Article pubs.acs.org/jced
Experimental Measurement and Correlation of Solubility Data and Thermodynamic Properties of Protocatechuic Acid in Four Organic Solvents Adel Noubigh,*,†,‡ Abdelkarim Aydi,†,§ and Manef Abderrabba† †
Laboratory of Physical Chemistry of Materials, Preparatory Institute for Scientific and Technical Studies of La Marsa, Carthage University 2070 La Marsa, Tunisia ‡ Department of Chemistry, Faculty of Science, Northern Borders University, Arar 91431, Kingdom of Saudi Arabia § Reactions and Process Engineering Laboratory INPL, 1 Rue Grandville, 54001 Nancy Cedex, France ABSTRACT: Using a thermostated reactor and UV/vis spectrophotometer analysis, the solubility of protocatechuic acid (PA) in ethanol, methanol, methyl acetate, and ethyl acetate was measured from (293.15 to 318.15) K at atmospheric pressure. The effect of solvent type and temperature on the solubility are discussed. The relative solubility of protocatechuic acid in different solvents was determined in descending order to be xmethanol > xethanol > xmethyl > xethyl. The experimental solubility data of the solutes (PA) in four pure solvents was correlated with the modified Apelblat equation and semiempirical Buchowski−Ksiazczak λh equation. Dissolution standard molar enthalpy ΔsolH° (kJ·mol−1), Gibbs energy ΔsolG° (kJ· mol−1), and entropy ΔsolS° (J·mol−1·K−1) of protocatechuic acid were estimated using the solubility data:
1. INTRODUCTION Protocatechuic acid (3,4- dihydroxybenzoic acid, PA) is one of the phenolic acid derivatives present in products of plants (olive oil, white wine, and so on)1,2 and in a variety of fruits3,4 and is frequently present in food.5,6 Protocatechuic acid is also found in many plants, spices, and nuts.7 This compound is antioxidant, anti-inflammatory, antimutagenic, antitumor, and anticancer.8−10 It is one of the biologically active components in some medicinal plants, including those used in natural medicine.11,12 The chemical structure of protocatechuic acid is shown in Figure1.
methyl acetate, and ethyl acetate over the temperature range from (293.15 to 318.15) K and compare them with the solubility data of protocatechuic acid in water presented in previous work.20,21 The solubility data were correlated with the modified Apelbalt equation and the Buchowski−Ksiazaczak λh equation. The dissolution thermodynamic properties were also calculated.
2. MATERIALS AND METHODS Protocatechuic acid (mass fraction purity > 97.0 %) was purchased from Sigma-Aldrich (Munich, Germany). There was no treatment before its use. It was later stored in a desiccator with P2O5 after the bottle had been opened. The methanol (mass fraction purity > 99.7 %), absolute ethanol (mass fraction purity > 99.8 %), methyl acetate (mass fraction purity > 99.6 %), and ethyl acetate (mass fraction purity > 99.6 %) for dissolving were analytical reagent grade and were supplied by Sigma-Aldrich. The solubility apparatus and method used are similar to those in our previous work.18−21 A 25 g amount of organic solvent was introduced into a double jacketed reactor. A magnetic stirrer was used to stir a solution of a selected liquid to which an excess of solid protocatechuic was added. A 0.2 μm pore syringe filter was used to take a sample of liquid phase. Then, the concentration was measured by UV/visible
Figure 1. Molecular structure of protocatechuic acid.
The solubility of solid compounds in organic solvents plays an important role in their industrial, pharmaceutical, separation, and environmental applications.13−15 Recently,16−24 some phenolic acid solubilities, both in pure and mixed solvents, have been measured as a function of temperature. However, to the best of our knowledge, there has not been any publication on the solubility of protocatechuic acid in organic and mixed solvents. Here we present values of the solubility of protocatechuic acid in methanol, ethanol, © XXXX American Chemical Society
Received: June 8, 2014 Accepted: December 30, 2014
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DOI: 10.1021/je500519y J. Chem. Eng. Data XXXX, XXX, XXX−XXX
Journal of Chemical & Engineering Data
Article calcd Table 1. Experimental (xexptl PA ) and Calculated (xPA ) Solubilities of Protocatechuic Acid (PA) in Pure Organic Solvents at Various Temperatures and at Atmospheric Pressurea
spectrophotometry (Beckman Coulter UV/vis spectrophotometer model DU-520) at 292 nm in order to check equilibrium. Because of its maximum absorbance, a wavelength of 292 nm was determined to be the most adequate for protocatechuic acid quantification. It was supposed that the system was at equilibrium when the concentration of protocatechuic acid in the liquid phase remained constant. Divers equilibria have been tested (from (1 to 6) h). It seems that 3 h is always sufficient to reach equilibrium. Two independent experiments were undertaken to check the protocatechuic acid solubility. The maximum deviation from the average value was found to be ± 5 %. Moreover, to verify the uncertainty of the measurement, a comparison of solubility data for gallic acid in ethanol and methanol between experimental values and literature values17 was made in the previous work.22,23 The average relative error of above the system was less than 0.02, so it was proven that this experimental technique was reliable. The mole fraction solubility xexptl PA in different solvents was calculated using experimental solubilities on the basis of the following equation: exptl x PA =
mPA /MPA mPA /MPA + mS /MS
T/K
(1)
where mPA and mS represent the masses of the solute and solvent, respectively, and MPA and MS are the molecular weights of the solute and solvent, respectively. Experimental (xexptl PA ) solubility of protocatechuic acid in the selected solvents are given in Table 1.
3. RESULTS AND DISCUSSION This additional study helps extend our solubility data basis of protocatechuic acid.14−17 The relationship between mole fraction solubility and temperature in different solvents is correlated with the modified Apelblat eq 2 and the λh eq 3. The solubility temperature dependence of solute (PA) in the selected solvents can be described by the modified empirical equation.25−27 ln x PA
B =A+ + C ln(T /K) (T /K)
293.15 298.15 303.15 308.15 313.15 318.15
3.374 4.508 6.538 9.298 12.611 16.721
293.15 298.15 303.15 308.15 313.15 318.15
3.024 4.099 4.950 6.650 8.465 10.812
293.15 298.15 303.15 308.15 313.15 318.15
0.563 0.843 1.150 1.665 2.254 3.003
293.15 298.15 303.15 308.15 313.15 318.15
0.478 0.671 0.906 1.300 1.791 2.329
xcalcd PA
× 10
2
xcalcd PA
RD
Methanol 3.327 0.013 4.666 −0.035 6.507 0.004 9.025 0.029 12.452 0.0126 17.092 −0.022 Ethanol 3.065 0.013 3.978 0.029 5.140 0.018 6.614 0.005 8.475 −0.001 10.818 −0.001 Methyl Acetate 0.581 0.001 0.820 0.001 1.152 0.000 1.610 0.001 2.236 0.000 3.090 0.001 Ethyl Acetate 0.479 −0.002 0.667 0.006 0.925 −0.021 1.275 0.019 1.748 0.024 2.385 −0.024
× 102
RD
3.251 4.685 6.631 9.206 12.520 16.655
0.036 −0.039 −0.014 0.009 0.007 0.004
3.026 3.989 5.196 6.685 8.495 10.662
−0.001 0.026 −0.029 −0.005 −0.003 0.014
0.574 0.823 1.164 1.626 2.243 3.056
−0.020 0.024 0.012 0.023 0.004 0.017
0.474 0.669 0.933 1.286 1.751 2.358
0.007 0.003 −0.030 0.011 0.022 −0.012
The standard uncertainty in the measured temperatures is 0.10 K. The relative standard uncertainty in the measured solubility in mole fraction is 2%.
Table 2. Apelblat Equation and λh Equation Fitting Parameters of Protocatechuic Acid (PA) in Different Pure Organic Solvents
(2)
Apelblat equation methanol ethanol methyl acetate ethyl acetate
A
B/K
C
104RMSD
102RAD
−116.738 −90.977 −121.194 −116.663
−11.442 −1.978 −2.239 −2.240
19.958 15.403 20.429 19.598 λh equation
2.467 2.355 6.297 3.374
1.963 1.138 1.645 1.615
methanol ethanol methyl acetate ethyl acetate
(3)
where xPA is the mole fraction solubility of protocatechuic acid, T and Tm are the experimental temperature and normal melting temperature, respectively, and λ and h are the model parameters obtained from the experimental solubility data in the systems which are listed in Table 3, respectively. As for the Buchowski−Ksiazaczak λh equation, recasting eq 3 into an exponential form yields 1 1 = [ehλ[(1/ T ) − (1/ Tm)] − 1] + 1 x PA λ
× 10
2
a
where T is the absolute temperature (K), A, B, and C are empirical parameters determined by the experimental solubility data which are listed in Table 2, and xPA is the mole fraction solubility of protocatechuic acid in organic solvents. The λh model developed by Buchowski et al.28,29 which is a semiempirical equation, is shown as follows: ⎡ 1 ⎛ λ(1 − x PA ) ⎞ 1 ⎤ ln⎜1 + − ⎥ ⎟ = λh⎢ (Tm/K) ⎦ x PA ⎣ (T /K) ⎠ ⎝
λh equation
Apelblat equation xexptl PA
Λ
h
104RMSD
102RAD
278.760 21.808 29.534 16.040
23.860 229.864 214.327 377.328
2.711 2.996 3.398 2.949
1.852 1.325 1.708 1.450
The root-mean-square deviations (RMSDs) for modified Apelblat and the λh equation are also listed in Tables 2 and 3. The rmsd is calculated according to the following formula: ⎡1 RMSD = ⎢ ⎢⎣ n
(4) B
⎤1/2
n
∑ i=1
calcd (x PA
−
exptl 2 ⎥ x PA )
⎥⎦
(5)
DOI: 10.1021/je500519y J. Chem. Eng. Data XXXX, XXX, XXX−XXX
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Table 3. Dissolution Standard Molar Enthalpy (ΔsolH°, kJ· mol−1), Gibbs Energy (ΔsolG°, kJ·mol−1), and Entropy (ΔsolS°, J·mol−1·K−1) of Protocatechuic Acid in Four Pure Organic Solvents T/K
ΔsolH°
293.15 298.15 303.15 308.15 313.15 318.15
48.772 49.603 50.433 51.263 52.093 52.924
ΔsolG°
ΔsolS°
8.299 7.602 6.891 6.166 5.427 4.676
138.062 140.871 143.632 146.349 149.021 151.652
measured and the calculated values are also regrouped in Table 1. The RD was determined by RD =
293.15 298.15 303.15 308.15 313.15 318.15 293.15 298.15 303.15 308.15 313.15 318.15
Ethanol 37.673 8.512 38.315 8.010 38.957 7.496 39.600 6.972 40.242 6.437 40.884 5.892 Methyl Acetate 49.846 12.557 50.696 11.914 51.545 11.256 52.395 10.585 53.245 9.899 54.095 9.201 Ethyl Acetate 47.818 13.025 48.633 12.425 49.450 11.810 50.265 11.155 51.080 10.543 51.895 9.890
exptl x PA
(6)
The relative average deviations (RADs) and the RMSDs are also reported in Tables 2 and 3. The RAD is calculated according to
Methanol
293.15 298.15 303.15 308.15 313.15 318.15
exptl calcd x PA − x PA
RAD =
1 n
n
∑ i=1
exptl calcd x PA − x PA exptl x PA
(7)
The experimental and calculated solubility data of protocatechuic acid in the selected solvents are reported in Table 1 and shown in Figure 3.
99.474 101.647 103.783 105.885 107.952 109.987 127.200 130.075 132.902 135.682 138.418 141.111 118.687 121.444 124.156 126.823 129.448 132.030
where n refers to the number of experimental points and xcalcd PA and xexptl PA are the solubility of protocatechuic acid calculated from eqs 2 and 3 and the experimental solubility data, respectively. Calculated and experimental solubilities in pure methanol, as is clear from Figure 2, show a very good agreement. The relative deviations (RDs) between the
Figure 3. Standard molar Gibbs energy of dissolution of protocatechuic acid in the selected pure organic solvents: ■, methanol; □, ethanol; ▲, methyl acetate; Δ, ethyl acetate.
Based on obtained results in the previous study20,21 and the present work shown in Tables 2 and 3, we can reach the following conclusions: all of the solubility curves are similar and have the same tendency. For each solvent studied, the equilibrium solubility mole fraction of protocatechuic acid increases with increasing temperature. On the other hand, the protocatechuic acid solubility in methanol is higher than ethanol, methyl acetate, and ethyl acetate. Starting with the solubility data, it can be seen that the highest solubilities were obtained in pure methanol and the lowest values were found in ethyl acetate. Similar results were reported by Daneshfar et al.17 in their study of solubility of gallic acid in methanol, ethanol, water, and ethyl acetate at different temperatures. The increase in PA solubility follows the descending order as follows: methanol > ethanol > methyl acetate > ethyl acetate. From Table 1 and Figure 2, we notice that, to correlate the experimental results of the solubility of protocatechuic acid in the selected solvents at different temperatures, eqs 2 and 3 can be used. Calculated solubilities of protocatechuic acid in methanol, ethanol, methyl acetate, and ethyl acetate show very
Figure 2. Solubility data of protocatechuic acid (xPA) in the four pure organic solvents: ■, methanol; □, ethanol; ▲, methyl acetate; Δ, ethyl acetate; line, calculated by eq 2 C
DOI: 10.1021/je500519y J. Chem. Eng. Data XXXX, XXX, XXX−XXX
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and methyl acetate at temperatures ranging from (293.15 to 318.15) K not previously reported in the literature have been obtained. The solubility of PA in selected solvents increases with an increase of the temperature. The solubility of PA in the different solvents is in descending order: methanol > ethanol > methyl acetate > ethyl acetate. The modified Apelblat equation is the most accurate mathematical representation over a limited temperature range for the system methanol and ethanol. The methyl acetate and ethyl acetate agrees well with the λh equation.
good accord with experimental results. The RDs calculated by modified Apelblat equation among all the values do not exceed 3.09 %, and the RADs, respectively, are 1.96 %, 1.14 %, 1.64 %, and 1.61 %. The RDs calculated by the λh equation was less than 4.56 %, and the RADs were 1.85 %, 1.32 %, 1.70 %, and 1.45 %. The agreement of solubilities calculated by the models can answer the demands of engineering application. From Table 2, the RMSDs of the modified Apelblat model were lower than those of the Buchowski−Ksiazaczak λh model. As stated previously, we can draw the conclusion that the modified Apelblat equation is the most accurate model for the systems of methanol and ethanol. Methyl acetate and ethyl acetate values agree well with the λh equation. The dissolution of a protocatechuic acid into a liquid is related to some thermodynamics at the value of energy, in particular, the standard molar Gibbs energy, molar enthalpy, and molar entropy of dissolution. To calculate these thermodynamic functions, the experimental solubility data fitted to eq 2 are used. The modification of the solution properties, which is due to the presence of the solute at its infinite dilution state at a given temperature, is reflected in these parameters. It is assumed that the activity coefficient of solute is equal to 1 in the hypothetical dilute ideal solution. The standard molar enthalpy, ΔsolH°, of dissolution of protocatechuic acid as a function of temperature can be expressed by30 ⎛ d ln x ⎞ ⎟ Δsol H ° = RT 2⎜ ⎝ dT ⎠ P
■
Corresponding Author
*Tel.: +216 98 934 601. Fax: + 216 71 74 65 51. E-mail: Adel.
[email protected];
[email protected]. Notes
The authors declare no competing financial interest.
■
The standard molar Gibbs energy, ΔsolG°, and entropy, ΔsolS°, of dissolution of protocatechuic acid can be calculated by31,32 (9)
⎛ d(ln x) ⎞ Δsol S° = R ⎜ ⎟ ⎝ d(ln T ) ⎠ P
(10)
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(8)
Δsol G° = −RT ln(x)P
AUTHOR INFORMATION
The values of the thermodynamic functions, ΔsolH°, ΔsolG°, and ΔsolS°, of the protocatechuic acid dissolution in different solvents are reported in Table 3. The standard molar Gibbs energy of dissolution versus temperature, depicted in Figure 3, is derived from experimental solubility data for each solution. Estimated enthalpies of dissolution from solubility measurements of protocatechuic acid in each solvent at different temperatures are comparable. It was found that, for all of the selected solvents, the enthalpy of dissolution is a linear function of temperature. Therefore, there was a constant heat capacity of solution. It can be seen from Table 3 that the dissolving process in the organic solvents in the experimental temperature range was endothermic, ΔsolH°, and ΔsolS° for protocatechuic acid dissolving in the selected solvents was relatively large. It can be seen in Table 3, that ΔsolG° was positive which indicated that the dissolution process of protocatechuic acid was nonspontaneous. Even though ΔsolS° is positive, the ΔsolH° is adequately positive to provide positive ΔsolG° values. Methanol and ethanol presented as better solvents for dissolving protocatechuic acid than methyl acetate and ethyl acetate.
4. CONCLUSION In this study, new findings regarding the solubility of protocatechuic acid (PA) in methanol, ethanol, ethyl acetate, D
DOI: 10.1021/je500519y J. Chem. Eng. Data XXXX, XXX, XXX−XXX
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