Article pubs.acs.org/jced
Measurement and Correlation for the Solid Solubility of Antioxidants Sodium L‑Ascorbate and Sodium Erythorbate Monohydrate in Supercritical Carbon Dioxide Tzu-Chi Wang* and Po-Chao Chang Department of Chemical and Materials Engineering and Master Program of Nanomaterials, Chinese Culture University, Taipei, Taiwan ROC ABSTRACT: In this research, the solubility of sodium L-ascorbate and that of sodium erythorbate monohydrate, both being antioxidants, in supercritical carbon dioxide are measured. Using semiflow type equipment, this experiment gathers the data of solubility for antioxidnats at (308.15, 313.15, and 318.15) K, over a range of pressures from (12 to 24) MPa. The data gleaned from the experiment are forward numerically processed, with the use of the semiempirical equations presented by Chrastil and by Mendez-Santiago and Teja, to make clear the correlation among various characteristics. The final results are satisfactory: when parameters with optimal fitting are picked to examine the correlation, the average absolute relative deviation (AARD) is under 4 %.
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INTRODUCTION In both laboratory and industrial environments, supercritical fluids play an indispensable role when a reaction cannot be activated in traditional solvents. Supercritical fluids, simply put, any substance at a temperature and pressure above its critical point, where distinct liquid and gas phases do not exist, including carbon dioxide, ethane, and xenon, intrigue researchers and engineers alike because they are suitable as a substitute for organic solvents in a range of industrial and laboratory processes from synthetic chemistry to analytical chemistry. Used in a wide variety of fieldsreaction and extraction, particle formation, and material processingthe supercritical fluid is broadly regarded as a green technology.1,2 With outstanding features, partly because of its appropriate operating range of temperature in extraction process, and partly because of its environmental friendliness, supercritical carbon dioxide is highly prized among a slew of popular supercritical fluids. Consequently, there is no doubt that the physical properties, such as diffusion coefficient or solubility, of supercritical fluid mixtures are of great importance for some industrial processes with the aim of effective design and smooth operation. Some authors3−5 have conducted research on the solubility of solids in supercritical CO2. Cortest et al. center on several antioxidantsascorbic acid, ascorbyl palmitate, α-tocopherol succinate acid, butyl hydroxyl anisole, dodecyl gallate, gallic acid, and propyl gallate, among othersmeasuring the solubility of these antioxidants in supercritical CO2 under different combinations of pressure and temperature.6 Wang et al. have their focus on antioxidants calcium L-ascorbate dehydrate and D-isoascorbic acid, collecting their solubility in various pressures and temperatures.7 © XXXX American Chemical Society
However, owing to the immense possibility of different combinations of chemical compounds, there is great need for more experimental data for specialty chemicals and generalized correlation models. Both sodium L-ascorbate (IUPAC name: sodium (2R)-2-[(1S)-1,2-dihydroxyethyl]-4-hydroxy-5-oxo-2Hfuran-3-olate, CAS number: 134-03-2) and sodium erythorbate monohydrate (IUPAC name: sodium [(5R)-5-(1,2-dihydroxyethyl)-4-hydroxy-5-methyl-2-oxo-3-furyl]oxysodium hydrate, CAS number: 63524-04-9) are, in addition to nutrient supplements, important antioxidants in preserving various kinds of food like meat, soups, and cakes. As a result, the solid solubility of them in supercritical CO2 has a variety of uses in the field of food manufacturing and testing. To facility the related applications in research or industry, this study centers on deriving essential solid solubility data, which are correlated with the use of either the semiempirical equation by Chrastil8 or the one by Mendez-Santiago and Teja,5 in diverse situations. Although many models have been proposed,9 the MendezSantiago and Teja model and Christal model are widely used in the literature. Fewer parameters, easy applicability, and reasonable range of error (within experiment’s range of error) are their advantages over other models. The researchers of this study set up similar semiflow apparatus to measure the solid solubility of sodium l-ascorbate and sodium erythorbate monohydrate, at (308.15, 313.15, and 318.15) K, in supercritical CO2 over the pressure range from (12 to 24) MPa. Received: October 1, 2014 Accepted: January 21, 2015
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DOI: 10.1021/je5009153 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
Journal of Chemical & Engineering Data
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Table 1. Physical Properties of Sodium L-Ascorbate and Sodium Erythorbate Monohydrate
Figure 1. Schematic diagram of the experimental apparatus.
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EXPERIMENTAL SECTION In this study, the researchers take great caution in the preparation of the relevant compounds to prevent the experimental results from being spoiled by impure chemicals. Carbon dioxide with a purity of at least 99.5 % is purchased from Chiau-Chung Gas Co. (Taiwan). Sodium L-ascorbate (C 6 H 7 NaO 6 ) and sodium erythorbate monohydrate (C6H7NaO6·H2O) with a purity of at least 98 % are procured from Sigma-Aldrich Co. All the solvents and carbon dioxide are used in the experiment unaltered. Table 1 lists the chemical structure and physical properties of sodium L-ascorbate and sodium erythorbate monohydrate. In addition to the chemicals, the experimental device is set up with no less rigorousness. A semiflow type apparatus, of which the configuration is shown in Figure 1, is constructed to measure the solid solubility in supercritical CO2. There are
three stages of the whole operation procedure: (1) the feeding, (2) the equilibrium, and (3) the analysis. For a start, CO2 in supercritical condition is injected into the apparatus in a measured manner for the later dissolution of the solid, which reaches equilibrium at the second stage. Then, the registered data are examined to calculate the coefficients of the models. At the first stage, the liquefied pure CO2, initially cooled to the temperature of 275.15 K with a cooler, is compressed to various specific pressures by an HPLC pump (manufactured by Thermo Separation Product), where a back pressure regulator is used for regulation. The compressed CO2 is subsequently fed into a preheating coil that is immersed in a water bath to maintain its temperature. The supercritical CO2 later flows through two interconnected cells: the pre-equilibrium and the equilibrium cell. The volume of each cell is 75 cm3, with the drug sample (5 g) mixed B
DOI: 10.1021/je5009153 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
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with glass beads. At the ends of the cell, glass wool or filter is applied to make it entrainment free for fear that any physical entrainment would spoil the experimental results. The temperature is taken by a calibrated thermocouple with a resolution of ± 0.1K. The Druck pressure transducer PTX 1400 is used to take the pressure with a resolution of ± 0.1 MPa. When the supercritical CO2 is released from the equilibrium cell, it is expanded through a needle valve and its pressure decreases to atmospheric pressure. To prevent sodium Lascorbate and sodium erythorbate monohydrate from freezing and blocking the pipeline, the temperature of the needle valve is maintained at 333.15 K with heating tape wrapping. Now the solid part is divided from the gas and melts in a flask of pure water. Water solvent is then applied to retrieve the residual solute in the line, and the water will later be purged by air. For the purpose of effective results, the solvability data are taken with great care. Ritter TG05, a wet test meter, is responsible for the measurement of the total volume of CO2 flow; the UV−vis (Cary 50 spectrophotometer) detector is for the water solution concentration in the flask. To ensure the instrument’s exactitude, the UV−vis detector is calibrated with solutions of specific concentrations. The absorption rates of sodium L-ascorbate and sodium erythorbate monohydrate have sharp peaks at the bands of (265.5 and 266.5) nm, observed in the UV−vis detector, respectively. Accuracy about the measurement is confirmed in each round of the experiment. For every specific condition of temperature and pressure, the relevant data are measured at least three times. In addition, measurements are made at various effluent CO2 flow rates between (4 and 5) L·h−1 at the atmospheric condition and at the equilibrium. Under such a condition, the flow rate does not influence the solid solubility measurement.
Table 2. Solubility of Sodium L-Ascorbate (2) in Supercritical Carbon Dioxide (1)a T/K
P/MPa
308.15
12 14 16 18 20 22 24 12 14 16 18 20 22 24 12 14 16 18 20 22 24
313.15
318.15
y2 8.494 1.165 1.394 1.546 1.690 1.853 1.964 9.639 1.386 1.757 2.067 2.312 2.508 2.801 7.833 1.496 2.227 2.636 3.268 3.484 3.840
× × × × × × × × × × × × × × × × × × × × ×
10−8 10−7 10−7 10−7 10−7 10−7 10−7 10−8 10−7 10−7 10−7 10−7 10−7 10−7 10−8 10−7 10−7 10−7 10−7 10−7 10−7
a
Standard uncertainties u are u(T) = 0.01 K, u(P) = 0.01 MPa, u(y2) = 4.016·10−9.
Table 3. Solubility of Sodium Erythorbate Monohydrate (2) in Supercritical Carbon Dioxide (1)a T/K
P/MPa
308.15
12 14 16 18 20 22 24 12 14 16 18 20 22 24 12 14 16 18 20 22 24
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RESULTS AND DISCUSSION After close examination of the isothermal solid solubility data of the two antioxidants in supercritical CO2, it is obvious that the solubility increases in accordance with the pressure along an isotherm; specifically, both antioxidants have similar solubility range, from 10−7 to 10−8. Tables 2 and 3 list the solid compounds’ mole fractions at equilibrium state. For simplicity’s sake, the subscripts 1 and 2 are used in this article to represent carbon dioxide and antioxidants (sodium L-ascorbate and sodium erythorbate monohydrate), respectively. It should be noted that the equilibrium solubility can be satisfactorily reproduced. Statistically, standard deviations of the experimental data are sufficiently small (Tables 2 and 3). The solid solubility measurement’s coefficient of variance, the standard deviation divided by mean value, is below 5 %, which indicates that the experiment can be performed repeatedly with high accuracy. The results are shown graphically in Figures 2 and 3. The effect of temperature on solid solubility can be derived from the crossover points in the relevant figures. The crossover pressures for sodium L-ascorbate and sodium erythorbate monohydrate are both determined to be 12 MPa. Two semiempirical equations, one by Chrastil and the other by Mendez-Santiago and Teja, are usually applied to find the correlation of the solid solubility in supercritical carbon dioxide. The former model shows a linear relationship between the solid solubility and the density of pure CO2 (component 1), both in their logarithmic form:
313.15
318.15
y2 3.327 3.839 4.503 5.016 5.581 6.384 7.168 3.136 4.329 5.353 6.307 7.248 8.471 1.031 2.787 5.140 6.635 8.045 9.629 1.085 1.184
× × × × × × × × × × × × × × × × × × × × ×
10−8 10−8 10−8 10−8 10−8 10−8 10−8 10−8 10−8 10−8 10−8 10−8 10−8 10−7 10−8 10−8 10−8 10−8 10−8 10−7 10−7
a
Standard uncertainties u are u(T) = 0.01 K, u(P) = 0.01 MPa, u(y2) = 1.226·10−9.
ln y2 = k ln ρ1 +
a +b T
(1)
where y2 is the solute concentration in the supercritical fluid (kg·m−3) and ρ1 is the density of the supercritical fluid (pure CO2; kg·m−3). The association number k and constants a and b are empirically fitted model parameters. The values of these C
DOI: 10.1021/je5009153 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
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The parameters for the two semiempirical equations and the correlation results are shown in Table 4. The average absolute Table 4. Correlated Results of Solid Solubility Data in Supercritical Carbon Dioxide model
parameters
CO2 + Sodium L-Ascorbate Chrastil k = 6.8, a = −9085.3, b = 16.3 Mendenz-Santiago and A = −14890.1, B = 161897.4, Teja b = 25.4 CO2 + Sodium Erythorbate Monohydrate Chrastil k = 6.6, a = −8144.3, b = 12.1 Mendenz-Santiago and A = −13899.7, B = 157947.1, Teja C = 21.3
Figure 3. Solubility for sodium erythorbate monohydrate (2) in supercritical carbon dioxide (1). ▲, 308.15 K; ●, 313.15 K; ◆, 318.15 K; ---, MST model; −, Chrastil model.
∑ k=1
the density of supercritical CO2 is nearly a straight line. This kind of result amounts to a guarantee that the parameters obtained by applying the MST model can be accurately extrapolated to other operating conditions.
|y2.exp − y2.calk | k y2.exp k
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(2)
CONCLUSION This study centers on finding the solid solubility data for antioxidants sodium erythorbate monohydrate and sodium Lascorbate (at (308.15, 313.15, and 323.15) K) in supercritical CO2 over the pressure ranging from (12 to 24) MPa. The solubility range of the two antioxidants is between 10−7 to 10−8. The crossover pressures for sodium l-ascorbate and sodium erythorbate monohydrate are determined at 12 MPa. With an average absolute relative deviation in solid solubility below 4 %, both the Chrastil and the Mendez-Santiago and Teja semiempirical equations give satisfactory correlation results for solid solubilities. In regard to the limited amount of
where the subscript k denotes the kth experimental data point for the solid solute. The latter semiempirical equation, developed by MendezSantiago and Teja, also has three parameters: T ln(y2 P) = A + Bρ1 + CT
3.18 3.28
Figure 4. Test of consistence for solubility data of sodium L-ascorbate (2) in supercritical carbon dioxide (1) using Mendez-Santiago and Teja (MST) model. ▲, 308.15 K; ●, 313.15 K; ◆, 318.15 K.
parameters can be obtained by minimizing the following objective function with regression: n
3.80 2.56
relative deviation for the calculated solid solubilities is shown to be below 4 %, which implies satisfactory accuracy of the experiment results. Figures 2 and 3 show the graphic correlated results of the two semiempirical equations. Literature5 has it that whether the experimental data are self-consistent can be examined by using the MST model. A typical example is shown in Figures 4 and 5, in which the plot of T ln(y2P) − TC against
Figure 2. Solubility for sodium L-ascorbate (2) in supercritical carbon dioxide (1). ▲, 308.15 K; ●, 313.15 K; ◆, 318.15 K; ---, MST model; −, Chrastil model.
100 AARD = n
AARD/%
(3)
By minimizing the same objective function shown in eq 2, the parameters A, B, and C can be computed using a simple least-square analysis. The value of ρ1 in eqs 1 and 3 is computed with the method 32 term modified Benedict-Webb-Rubin (MBWR-32) equation.10 D
DOI: 10.1021/je5009153 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
Journal of Chemical & Engineering Data
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Figure 5. Test of consistence for solubility data of sodium erythorbate monohydrate (2) in supercritical carbon dioxide (1) using MendezSantiago and Teja (MST) model. ▲, 308.15 K; ●, 313.15 K; ◆, 318.15 K.
antioxidants that are added to the food for preservation, the supercritical fluid technique is a treatment safe enough for those compounds.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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REFERENCES
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