Temperature Effect on the Extraction of Carboxylic Acids by Amine

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Ind. Eng. Chem. Res. 2004, 43, 7608-7617

Temperature Effect on the Extraction of Carboxylic Acids by Amine-Based Extractants Riki Canari and Aharon M. Eyal* Casali Institute of Applied Chemistry, The Hebrew University of Jerusalem, 91904 Jerusalem, Israel

The extraction of carboxylic acids by extractants that contain lipophilic amines is dependent on temperature. That phenomenon is used in industry, where extraction is conducted at approximately ambient temperature and back-extraction occurs at an elevated temperature. In this work, stronger temperature effects are observed in cases where the alkyl amine is relatively weak (pHhn 3), polycarboxylic acid is extracted, the concentration of the amine and/or the carboxylic acid in the organic phase is low, the amine and/or acid are bulky, and the diluent of the amine has low polarity. This shows that the temperature effect is strong in systems where the ionpair interaction between the protonated amine and the acid’s anion is relatively weak (weak acid and amine), where ion-pair stabilization is dependent on the aggregates’ formation (hydrophilic acids, low-polarity amine, low concentration) and where the formation of those aggregates is hindered (bulky amines and acids). 1. Introduction Water-immiscible amines were observed to be selective and powerful extractants for separating carboxylic acids from their dilute solutions. The distribution coefficient in the extraction of citric acid by amine-based extractants is higher, by 1 or 2 orders of magnitude, compared to those for extraction with other solvents, such as ketones, alkanols, esters, amides, and organic alkyl phosphates.1 These high distribution coefficients enable high separation yield, using a small number of extraction stages and low phase ratios between the solvent and the feed solution. However, this effective extraction conflicts with the recovery of the extracted acid from the extractant. This is particularly true for recovery by back-extraction into an aqueous phase, which is required if the extracted acid is needed in free acid form. Therefore, strong extractants lead to dilute back-extracts. A solution to this practical problem was suggested by Baniel et al.,2,3 who found that the distribution coefficient for the extraction of citric acid by tertiary amines decreases when the temperature is increased. They developed and implemented a “temperature-swing” process for the recovery of citric acid from its fermentation broth. The acid is extracted from the solution by a tertiary amine, such as tridodecylamine, in a suitable diluent at approximately ambient temperature and is back-extracted at an elevated temperature of 80-140 °C. Wennersten4 tested the temperature effect on the extraction of citric acid by various extractants. The focus of his work was on extractants that contain a phosphoryl group, e.g., n-tributyl phosphate and trioctyl phosphine oxide. In a later publication, Wennersten5 examined the extraction of citric acid using C8-C10 tertiary amine (Alamine-336, or A-336) in various diluents at 25 and 60 °C. Wennersten concluded that the formation of the amine-acid complex is strongly dependent on temperature. * Author to whom correspondence should be addressed. Tel: 972-2-6585843. Fax: 972-2-6584533. E-mail: eyala@ cc.huji.ac.il.

Tamada and others6,7 showed that increasing the temperature decreases the extraction of succinic and lactic acid by tridodecylamine in methyl isobutyl ketone (MIBK) or in chloroform. Based on van’t Hoff’s equation, they calculated the enthalpy (∆H) and the entropy (∆S) for the association in these extraction systems. Tamada’s research concluded that molar complexation of a 1:1 acid:amine ratio is much more exothermic and involves a much greater loss of entropy than the formation of 2:1 or 3:1 complexes. Eyal et al.8 examined the effect of temperature on the extraction of mineral acids by tertiary amines in kerosene and octanol. They concluded that the temperature effect increases as the branching of amine, with its dilution, increases and as the polarity of the diluent decreases. Sadaka and Garcia9 tested the extraction of shikimic and quinic acid by tridodecylamine in heptanol at various temperatures. Ratios of 2-3 were observed between the distribution coefficients at 5 °C and those at 60 °C. Sadaka and Garcia9 proposed extracting the acids at low temperatures and recovering them at high temperatures. In addition, they suggested adding a “displacer” (oleic acid) to the organic phase in the backextraction stage. The authors did not address the recovery of the oleic acid from the extractant. Eyal and co-workers proposed a temperature-swing process for the recovery of ascorbic acid10 or erythorbic acid11 from their fermentation liquors, using secondary or tertiary alkylamine in a diluent. Particularly hightemperature effects were observed for these acids. This phenomenon is utilized to generate concentrated backextracts from dilute fermentation liquors. The aforementioned articles do not contribute to the prediction of what systems would have greater temperature effects, nor do they supply a satisfactory explanation to the phenomenon. In our previous articles,12,13 we developed a theory dividing the extraction mechanisms of acids with aminebased extractants into two main categories: (i) ion-pair formation and (ii) hydrogen bonding and solvation. The ion-pair formation is the dominant mechanism in those

10.1021/ie034127j CCC: $27.50 © 2004 American Chemical Society Published on Web 10/07/2004

Ind. Eng. Chem. Res., Vol. 43, No. 23, 2004 7609 Table 1. Acids Used in the Experiments, and Their Structure, pKa Value, Producer, and Purity acid

formula

pKaa

producer

purity

acetic acid propionic acid n-butyric acid isobutyric acid lactic acid gluconic acid oxalic acid malonic acid succinic acid D,L-maleic acid D,L-malic acid glutaric acid 2-oxoglutaric acid citric acid

CH3-COOH CH3-CH2-COOH CH3-CH2-CH2-COOH CH3-CH(CH3)-COOH CH3-CHOH-COOH H2COH-[HC(OH)]4-COOH COOH-COOH(2H2O) COOH-CH2-COOH COOH-CH2-CH2-COOH COOH-CH:CH-COOH (cis) COOH-CH2-HC(OH)-COOH COOH-(CH2)3-COOH COOH-C(O)-CH2-CH2-COOH COOH-CH2-(C(OH)-COOH)-CH2-COOH

4.75 4.87 4.81 4.84 3.83b 3.76b 1.27, 4.19 2.83, 5.69 4.2, 5.61 1.93, 6.07 3.22, 5.11 4.42, 5.41 2.57c 3.08, 4.74, 5.4

Frutarom BDH BDH Riedel de Haan Merck Merck J. T. Baker Merck J. T. Baker BDH BDH Fluka Merck Merck

99.7% 99% 99% 99% 90% 50% 99.9% 99% 99.4% 99% 99% 98% 99% extra pure

a

From Handbook of Chemistry and Physics.19

b

From IUPAC.20

cases where the amine extractant has an apparent basicity greater than that of the anion of the extracted acid. (The amine’s apparent basicity is determined by Grinstead’s method14 as the pH of half neutralization (pHhn) of the extractant.) On the other hand, in cases of relatively weak extractants, compared to the anion of the extracted acid (pHhn < pKa), extraction is conducted either through hydrogen bonding or through solvation interactions. In these latter mechanisms, the degree of extraction is mainly determined by the concentration of the undissociated fraction of the acid and, thus, is strongly dependent on the pKa value of the acid. This theory successfully explains the extraction of monoprotic acids13 and diprotic acids,15 the selectivity in acids’ extraction from multi-acid systems,16,17 and the effect of the anion concentration in the aqueous phase.18 The present article summarizes a study of the temperature effect on the extraction of carboxylic acids, as determined by the effect of the concentrations and the structures of the acid, of the amine and of the diluent. The results are analyzed according to the extraction mechanisms. 2. Experimental Section 2.1. Materials. The acids used in the experiments, and their structure, pKa values, producer, and purity are presented in Table 1. The amines used were as follows: (i) a primary amine of the structure R1-R2R3-C-NH2 and a total of 18-22 C atoms (Primene JMT, from Rohm & Haas); and (ii) a straight chain tertiary amine, tridodecylamine, Alamine-304 (or A-304) (from Henkel, technical grade). The following diluents were used: 1-octanol, which is also referred as an enhancer (Merck, 99%), and low-aromatics kerosene (Parasol, Paz). 2.2. Methods. 2.2.1. Extraction Experiments. The organic phases were prepared by mixing the calculated quantities of the amine and the enhancer (e.g., 0.5 mol/ kg and 10%, respectively) in kerosene. Seven-gram aliquots of the organic phases were mixed with 100 g of aqueous solutions of the acid, for 20 min, at the selected temperature. That mixing was determined to be sufficient for reaching equilibrium. The phases were then separated and the acid concentrations were determined in both, by titration with 0.1 N NaOH, using phenolphthalein as the indicator and 2-propanol as the co-solvent for the organic phases. Most contacts were performed in duplicate. Deviations of 2/3. Up to that point, extraction decreases as the temperature increases, whereas above that point, extraction increases. This phenomenon can be explained by the change in the extraction mechanisms. At the lower concentration range, the increase in the thermal energy disturbs the interaction in the organic phase between the protonated amine and the acid’s anion, thus decreasing the extraction. However, at a higher concentration, more acid is added to the organic phase via hydrogen bonding or solvation, which, similar to dissolution, increases when the temperature is increased. As a result, the effect of the temperature is reversed. Table 2 shows a similar phenomenon in the extraction of hydrochloric, citric, succinic, lactic, shikimic, and quinic acids. In all of these

Table 8. Magnitude of Two Peaks (1710 and 1580 cm-1) in the IR Spectra of 0.5 M Alamine-304 and 10% Octanol in Kerosene Loaded with Acetic and Propionic Acids acid concentration in organic phase (mol/kg)

a

Relative Peak Magnitudea v 1500-1600 cm-1 v 1700-1730 cm-1

0.051 0.070 0.513

acetic acid M M L

S S L

0.101 0.366 1.38

propionic acid S M M

S M L

S, small; M, medium; and L, large.

cases, the extraction decreases when the temperature is increased, up to the concentration where the slope of the distribution curve changes (typically to a plateau). At higher concentrations, there is either no effect or the extraction increases as the temperature increases. In all of these cases, ion-pair formation is expected to serve as the main extraction mechanism at the lower acid concentrations (below the plateau), because the pHhn(Cl) of the extractant is greater than that of the acid’s pKa1 value. At higher concentrations, on the other hand, the extraction is via hydrogen bonding or solvation. The extraction of monocarboxylic acids with no other functional group is different. The distribution curves do not show the typical sharp decrease in slope, and the direction of the temperature effect does not reverse. Kertes and King26 noted that a very slight temperature effect, if any, exists in the distribution coefficient of propionic acid between water and dibutyl ether, nheptanol, or acetophenone at temperatures of 20-90 °C. All these solvents extract the acid through hydrogen bonding or solvation. In the cases examined in the present article, the apparent basicity of the amine, pHhn(Cl, 10% octanol) ) 4.47, is slightly lower than the pKa value of acetic, propionic, butyric, and isobutyric acids: 4.75, 4.81, 4.84, and 4.87, respectively. Thus, part of the acid is expected to be ionpaired to the amine, but most of it is expected to be hydrogen-bonded or solvated, as in the cases studied by Kertes and King.26 The IR spectra of the loaded organic phases are in agreement. Table 8 summarizes the magnitude of two important peaks in the IR spectra of extractants composed of 0.5 mol/kg A-304 and 10% octanol in kerosene, which are loaded with acetic or propionic acid. The two peaks are located at 1710 and 1580 cm-1. The first is attributed to the nonsymmetric carbonylic bond and indicates the presence of the acid in its undissociated form. The second peak refers to the symmetric carbonylic bond and indicates a dissociated acid. The table shows that, even in those cases where the amine concentration in the organic phase is higher than that of the acid, some of the acid is in its undissociated form, i.e., extracted through solvation or hydrogen-bonding interactions. The other part is in its dissociated form,

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ion-paired to the amine. Therefore, unlike in cases of stronger acids, both mechanisms contribute to extraction through the entire concentration range. There is no concentration where the mechanism changes drastically and, therefore, there is no reversion of the temperature effect. 3.3.2. Dependence of the Temperature Effect on the Acids’ pKa Value. Ion-pair formation is the main extraction mechanism when extracting maleic, oxoglutaric, malonic, citric, malic, gluconic, and lactic acids by 0.5 mol/kg A-304 and 10% octanol in kerosene. This is true because the pH half neutralization of the extractant is greater (pHhn(Cl, 10% octanol) of 4.47) than the pKa1 values of the acids (which are 1.93, 2.57, 2.83, 3.01, 3.22, 3.75, and 3.86, respectively). In these cases, the higher the acidity of the extracted acid, the stronger the interaction between the amine and the proton and, therefore, the higher the extraction (see Figure 3). In these systems, the temperature effect increases as the pKa values of the acid and the number of the carboxylic groups increase and as the acid concentration decreases. Thus, a high-temperature effect was observed in the extraction of weak dicarboxylic acids such as succinic and glutartic acid, and especially in the extraction of citric acid (which is a tricarboxylic acid) at the lower acid concentration range. We suggest that the temperature effect is related to the ability of ion pairs to stabilize by aggregation in a relatively low-polarity diluent. Previous studies27 have shown that such aggregation greatly contributes to extraction. Higher pKa values in that range means a weaker acid and, thus, a weaker interaction between the protonated amine and the anion. Increasing the number of the carboxylic groups on an acid leads to a bulkier molecule, which interferes with aggregation. Furthermore, it increases the polarity of the ion pair and thereby its dependence on aggregation. Temperature elevation decreases the stability of the aggregates, which leads to a decrease in the extraction. In addition, decreasing the acid concentration in the organic phase decreases the polarity of that phase and the number of ion pairs available to form aggregates and, thus, decreases the ability to stabilize the ion pairs. The temperature effect in cases of monoprotic acids is small, compared to that of dicarboxylic acids with about the same pKa1 value (Z30/Z75 values of ∼2 and ∼4, respectively). This is probably the result of lower polarity of their ion pair and, thus, less dependence on aggregation for stabilization in the relatively nonpolar diluent. 3.3.3. Dependence of the Temperature Effect on the Structures of the Amine and the Diluent and on Their Concentration. Grinstead and Davis27 concluded that aggregates of lipophilic primary-amine ion pairs exist in very dilute amine hydrochloride solutions, starting at ∼10-5 M, whereas those of lipophilic tertiary-amine hydrochlorides only start at concentrations of >10-2 M. They explained this phenomenon by the lower hindrance for the aggregation in the case of primary amines, which carry only one hydrocarbon chain, and by the two H atoms on the N atom of these amines, which are available for hydrogen bonding to form ionpair aggregates. As a result, the primary amines act as stronger bases and extract better. These two types of amines are also different in their sensitivity to temperature. The temperature effect is much smaller in the case of the primary amine (see Table 3).

A strong temperature effect was observed in the extraction of strong mineral acids using bulky amines,8 such as tris(2-ethylhexyl) amine. In those systems, the effect was increased with amine dilution and with decreased polarity of the diluent. In correlation, extraction of the carboxylic acids by amines in protic or polar diluents strongly enhances the extraction but reduces the temperature effect (see Table 5). The same effect was observed when the amine and octanol concentrations were increased (see Table 4). These results are all in agreement with the explanation we propose. High-temperature effects are observed in cases where there is maximal dependence on ion-pair aggregation for stabilization and where such aggregation is hindered. This explains the high-temperature effect in cases of bulky amines that sterically hinder the formation of ion-pair aggregates. This is true for cases of diluents with low polarity that push toward high aggregation, and for cases with low amine concentrations that decrease the ion-pair concentration in the organic phase. In summary, the analysis of our results and those found in the literature show that a high-temperature effect is observed in systems where dependence on stabilization by ion-pair aggregation is high and such aggregation is hindered. Therefore, a high-temperature effect is found in cases where weak and polyprotic acids are extracted, in cases of low acid concentration in the organic phase, in cases of extraction by sterically hindered amines with less functions available for hydrogen bonding between ion pairs, and in cases of extraction by extractants with nonpolar and nonprotic diluents. To support our explanation, we examined the effect of temperature on the anion exchange reaction in resins and on the basicity of water-soluble amines. 3.4. Effect of Temperature on the Basicity of Water-Soluble Amines. Perrin28 determined the effect of temperature on the ionization constants of conjugate acids of organic bases in water, - d(pKa)/dT, and concluded that the base weakens when the temperature is increased. This is in agreement with the temperature effect on the extraction of acids. In addition, Perrin concluded that, over a wide range of pKa values, the temperature effect varies linearly with pKa, as can be seen in Figure 5 (diamond-shaped symbols). However, this figure shows a very large deviation from linearity, especially for the cases of aliphatic amines (circular symbols), which are the subject of our interest. This figure shows a decrease of ∼0.2-0.3 pH units in the pKa value of these amines for a 10 °C temperature increase. The values of - d(pKa)/dT are presented in Figure 6, as a function of the substitution degree, rather than as a function of the pKa (as in Figure 5). This figure shows that the temperature effect on pKa increases in the following sequence: primary > secondary > tertiary, which is in correlation with their Hoy’s hydrogen bond parameter:29 ∼7, 6, and 4 MPa1/2, respectively. That sequence is opposite to that which we determined for extraction with water-insoluble amines, but it does not contradict our explanation. Unlike that observed in aqueous phases, in the lipophilic phase, there is a maximal dependence on aggregation of the ion pairs for the stabilization. Such aggregation is hindered in the case of tertiary amines. In aqueous solutions, on the other hand, ions stabilized by hydration. Therefore, complicated structures of primary amines hydrogen-bonded to water molecules in the aqueous

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Figure 5. Effect of temperature on the ionization constants of conjugate acids of organic bases in water28 and in resins,30 as a function of their pKa value. (Note: The effect of temperature involves a decrease of - d(pKa)/dT per 1 °C.) Table 9. Basicity and Temperature Effect of Basic Ion-Exchange Resinsa amine molecule dimethyl dipropyl ethyl Figure 6. Effect of temperature on the ionization constants of conjugate acids of some aliphatic amines in water.28 (Note: The effect of temperature involves a decrease of -d(pKa)/dT per 1 °C.)

phase are sensitive to temperature elevation. As a result, the temperature effect is greater in the case of less substitution on the N atom. 3.5. Effect of Temperature on Anion-Exchange Reactions in Resins Carrying Amine Functional Groups. Weiss and co-workers30,31 observed that the adsorption of HCl on anion-exchange resins decreases when the temperature is increased, in agreement with the temperature effect on the extraction. Figure 5 and Table 9 present the temperature effect as a function of resin basicity. Weiss et al. synthesized the resins by reacting chloromethylated, 2%-3% cross-linked polystyrene beads with amines. The beads were supplied by two manufacturers: Permutit and Dow Chemicals. (In Figure 5, the triangles represent Dow beads and the squares represent the Permutit beads.) Their results show no correlation between the basicity and the temperature effect. More surprisingly, their results show that the resins, which were synthesized using Permutit beads, are more basic and show a highertemperature effect than those synthesized using Dow beads.

bead manufacturer

pKa

temperature effect

Permutit Dow Chemicals Permutit Dow Chemicals Permutit Dow Chemicals

7.6 7 6.3 5.2 8 6.9

0.2 0.1 0.2 0.07 0.28 0.12

a The temperature effect is defined as the difference in the pH when the resin was half loaded with HCl for a 10 °C increase in temperature. The resins were synthesized by reacting amine molecules and chloromethylated 2%-3% cross-linked polystyrene beads supplied by two manufacturers (Permutit and Dow Chemicals). Data have been taken from Weiss et al.30

We suggest that, as in the cases of extraction with lipophilic amines and of the basicity of water-soluble amines, the effect of temperature is mainly observed by affecting complex intermolecular interactions. The resin case is similar to that of water-soluble amines in that the interaction occurs in an aqueous medium, so that stabilization is gained through hydration and not through ion-pair aggregation, as in the extraction case. However, it is different from both cases, in that the amine functions have limited freedom to move, so that the intermolecular interactions are affected by the bead structure. That is probably the reason for the difference in the magnitude of temperature effect between ion exchangers formed from different beads. 4. Conclusion The extraction of carboxylic acids by extractants that contain lipophilic amines is dependent on temperature.

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That phenomenon is used for industrial extraction of carboxylic acids in temperature-swing processes, where extraction is conducted at approximately ambient temperature and back-extraction is performed at an elevated temperature. In those cases where ion-pair formation is the dominant mechanism, temperature elevation decreases the extraction. In the range where acid concentration in the organic phase is above-stoichiometric, the extraction mechanism changes to hydrogen bonding or solvation and the direction of the temperature effect is reversed. In cases where weaker carboxylic acids are extracted, and where hydrogen bonding makes an important contribution also at below-stoichiometric extraction, the reversion in the direction of the temperature effect is not observed. Use of the van’t Hoff analysis leads to conflicting results, which indicates that, in cases relevant to the present study, enthalpy, entropy, or both are dependent on temperature. Temperature effects on the acids’ pKa valuess and solubility, and on water co-extraction, do not explain the temperature effect on extraction. A study of several extraction systems shows that a stronger temperature effect is observed in cases where (i) the alkylamine is a relatively weak one (pHhn 3), (iv) polycarboxylic acid is extracted, (v) the concentration of the amine and/or the carboxylic acid in the organic phase is low, (vi) the amine and/or acid are bulky, and (vii) the diluent of the amine is of low polarity. These results show that the temperature effect is strong in systems where the ion-pair interaction between the amine and the extracted acid is relatively weak, where extraction is dependent on the stabilization of the ion pair by aggregates, and where the formation of these aggregates is hindered. Polar ion pairs are dependent on aggregation for stabilization in nonpolar diluents, particularly when the extracted acid carries additional polar functions, as in polycarboxylic acids. Decreasing the concentration of the amine and of the extracted acid in the organic phase decreases the ability to aggregate. Bulky amine and/or acid are sterically hindered, which interferes with aggregation. The basicity of water-soluble amines in aqueous solutions and adsorption of acids on anion-exchange resins decrease when the temperature is increased, in agreement with the temperature effect on extraction. In all three cases, the effect of temperature is mainly through affecting complex intermolecular interactions.

Literature Cited (1) Yu-Ming, J.; Dao-chen, L.; Yuan-Fu, S. Study on Extraction of Citric Acid. In Proceedings of International Solvent Extraction Conference; Library of Congress Cataloging in Publication Data: Denver, CO, 1983; pp 517-518. (2) Baniel, A. M.; Blumberg, R.; Hajdu, K. Recovery of Acids from Aqueous Solutions. British Patent No. 1,426,018, 1973. (3) Baniel, A. M.; Blumberg, R.; Hajdu, K. Recovery of Acids from Aqueous Solutions. U.S. Patent No. 4,275,234, June 23, 1981. (4) Wennersten, R. A New Method for the Purification of Citric Acid by Liquid-Liquid Extraction. Proceedings of International Solvent Extraction Conference (ISEC); Liege University Press: Liege, Belgium, 1980; Vol. 2, p 63.

(5) Wennersten, R. The Extraction of Citric Acid from Fermentation Broth Using a Solution of Tertiary Amine. J. Chem. Technol. Biotechnol. 1983, 33B, 85-94. (6) Tamada, J. A. Extraction of Carboxylic Acids by Amine Extractants. Ph.D. Dissertation, Lawrence Berkeley Laboratory, University of California, Berkeley, CA, 1989. (7) Tamada, J. A.; King, C. J. Extraction of Carboxylic Acids by Amine Extractants. 3. Effect of Temperature, Water Coextraction and Process Considerations. Ind. Eng. Chem. Res. 1990, 26, 1933-1938. (8) Eyal, A.; Hazan, B.; Bloch, R. Recovery and Concentration of Strong Mineral Acids from Dilute Solutions Through LLX III. A “Temperature Swing” Based Process. Solvent Extr. Ion Exch. 1991, 9 (2), 223-236. (9) Sadaka, M. G.; Garcia, A. A. The Effect of Temperature on Forward/Bach Extraction Using an Amine Extractant for the Recovery of Cyclic Hydroxy Carboxylic Acids. Sep. Sci. Technol. 1998, 33 (11), 1667-1680. (10) Eyal, A. M.; Hazan, B. Process and Compositions for the Recovery of Ascorbic Acid, U.S. Patent No. 6,037,480, March 14, 2000. (11) Eyal, A. M.; Vitner, A.; Reuveni, T.; Hazan, B. Process for the Production of Erythorbic Acid, U.S. Patent No. 6,172,242, January 9, 2001. (12) Eyal, A. M.; Canari, R. pH Dependence of Carboxylic and Mineral Acid Extraction by Amine-Based Extractants: Effect of pKa, Amine Basicity and Diluent Properties. Ind. Eng. Chem. Res. 1995, 34, 1789. (13) Canari, R.; Eyal, A. M. The Extraction of Carboxylic Acid by Amine-Based ExtractantssApparent Extractant Basicity According to the pH Half-Neutralization. Ind. Eng. Chem. Res. 2003, 42, 1285. (14) Grinstead, R. R. Base Strengths of Amine in Liquid-Liquid Extraction Systems. In Proceedings of International Solvent Extraction Conference (ISEC); Dyrssen, D., Liljenzin, J. O., Rydberg, J., Eds.; North-Holland, Amsterdam, 1966; p 427. (15) Canari, R.; Eyal, A. M. The Effect of pH on Dicarboxylic Acids Extraction by Amine-Based Extractants. Ind. Eng. Chem. Res. 2003, 42, 1293. (16) Canari, R.; Eyal, A. M. Selectivity in Mono-Carboxylic Acids Extraction from Their Mixture Solutions Using an Amine Based ExtractantsThe pH Effect. Ind. Eng. Chem. Res. 2003, 42, 1301. (17) Canari, R.; Eyal, A. M. Selectivity in Extraction of Lactic, Malic, Glutaric and Maleic Acid from Their Binary Solutions Using an Amine-Based ExtractantsThe pH Effect. Ind. Eng. Chem. Res. 2003, 42, 1308. (18) Canari, R.; Eyal, A. M. Acids Extraction by Amine-Based Extractants: An Analysis of the Effect of Anion Concentration in the Aqueous Phase Using the Donnan Model. Ind. Eng. Chem. Res. 2003, 42, 1315. (19) Lide, D. R., Ed. Handbook of Chemistry and Physics, 73rd ed.; CRC Press: Boca Raton, FL, 1992-1993; pp 8-41. (20) Serjeant, E. P.; Dempsey, B. Ionisation Constants of Organic Acids in Aqueous Solution. In IUPAC Series. Pergamon Press: New York, 1979; pp 45, 177. (21) Maskill, H. Temperature Dependence of Chemical Equilibria. In The Physical Basis of Organic Chemistry; Oxford University Press: New York, 1985; pp 142-143. (22) Bender, P.; Biermann, W. J. Heat of Neutralization Studies at High Acid-Base Concentrations. I. Sodium HydroxideHydrochloric Acid. J. Am. Chem. Soc. 1952, 74, 322-325. (23) Bizek, V.; Horacek, J.; Rericha, R.; Kousova, M. Amine Extraction of Citric Acid with 1-Octanol/n-Heptane Solutions of Trialkylamine. Ind. Eng. Chem. Res. 1992, 31, 1554-1562. (24) Perrin, D. D. Temperature Effect. In pKa Prediction of Organic Acids and Bases; Dempsey, B., Serjeant, E. P., Eds.; Chapman and Hall: New York, 1981; pp 7-8. (25) Apelblat, A.; Manzurola, E. Solubility of Oxalic, Malonic, Succinic, Adipic, Maleic, Malic, Citric, and Tartaric Acids in Water from 278.15 to 338.15 K. J. Chem. Thermodynam. 1987, 19, 317320. (26) Kertes, A. S.; King, C. J. Extraction Chemistry of Fermentation Product Carboxylic Acids. Biotechnol. Bioeng. 1986, 28, 269-282. (27) Grinstead, R. R.; Davis, J. C. Base Strengths of AmineAmine Hydrochloride Systems in Toluene. J. Phys. Chem. 1968, 72 (5), 1630.

Ind. Eng. Chem. Res., Vol. 43, No. 23, 2004 7617 (28) Perrin, D. D. The Effect of Temperature on pK Values of Organic Bases. Aust. J. Chem. 1964, 17, 484-488. (29) Barton, A. F. M. Handbook of Solubility Parameters and Other Cohesion Parameters, Second Edition; CRC Press: Boca Raton, FL, 1991; pp 123-141. (30) Weiss, D. E.; Bolto, B. A.; McNeill, R.; Macpherson, A. S.; Siudak, R.; Swinton, E. A.; Willis, D. An Ion-Exchange Process with Thermal Regeneration II. Properties of Weakly Basic Resins. Aust. J. Chem. 1966, 19, 561-587.

(31) Bolto, B. A.; Weiss, D. E. The Thermal Regeneration of Ion-Exchange Resins. In Ion Exchange and Solvent Extraction. A Series of Advances; Marinsky, J. A., Marcus, Y., Eds.; Marcel Dekker: New York, 1977; Vol. 7, pp 221-286.

Received for review September 16, 2003 Revised manuscript received August 25, 2004 Accepted August 27, 2004 IE034127J