢
d0
c ⎣ d0 ⎦
is the density of solvent (MeOH + water mixture).
Results and discussion
The densities, of the solutions of oxalic acid and its
salts, viz., ammonium oxalate, sodium oxalate and
potassium oxalate in MeOH + water (3, 6, 9, 12 and
15% by weight of MeOH) at 298.15 K, have been
used to calculate the apparent molar volumes ( φ v ) of
oxalic acid and its salts using Eq. (1). The plots of φ v
versus square root of molar concentration were found
NOTES
to be linear and without scatter, with negative slopes
in different compositions of MeOH + water. A
representative plot for ammonium oxalate in different
compositions of MeOH + water at 298.15 K is shown
in Fig. 1.
The partial molar volumes ( φ v0 ) and the
experimental slopes (Sv) were calculated using leastsquares fits to the linear plots of experimental values
of φ v versus square root of molar concentration, (c½),
using Masson equation, φ v = φ v0 + Sv c½ , where φ v0
0
= V 2 is the partial molar volume and Sv the
experimental slope. The values of φ v0 and Sv (with
standard errors) obtained in different compositions of
water-rich binary aqueous mixtures of methanol at
298.15 K are recorded in Table 1.
It is evident from Table 1 that the values of Sv
become negative as the system changes from aqueous
Fig. 1 ⎯ Plots of φv versus c1/2 for ammonium oxalate in different
compositions of MeOH + water at 298.15 K.
807
to aqueous mixture of methanol at 298.15 K. In other
words the addition of a small quantity of MeOH
(3% w/w) to water changes the sign of Sv from
positive in water to negative in MeOH + water. The
negative values of Sv, for oxalic acid and its salts in
water + methanol mixtures at 298.15 K, suggest the
presence of weak solute-solute/ion-ion interactions. It
is also clear from Table 1 that the value of Sv, of
course negative, further increases in magnitude for an
individual solute as the composition of methanol in
water increases thereby showing that solute-solute
interactions, though weak, further improve with the
increase of methanol content in water at 298.15 K,
which may be attributed to the decrease in solvation.
In other words, solvent-solvent (water-methanol)
interactions increase with the increase of methanol in
water, i.e., methanol has more affinity for water as
compared to the added solute.
A comparison of results obtained in methanol +
water with those obtained in only water suggests a
possible explanation for the presence of negative
values (i.e., negative slopes) for oxalic acid and its
salts. Although at infinite dilution, oxalic acid and its
salts are completely dissociated in the different
compositions of methanol + water, the situation
would be different at higher concentrations of oxalic
acid and its salts. Due to the moderate dielectric
constant of MeOH + water mixtures, these solutes
remain completely ionized even at fairly high
concentrations. Therefore an appreciable interionic
penetration occurs and this gives rise to a negative
slope (i.e. weak ion-ion interactions and strong ionsolvation) in the φ v versus c½ curves for these solutes.
These negative values of SV, in different compositions
of water + MeOH, also suggest the presence of
cation-anion penetration19 which occurs due to
competition between ions to occupy the void space of
the large solvent molecules.
It is clear from Table 1 that the φ v0 values are
positive and large in magnitude, for oxalic acid and its
salts, in the entire composition range of water +
methanol, at 298.15 K, thereby showing the presence
of strong ion-solvent interactions. Table 1 also shows
that the value of φ v0 decreases, for an individual
solute, with the increase of methanol composition in
water, showing that ion-solvent interactions decrease
on the addition of MeOH in water. A quantitative
comparison of φ v0 values shows that these are much
INDIAN J CHEM, SEC A, JUNE 2009
808
0
Table 1⎯Partial molar volumes ( φ v0 ), experimental slopes (Sv) and partial molar volumes of transfer ( ∆V 2,tr ) for oxalic acid and its salts
in water and MeOH + water mixtures at 298.15 K (Standard errors are given in parentheses)
MeOH + water (% w/w)
Oxalic acid
0 (water)
3
6
9
12
15
Ammonium oxalate
0 (water)
3
6
9
12
15
Sodium oxalate
0 (water)
3
6
9
12
15
Potassium oxalate
0 (water)
3
6
9
12
15
φ v0 × 10-6 (m3 mol-1)
Sv × 10-6(m3 dm3/2 mol-3/2)
∆V 2,tr × 10-6(m3 mol-1)
52.85 (± 0.38)
103.45 (± 0.28)
101.89 (± 0.04)
99.74 (± 0.05)
95.62 (± 0.09)
92.26 (± 0.09)
0.865 (± 0.019)
-0.683 (± 0.014)
-0.654 (± 0.002)
-0.619 (± 0.002)
-0.529 (± 0.005)
-0.441 (± 0.004)
50.60
49.04
46.89
42.77
39.41
59.13 (± 0.10)
104.93 (± 0.10)
102.02 (± 0.05)
94.76 (± 0.03)
82.87 (± 0.03)
77.51 (± 0.02)
0.775 (± 0.002)
-0.664 (± 0.005)
-0.618 (± 0.002)
-0.593 (± 0.002)
-0.342 (± 0.005)
-0.313 (± 0.004)
45.80
42.89
35.63
23.74
18.38
8.53 (± 0.05)
83.27 (± 0.15)
75.22 (± 0.15)
57.96 (± 0.19)
49.42 (± 0.08)
44.42 (± 0.24)
0.952 (± 0.005)
-1.765 (± 0.008)
-1.688 (± 0.007)
-1.294 (± 0.010)
-1.164 (± 0.004)
-1.111 (± 0.012)
74.74
66.69
49.43
40.89
35.89
15.77 (± 0.29)
117.06 (± 0.48)
109.25 (± 0.27)
99.31 (± 0.22)
94.57 (± 0.17)
90.50 (± 0.23)
1.514 (± 0.015)
-1.479 (± 0.025)
-1.349 (± 0.002)
-1.179 (± 0.002)
-1.125 (± 0.005)
-1.073 (± 0.004)
101.29
93.48
83.54
78.80
74.73
larger in magnitude than those of Sv values for all the
solutes. This suggests that ion-solvent interactions
dominate over the ion-ion interactions in water +
methanol mixtures at 298.15 K.
0
The volumes of transfer ( ∆V 2,tr ) have been
0
∆V 2,tr
calculated by using the expression:
=
φ (MS) − φ (w), where φ (MS) and φ (w) are the
0
v
0
v
0
v
0
v
partial molar volumes of oxalic acid and its salts in the
mixed solvent (MeOH + water) and water, respectively
0
(Table 1). The value of ∆V 2,tr , for a particular solute,
decreases with the increase of methanol content in
0
water at 298.15 K. The decrease in φ v0 and ∆V 2,tr
may be attributed to the increase in electrostriction in
the presence of methanol. Thus the electrostriction
effect, which brings about shrinkage in the volume of
the solvent, goes on increasing with the increase of
methanol content in the mixed solvent. On comparing
0
the values of φ v0 for an individual solute in water with
those in binary aqueous mixtures of methanol, a large
increase in the values of φ v0 is observed on addition of
only a very small quantity of methanol (3% w/w) to
water. This indicates that electrostriction, which existed
in aqueous solution, is reduced considerably on
addition of methanol to water at 298.15 K. Since
electrostriction primarily reflects solute-solvent
interactions, it may be inferred from these results that
solute-solvent interactions improve considerably on the
change of solvent from water to water + methanol.
Since the oxalate ion is common in oxalic acid and
0
its salts, therefore, from the values of ∆V 2,tr it may be
inferred that in a particular composition of water +
methanol, the electrostriction for H+, NH +4 , Na+ and
K+ cations, follows the following order: NH +4 > H+ >
Na+ > K+.
NOTES
809
This indicates that the K+ ion is preferentially
solvated by methanol + water and the order of
preferential solvation of these cations is as follows:
K+ > Na+ > H+ > NH +4 .
0
Further, since the value of ∆V 2,tr , for an
individual solute, decreases with the increase in
composition of methanol in water for oxalic acid and
its salts, therefore, the extent of preferential solvation
for a particular cation decreases with the increase of
methanol content in water, which may be attributed to
the increase in solvent-solvent interactions between
water and methanol.
Since the behaviour of the individual electrolyte
was found to be linear and identical in different
compositions of MeOH + water at 298.15 K, only one
system (3% w/w) has been selected for studying the
effect of temperature. The densities were determined
for various concentrations of oxalic acid and its salts
in 3% (w/w) MeOH + water at different temperatures
(298.15, 303.15, 308.15, 318.15 and 318.15 K).
Linear plots of φ v versus c½ were obtained at
different temperatures for individual solute. A sample
plot for potassium oxalate is shown in Fig. 2.
The values of limiting apparent molar volumes ( φ v0 )
and the experimental slopes (Sv) at different temperatures
have been obtained by using least square fits to the plots
of φ v versus c½ using Eq. (2). These values along with
standard errors are reported in Table 2.
It is evident from Table 2 that the values of Sv are
negative for oxalic acid and its salts in 3% (w/w)
MeOH + water mixture, at all temperatures. This
reflects weak ion-ion interactions in the entire
temperature range in MeOH + water system. The
values of Sv, for oxalic acid and its salts in 3% (w/w)
MeOH + water, decrease in magnitude with the rise in
temperature (Table 2). This suggests that ion-ion
interactions further weaken with the rise in
temperature, which may be attributed to the increase
in solvation of individual solute in MeOH + water
system.
The values of φ v0 increase with the increase in
temperature, for oxalic acid and its salts in MeOH +
water, thereby showing that ion-solvent interactions
are further strengthened with the increase in
temperature. The increase in φ v0 may be attributed to
the increase in solvation.
Fig. 2 ⎯ Plots of φv versus c1/2 for potassium oxalate in 3 % (w/w)
MeOH + water at different temperatures.
The temperature dependence (in K) of φ v0 in 3%
(w/w) MeOH + water for oxalic acid and its salts can
be expressed by:
φ v0 = − 921.86 (± 2.80) + 6.00 (± 0.00) T
– 0.009 (± 0.001) T2
for oxalic acid,
… (4)
φ v0 = − 2369.88 (± 3.65) + 15.16 (± 1.24) T
– 0.023 (± 0.002) T2
for ammonium oxalate,
... (5)
φ v0 = - 2951.13 (± 4.58) + 18.47 (± 2.35) T
– 0.208 (± 0.001) T2
for sodium oxalate, and
... (6)
φ v0 = − 3753.98 (± 2.56) + 23.87 (± 2.22) T
– 0.037 (± 0.003) T2
for potassium oxalate.
… (7)
INDIAN J CHEM, SEC A, JUNE 2009
810
Table 2⎯Partial molar volumes ( φ v0 ), experimental slopes (Sv) and the partial molar volume expansibilities ( φ E0 ) for oxalic acid and its salts
in 3% (w/w) MeOH + water at different temperatures. [Standard errors are given in parentheses]
Sv × 10-6 (m3 dm3/2 mol-3/2)
φ 0 × 10-6 (m3 mol-1)
φ 0 × 10-6 (m3 mol-1 K-1)
Temp. (K)
v
Oxalic acid
298.15
303.15
308.15
313.15
318.15
Ammonium oxalate
298.15
303.15
308.15
313.15
318.15
Sodium oxalate
298.15
303.15
308.15
313.15
318.15
Potassium oxalate
298.15
303.15
308.15
313.15
318.15
E
103.45 (± 0.28)
108.04 (± 0.21)
111.77 (± 0.40)
119.54 (± 0.26)
122.41 (± 0.27)
-0.683 (± 0.014)
-0.748 (± 0.010)
-0.825 (± 0.020)
-0.939 (± 0.013)
-0.955 (± 0.015)
0.875
0.789
0.703
0.617
0.531
104.93 (± 0.10)
110.16 (± 0.17)
115.65 (± 0.27)
119.67 (± 0.22)
122.86 (± 0.31)
-0.664 (± 0.005)
-0.701 (± 0.194)
-0.783 (± 0.290)
-0.831 (± 0.180)
-0.892 (± 0.800)
1.443
1.213
0.983
0.753
0.523
83.27 (± 0.15)
86.87 (± 0.08)
94.23 (± 0.23)
101.45 (± 0.25)
106.03 (± 0.32)
-1.765 (± 0.008)
-1.782 (± 0.003)
-1.871 (± 0.012)
-1.955 (± 0.013)
-2.044 (± 0.016)
1.889
1.611
1.333
1.055
0.777
117.06 (± 0.48)
121.32 (± 0.30)
129.09 (± 0.19)
136.82 (± 0.45)
140.94 (± 0.23)
-1.479 (± 0.025)
-1.493 (± 0.015)
-1.599 (± 0.010)
-1.669 (± 0.024)
-1.716 (± 0.012)
2.101
1.736
1.371
1.006
0.641
The partial molar volume expansibilities
φ = ∂φ v0 / ∂T p , calculated using Eqs (4) to (7) for
0
E
[
]
oxalic acid and its salts (in 3% (w/w) MeOH + water)
have also been recorded in Table 2. The value of φ E0
decreases with the increase in temperature for oxalic
acid and its salts in 3% (w/w) MeOH + water,
indicating that the behaviour of oxalic acid and its
salts is just like common electrolytes20, where the
partial molar volume expansibilities decrease with the
rise in temperature. The decrease in φ E0 may be
ascribed to the absence of “caging or packing effect”.
The variation of φ E0 with temperature for oxalic acid
and its salts has been found to be linear in 3% (w/w)
MeOH + water (Fig. 3). Comparison of φ E0 values for
oxalic acid and its salts in water5 and in 3% (w/w) MeOH
+ water shows that the behaviour of oxalic acid and its
salts does not change with the change in system from
water to MeOH + water, i.e., when water is mixed with
other protic solvent.
During the past few years, it has been emphasized by
number of workers that Sv is not the sole criteria for
determining the structure making or breaking nature of
Fig. 3 ⎯ Variation of φE0 with temperature in 3 % (w/w) MeOH +
water system.
NOTES
any solute. On the other hand, the sign of
[∂ 2φ v0 / ∂T 2 ] p for various solutes gives an indication
of long range structure making or breaking capacities
of the solutes in aqueous solutions. The structure
making
solutes
have
positive
values
of
2 0
2
[∂ φ v / ∂T ] p , while structure breaking solutes show
negative values. In the present study it is observed
from expressions (4) to (7) that [∂ 2φ v0 / ∂T 2 ] p for the
solutions of oxalic acid and its salts is negative,
thereby showing that oxalic acid and its salts act as
structure breakers in MeOH + water system. In other
words, the addition of oxalic acid and its salts to
MeOH + water causes a breakdown of the structure of
MeOH + water system, i.e., modify the structure of
methanol + water system.
References
1 Malasane P R & Aswar A S, Indian J Chem, 44A (2005)
2490.
2 Salabat A, Shamshiri L & Sahrakar F, J Mol Liq, 118 (2005)
67.
3 Parmar M L & Banyal D S, Indian J Chem, 44A (2005)
1582.
811
4 Zhao C W, Ma P S & Li J D, J Chem Thermodyn, 37 (2005)
37.
5 Parmar M L & Guleria M K, J Indian Chem Soc, 82 (2005)
648.
6 Warminska D, Krakowiak & Grzybkowski, J Mol Liq, 116
(2005) 61.
7 Man Singh, J Chem Sci, 118 (2006) 269.
8 Parmar M L & Guleria M K, J Mol Liq, 126 (2006) 48.
9 Huque M, Siddique I A & NizamUddin Md, J Chem
Thermodyn, 38 (2006) 1474.
10 Parmar M L & Thakur R C, J Mol Liq, 128 (2006) 85.
11 Chemiselewska A, Wypych-Stasiewicz A & Bold A, J Mol
Liq, 130 (2007) 42.
12 Parmar M L & Attri S C, J Mol Liq, 136 (2007) 38.
13 Swenson D M & Wooley E M, J Chem Thermodyn, 40
(2008) 54.
14 Cox B G & Waghorne W E, Chem Soc Rev, 9 (1980) 1381.
15 Lau Y K, Saluja P P S & Kebearle P, J Am Chem Soc, 102
(1980) 7429.
16 Covington A K & Dickinson T, Physical Chemistry of
Organic Solvent Systems, (Plenum, New York) 1973.
17 Parmar M L & Dhiman D K, Indian J Chem, 40A (2001)
1161.
18 Parmar M L & Thakur R C, Indian J Chem, 48A (2009) 57.
19 Parmar M L & Mahajan S, Acta Cienc Indica, 1 (1984) 31.
20 Millero F J in : Structure and Transport Processes in Water
and Aqueous Solutions, edited by R A Horne (Wiley
Interscience, New York) 1971, Chap. 15, p. 622.