Extraction of Carboxylic Acids by Amine-Based Extractants: Apparent

Riki Canari and Aharon M. Eyal*. Casali Institute of Applied Chemistry, The Hebrew University of Jerusalem, 91904 Jerusalem, Israel. This article anal...
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Ind. Eng. Chem. Res. 2003, 42, 1285-1292

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APPLIED CHEMISTRY Extraction of Carboxylic Acids by Amine-Based Extractants: Apparent Extractant Basicity According to the pH of Half-Neutralization Riki Canari and Aharon M. Eyal* Casali Institute of Applied Chemistry, The Hebrew University of Jerusalem, 91904 Jerusalem, Israel

This article analyzes the extraction of hydrochloric, trichloroacetic, propionic, lactic, and acetic acids by nine amine-based extractants, including Primene JMT, tris(2-ethylhexyl)amine (TEHA), and tri-n-octylamine (TOA), in various diluents. The extractants were half-loaded with the acids, and the pH values (pHhn) of the aqueous phases were determined. The IR spectra of the organic phases were also recorded. The results were analyzed according to our theory, which was developed in a previous article, and a good correlation was found. Ion-pair formation is the main mechanism for systems in which the basicity of the amine is higher than that of the extracted acid’s anion, and H-bonding of the undissociated acid becomes important only for the extraction of weak acids by weak bases. Thus, one can predict the acid form in the organic phase from the difference between the apparent basicity of the extractant on extracting HCl, pHhn(HCl,D), and the pKa of the extracted acid. 1. Introduction Long-chain aliphatic amines are known to be efficient, selective, and reversible extractants for the separation of carboxylic acids.1-6 All of these properties derive from the extraction mechanism of the acid. The mechanism depends on a number of parameters, including the temperature; the pH (which will be discussed in the following); and the properties and concentrations of the amine, acid, diluents, and other components in the system. Of the above factors, amine basicity is one of the most important parameters controlling the extraction, but there is no simple independent measure for such basicity. Measured by standard methods, the pKa values for water-soluble alkylamines are about 9-11.7 Similar methods are not applicable to water-immiscible amine systems. That is true because the standard electrochemical potential of a hydrogen electrode (H2/H+) in an organic phase is not necessarily 0, as it is in water (defined by IUPAC in 1953). Moreover, calculating the activity coefficients of ions is much more complicated in an organic phase than in water, where calculations can be done according to the Debye-Hu¨ckel equations. Marcus et al.8 proposed a method that uses the dielectric constants of the components to determine the standard potential of a nonaqueous solution. They noted that other scientists have used at least 10 terms to calculate the standard potential of an ion. Moreover, the pH determination in an organic medium, even in a very polar one, is a most complicated

procedure, as described by Sen and Gibbons,9 who measured lactic acid acidity in 10, 20, 30, 50, 60, 70, 80, and 90% methanol in water at four different temperatures. For water-immiscible amines, the ones of interest to our case, basicity has been determined indirectly, through the efficiency of extraction.10,11 The problem with this approach is that the extraction efficiency is dependent on a number of parameters, as mentioned above. Hence, there is no simple way to assign independent (intrinsic) basicity values to the amines of interest. Yet, for the selection of amines for process optimization, the assignment of basicity values to amines is required. Grinstead and co-workers10,11 used extraction efficiencies to compare the relative basicities of various waterimmiscible amines. The equation used for the reaction

* To whom correspondence should be addressed. Tel.: 9722-6585843. Fax: 972-2-6584533. E-mail: [email protected].

where R3N and KaB represent the amine and the apparent base strength of the protonated amine at some

R3NH+Cl-org T R3Norg + H+aq + Cl-aq was

[H+]aq[Cl-]aq[R3N]org

KaB )

[R3NH+Cl-]org

(1)

or

log

(

)

[R3NH+Cl-]org [R3N]org

) pKaB - pH + log([Cl-]aq)

10.1021/ie010578x CCC: $25.00 © 2003 American Chemical Society Published on Web 03/08/2003

(2)

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specified concentration, respectively. Grinstead et al. formed water-immiscible amine hydrochlorides by equilibrating amines with aqueous solutions containing equivalent amounts of hydrochloric acid. They then added NaOH at one-half the equivalent amount to achieve [R3NH+]org ) [R3N]org. Thus, according to eq 2, the pH of the aqueous phase at this point, i.e., the pH of halfneutralization (pHhn(HCl)), is equal to pKaB (the chloride concentration in the aqueous phase was 1 M). The approach of Grinstead et al. involves some difficulties, including the implicit assumption that one can use the proton activity in the aqueous phase as the proton activity in the organic phase in calculating the basicity of water-immiscible amines. These aspects are discussed in an accompanying article.12 Yet, as suggested in the following, pHhn(HCl) is a very useful tool. Grinstead et al.10,11 measured the pH of half-neutralization of many alkylamines in toluene (pHhn(HCl,toluene)) and found values of 2-6, 4-7, and 6.5-8.5 for tertiary, secondary, and primary amines, respectively. They concluded that steric hindrance has a critical effect on the pHhn values within the groups. HCl was used as the reference acid for determining the amine basicity, because its extraction through other mechanisms is negligible when in equilibrium with dilute aqueous solutions.12 For the same reasons, HCl was chosen for our work. In a previous article,1 we used Grinstead’s method to measure the apparent basicity of extractants. We found the pHhn values to increase with increasing polarity of the diluent, particularly with increasing protic contribution. We measured the pHhn(HCl) of 0.5 mol/kg Alamine 336 in kerosene, xylene, nitrobenzene, tributyl phosphate, and 1-octanol. The results were 3.50, 3.52, 3.62, 3.88, and 4.16, respectively. For 0.3 mol/L Alamine 336 in 1-octanol and chloroform, we found pHhn(HCl) values of 4.21 and 4.13, respectively. The pHhn of a given amine in a given diluent D in the extraction of HCl (pHhn(HCl,D)) includes contributions resulting from the properties of (1) the amine (such as the degree of substitution on the nitrogen atom and the steric hindrance), (2) the diluent, and (3) the extracted acid. Hydrocarbons, when used as diluents, have very weak interactions with the amine and with the aminehydrochloride ion pairs. Therefore, pHhn(HCl,kerosene) is referred to here as the standard basicity of the amines. In contrast, polar, and particularly protic, diluents stabilizing the ion pair through solvation and through H-bonds have strong effects on the extraction mechanism and efficiency. Thus, pHhn(HCl,polar or protic diluent) is higher than the standard basicity. There are a number of characteristics in which the extraction of carboxylic acids differs significantly from that of HCl, including acidity, H-bonding capabilities, and steric hindrance. Thus, as indicated in our previous article,1 the apparent basicity was found to be very sensitive to the nature of the anion of the extracted acid. Still, the basicity of the amine is a key parameter. In our previous article,1 we proposed a theory in which the interactions in extraction systems containing a water-immiscible amine and a carboxylic acid are viewed as a competition between the amine and the anion of the carboxylic acid for the available protons. In cases where the amine is a much stronger base than the anion of the extracted acid, i.e., pKaB . pKaA, the amine binds the proton and forms a positively charged protonated amine, which binds the anion of the

extracted acid. Extraction based on such a mechanism is referred to as ion-pair formation, as in the case of HCl extraction (see eqs 1 and 2). The basicity of the carboxylic acid anion is known from the acid pKaA. For the amine, one can use pHhn(HCl,D), as will be shown in the following by an IR study. On the other hand, if the amine is a much weaker base, i.e., pKaB , pKaA, the contribution of ion-pair formation is small, and extraction by the amine is mainly affected by H-bonding or solvation of undissociated acid molecules. Hence

R3Norg + HAaq T R3N‚‚‚HAorg KH )

[R3N‚‚‚HA]org [R3N]org[HA]aq

(3)

where KH is the H-bond formation mass-action constant. One of the most important parameters affecting the extraction efficiency is the pH. The effect of the pH on extraction through H-bonding is calculated by the combination of eq 3 and the acid-dissociation equation according to

log

(

)

[R3N‚‚‚HA]org [R3N]org

)

log KH + log([A-]aq) + pKaA - pH (4)

The pH is a very important parameter in the fermentative production of carboxylic acids. Fermentation of most carboxylic acids is product-inhibited and slows dramatically when the pH is lowered to values below the pKa of the acid.1,2,14,15 A base is usually added to the fermentor to maintain the pH above the pKa of the fermented acid. Thus, the separation of carboxylic acids from relatively high pH fermentation broth is of great interest. Of particular interest is the extraction of lactic acid (pKa of 3.87) from fermented broth at pH > 4.5.3,16-19 Improving citric acid production is also of interest.20,21 Furthermore, improving the distribution of mineral and carboxylic acids between aqueous solutions of relatively high pH and amine-based extractants, is important in salt splitting processes.22,23 In our previous article, we studied the effect of pH on the extractions of hydrochloric, propionic, and dichloroacetic acid by trialkylamine and of propionic acid by Primene JMT. Using our theory, we analyzed these results and those of Tung and King4 and of Yang et al.3 The results showed a good correlation with the theory. The aim of the present article is to further investigate the extraction of carboxylic acids by amine-based extractants. More specifically, we determined the values of the apparent basicity of nine amine-based extractants in more than 40 extraction systems via their pHhn values as a function of the properties of the amines, the diluents, and the extracted acid. The results were analyzed by our theory, and the analysis was confirmed by IR spectra of the organic phases of the studied systems. Another objective was to investigate the effect of pH on the extraction of some monoprotic carboxylic acid to analyze the results using our theory. 2. Experimental Section Materials. The acids used were hydrochloric acid (Frutarom, analytical grade, ∼32%), propionic acid (Pro; BDH, 99%), trichloroacetic acid (BDH, 99%), lactic acid

Ind. Eng. Chem. Res., Vol. 42, No. 7, 2003 1287 Table 1. pHhn Values of the Extractants Loaded with Hydrochloric Acid

a

extractant

pHhn loaded with HCl

TEHA + kerosene TEHA + 20% octanol TEHA + octanol TOA + kerosene TOA + 20% octanol TOA + octanol JMT + kerosene JMT + 20% octanol JMT + octanol

0.6a 1 1.8 2.85a 4.5 6.06 6.84 6.97 7.26

Two organic phases

(Merck, 90%, extra pure), and acetic acid (Frutarom, 99.7%, analytical grade). The amines used were a C18C22 primary amine of the form RR′R′′CNH2, Primene JMT (Rohm & Haas, technical grade), the branched tertiary amine tris(2-ethylhexyl)amine (TEHA; Hoechst, technical grade), and the straight-chain tertiary amine tri-n-octylamine (TOA; Hoechst, technical grade). The diluents used were 1-octanol (Merck, 99%) and the lowaromatics kerosene Parasol (Paz). Methods. Nine extractants were prepared from the three amines and three diluents. Portions of 0.5 mol/kg TEHA, TOA, and JMT were separately dissolved in kerosene, 1-octanol, or a 1-octanol + kerosene mixture with an octanol concentration of 20% of the total organic phase. The latter diluent is denoted in the following as “20% octanol”). pHhn Determination. Extractant (15 g containing 7.5 mmol of amine) was equilibrated with 5 g of an aqueous phase containing 3.8 mmol of the extracted acid (this amount allows the amine to be half-loaded). The aqueous phase also contained the sodium salt of the acid at a concentration of 0.25 mol/kg to achieve similar anion concentrations in the two phases. (In a few systems, the acid concentration in the organic phase was less than 0.25 mol/kg; see Table 4.) The two phases were then separated, and the pH of the aqueous phase was determined. IR Spectra. IR spectra of the organic phases were obtained with a Nicolet 510 FTIR spectrometer. 3. Results and Discussion pHhn(HCl) and IR Spectra. Table 1 presents the pHhn(HCl) values of the nine extractants loaded with hydrochloric acid. The table shows that the pHhn values decrease in the following sequence: primary amine > straight-chain tertiary amine > branched tertiary amine. The amine basicity sequence is opposite to that obtained in the gaseous phase, where the inductive effect of the hydrocarbon chains bound to the nitrogen atom is the dominant parameter.24 In the organic phase, steric hindrance was found to have a great effect on the pHhn,11 which is why the branched tertiary amine was found to act as the weakest base. The table also shows that the pHhn(HCl) value increases with increasing concentration of the protic diluent. The protic diluent has a significant effect on the stabilization of the ion pairs in the organic phase. Still, the pHhn(HCl) values for the amine are much lower than the pKa values for water-soluble alkylamines, which are about 9-11. The same extractants were tested in the extraction of monocarboxylic acids. IR spectra of the loaded extractants were examined for correlations between the

Figure 1. IR spectra of (a) TEHA in octanol and (b) TEHA in octanol and water.

acid species in the extractant, the pHhn(HCl) value, and the extraction mechanism. Two absorptions in the IR spectra are relevant to this purpose: (1) the peak at about 1545-1675 cm-1 is of the symmetric carbonylic bond, which indicates that the carboxylic group is in its dissociated form, and (2) the peak at about 1710-1750 cm-1 is of the nonsymmetric carbonylic bond, which corresponds to the undissociated carboxylic group. A degree of complication is introduced by an additional peak at about 1650 cm-1 that is present in some of the IR spectra and is related to the presence of water in the organic phase.25 This can be seen in Figure 1. Trace a of Figure 1 shows the spectrum of an organic phase containing 0.5 mol/kg TEHA in 1-octanol without water, and trace b shows the same extractant after it has been put into contact with water. As can be seen, the relevant peak is observed only in the presence of water. (In the following, only a few spectra are presented, so as not to overload the article with too many figures.) Propionic Acid. A peak at 1710 cm-1 is found in the three spectra of the loaded TEHA in kerosene, in 20% 1-octanol, and in 1-octanol. This peak is attributed to the nonsymmetric carbonylic bond and indicates the presence of propionic acid in its undissociated form. Such a peak also appears in the spectrum of TOA in kerosene loaded with propionic acid (at about 1720 cm-1) (Figure 2a). However, in the spectrum of propionic acid loaded TOA in 20% 1-octanol (trace b of Figure 2), a peak at 1588 cm-1 is also observed. This peak indicates that some of the acid is in its dissociated form, while the rest is in its undissociated form. The spectrum of propionic acid loaded TOA in 1-octanol (trace c of Figure 2) shows a larger peak at 1588 cm-1 and a smaller one at about 1720 cm-1, indicating that the ratio of the dissociated acid to the undissociated acid has increased. (This spectrum also shows a peak indicating the presence of water.) The spectrum of propionic acid

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Figure 2. IR spectra of 0.25 mol/kg propionic acid in (a) 0.5 mol/ kg TOA in kerosene, (b) 0.5 mol/kg TOA in 20% 1-octanol in kerosene, and (c) 0.5 mol/kg TOA in 1-octanol.

loaded JMT in kerosene and those of JMT in 20% 1-octanol and in 1-octanol do not show any peak in the higher range, only the one at about 1560 cm-1. Therefore, the propionic acid is present in these extractants mainly in its dissociated form. Trichloroacetic Acid. The spectrum of trichloroacetic acid in the weakest amine-based extractant of this experiment, TEHA in kerosene, shows a very small peak at about 173 cm-1 and a large peak at 1675 cm-1. To identify the bonds associated with these peaks, trichloroacetic acid was added to kerosene with no amine. The resulting spectrum exhibited a peak at 1750 cm-1. As no dissociated form of the acid is expected in such an extractant, this peak corresponds to the nonsymmetric carbonylic bond of this acid. Hence, the large peak at 1675 cm-1 in the spectrum of TEHA in kerosene corresponds to the presence of dissociated acid, while the smaller one, at about 1730 cm-1, corresponds to the presence of undissociated acid. Therefore, even in this weak extractant (TEHA in kerosene), the trichloroacetic acid is present mainly in its dissociated form. The peak of the dissociated acid is the only acid peak present in the spectra of the trichloroacetic acid loaded extractants that are more basic than TEHA in kerosene, i.e., TEHA in 20% 1-octanol, TEHA in 1-octanol, and TOA and JMT in the various diluents. Lactic Acid. The spectra of lactic acid in TEHA in 20% 1-octanol and TEHA in 1-octanol show only a peak at 1719 cm-1, that of the symmetric carbonylic bond, indicating that lactic acid is present in these extractants in its undissociated form. Such a peak is not present in the spectrum of lactic acid loaded TOA in 20% 1-octanol, but a large peak can be observed at 1592 cm-1, indicating that lactic acid is present in this extractant mainly in its dissociated form. (The spectrum of the amine in 1-octanol shows a peak at 1650 cm-1, which corresponds

to the presence of water.) The spectra of the stronger extractants (JMT in the diluents) loaded with lactic acid show, in the relevant area, only the peak of the dissociated acid. Acetic Acid. The spectra of acetic acid loaded extractants show an increase in the peak at 1573 cm-1 in the sequence TEHA in 20% 1-octanol < TOA in 20% 1-octanol < TOA in 1-octanol. The peak at 1715 cm-1 increases in the opposite order. Thus, the proportion of the dissociated acid increases with increasing basicity of the extract. The spectra of the stronger extractants (JMT in the diluents) loaded with the acetic acid show, in the relevant area, only the peak of the dissociated acid. Dependence of the Extraction Mechanism on Relative Basicities, pHhn(HCl) - pKa. Table 2 summarizes the relevant peaks in the IR spectra of the nine extractants loaded with the various acids and the related chemical bonds. The table also shows the values of the difference between the pHhn(HCl,D) and the pKa of the extracted acids. The bold values represent the cases in which the IR spectra indicate the formation of undissociated acid in the organic phase. According to our theory,1 pHhn(HCl,D) gives a good indication for the basicity of the amine, pKaB, and thereby for the acid form in the organic phase. Table 2 shows that, where pHhn(C,D) - pKaA is close to 0 (the amine basicity is close to that of the anion of the extracted acid), both the dissociated and undissociated forms of the acids are present in the organic phase. In systems where this difference is negative (the amine basicity is lower than that of the anion of the extracted acid), the main acid form in the organic phase is undissociated, whereas when the difference is positive, the acid in the organic phase is mainly dissociated. These results are in agreement with our expectations and confirm that the pH of half-neutralization values determined with hydrochloric acid using the same diluent reflect the extractant’s basicity. The difference between the pHhn(HCl,D) and the pKa of the extracted acid predicts the strength of the interaction in addition to the extraction mechanism, which is important in process design. The greater the difference, the stronger the interaction between the amine and the acid and, thus, the better the extraction efficiency. Yet, for the same reason, the efficiency of the back-extraction is lower. Another important parameter is the dependence of the interaction strength on the temperature. In cases involving the ion-pair formation mechanism, the interaction typically weakens significantly as the temperature increases, which improves back-extraction. This phenomenon is not observed in cases where the H-bonding mechanism dominates. Thus, efficient extraction and back-extraction are expected in cases where the pHhn(HCl) greater than the pKa of the extracted acid, but not more than by 2 units. That is the reason for choosing straight-chain tertiary amines for the industrial extraction of citric acid (for which pHhn(HCl,D) - pKa ) 1-2 pH units). In addition, pHhn(HCl) is very sensitive to the diluent structure and concentration, thus allowing it to be tailored to specific needs. Dependence of the Apparent Basicity on the Extraction of Monocarboxylic Acids. As mentioned above, hydrochloric acid was used as the reference acid for determining the amine basicity, because it is mainly extracted through the ion-pair mechanism. However,

Ind. Eng. Chem. Res., Vol. 42, No. 7, 2003 1289 Table 2. Extractions of Trichloroacetic, Propionic, Lactic, and Acetic Acid by the Nine Extractants: Relevant IR Peaks and Values of the Difference pHhn(HCl) - pKaA extractant kerosene TEHA + kerosene TEHA + 20% octanol TEHA + octanol TOA + kerosene TOA + 20% octanol TOA + octanol JMT + kerosene JMT + 20% octanol JMT + octanol TEHA + kerosene TEHA + 20% octanol TEHA + octanol TOA + kerosene TOA + 20% octanol TOA + octanol JMT + kerosene JMT + 20% octanol JMT + octanol

bonded acid form

peak(s) (cm-1)

pHhn(HCl) - pKaA

trichloroacetic (pKa ) 0.7) 1750 1730, 1675 COOH, COO1675 COO1675 COO1675 COO1675 COO1675 COO1675 COO1675

COOlactic acid (pKa ) 3.87)

1719 1719, (1650)

COOH COOH

1592 1592 1600 1600 1600, (1650)

COOCOOCOOCOOCOO-

Table 3. pHhn Values of the Extractants Loaded with Hydrochloric, Trichloroacetic, Propionic, Lactic, and Acetic Acids extractant

trichloroacetic propionic lactic acetic

TEHA + kerosene TEHA + 20% octanol TEHA + octanol

2.82 3.58 4.68

TOA + kerosene TOA + 20% octanol TOA + octanol

6.45 7.25 8.24

JMT + kerosene JMT + 20% octanol JMT + octanol

9.2 9.73

5.4 6.38

4.1 5.58

7.2 7.27 7.93

6.61 6.77 7.36

6.57 6.84 7.57

interactions of this ion pair, R3NH+Cl-, with other ion pairs in the solution, with the diluent, and with coextracted water molecules have a significant effect on the extraction11 and, thus, on the pHhn. The interactions are expected to be different when another anion is paired with the protonated amine. How should these interactions affect the pHhn value? The assumptions in our previous article1 were that, comparing two acids, HX and HA, of the same acidity, the one with an anion more compatible with the organic phase, say HA, is expected to be preferentially extracted. This effect is expected to be stronger in a diluent of low polarity. In the present article, we examined these assumptions. Table 3 and Figure 3 present the pH values of the aqueous phases that are in equilibrium with the above extractants when half-loaded with various monocarboxylic acids (limited to cases in which the IR spectra indicate the formation of dissociated acid in the organic phase). These results show that the basicity sequence of the extractant is similar in the various acids, but the apparent basicity is sensitive to the properties of the anion. In most of the cases, the pHhn value decreases in the order extractant loaded with trichloroacetic acid . propionic acid > hydrochloric acid > acetic acid ≈ lactic acid. The results show that the extractant’s apparent basicity is higher by about 2 pH units when loaded with trichloroacetic acid than when loaded with the other acids. The apparent amine basicity of Primene JMT loaded with trichloroacetic acid is almost the same as that determined for water-soluble amines (i.e., 9-11).

peak(s) (cm-1)

bonded acid form

pHhn(HCl) - pKaA

propionic acid (pKa ) 4.87) -0.1 0.3 1.1 2.15 3.8 5.36 6.14 6.27 6.56

1710 1710 1710, (1650) 1710, 1588 1710, 1588 1588, (1650) 1565 1565 1565

-3.27 -2.87 -2.07 -1.02 0.63 2.19 2.97 3.1 3.39

1715 1715 1715 1710, 1573 1573, (1650) 1555 1555 1555, (1650)

COOH COOH COOH (H2O) COOH, COOCOOH, COOCOOH, COO- (H2O) COOCOOCOOacetic acid (pKa ) 4.75) COOH COOH COOH COOH, COOCOOCOOCOOCOO-

-4.27 -3.87 -3.07 -2.02 -0.37 1.19 1.97 2.1 2.39 -4.15 -3.75 -2.95 -1.9 -0.25 1.31 2.09 2.22 2.51

This high amine basicity can be explained by the high acidity of trichloroacetic acid as compared to the acidities of the other tested carboxylic acids (pKa of 0.7 compared to about 4-5). The stronger the acid, the stronger the association of the proton with the nitrogen’s electron pair to form the positively charged group R3NH+. Tamada et al.5,26,27 examined the extraction of carboxylic acids by amine extractants. They showed that the extraction of carboxylic acids increases with increasing acidity of the extracted acid. Another important parameter is the hydrophilic/hydrophobic properties of the anion. The higher the anion’s hydrophobicity, the better the ion-pair solvation in the organic phase. Thus, high pHhn values can be observed when the amine is loaded with propionic acid and acetic acid. The results show that the pHhn values are higher for amines loaded with trichloroacetic acid than for those loaded with hydrochloric acid (Table 1), even though the latter is a stronger acid. This can be explained by the charge density of the anion, which is higher in the case of hydrochloric acid. Diamond et al.28 showed that the higher the anion charge density, the lower the extractant loading. The authors explained this phenomenon by increasing hydration in the aqueous phase, which leads to higher stabilization there and, thus, lower extraction with increasing charge density. Extraction through H-Bonding or Solvation. As mentioned above, Table 2 also presents the pHhn values for cases where the undissociated acid is the main form in the organic phase. In those cases, the anion basicity is stronger than the basicity of the amine. Thus, the extraction is mainly effected through the H-bonding of undissociated acid molecules. Table 4 presents the log KH values calculated according to eq 4 for these cases. The table shows that KH increases with decreasing acidity of acid and with increasing hydrophobicity. It also shows that KH increases with increasing 1-octanol concentration. The obtained log KH values are in the range of -0.36 to 0.46. These values are lower than the pKa’s of the acids (3.78, 4.75, and 4.87 for lactic, acetic, and propionic acid, respectively). Thus, according to eq 4, the determined pH is close to the acid’s pKa value. Effect of pH. To this point, we have been dealing with systems of a single concentration of the extracted

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Figure 3. pHhn values of 0.5 mol/kg TEHA, TOA, and JMT in octanol, 20% octanol, and kerosene when loaded with trichloroacetic, propionic, hydrochloric, acetic, or lactic acid. Table 4. log KH Values Calculated According to Eq 4 in Cases Where the Undissociated Acid Is the Main Form in the Organic Phasea system TEHA + 20% 1-octanol + lactic acid TEHA + kerosene + acetic acid TEHA + 20% 1-octanol + acetic acid TEHA + 100% 1-octanol + acetic acid TEHA + kerosene + propoionic acid TEHA + 20% 1-octanol + propoionic acid TEHA + 100% 1-octanol + propoionic acid a

[HA]org (mol/kg) pHhn log KH 0.072 0.049 0.17 0.19 0.16 0.20 0.21

3.1 3.95 4.26 4.64 4.39 4.84 5.36

-0.94 -1.16 -0.184 0.266 -0.205 0.396 0.95

Based on IR spectroscopy.

acid in the organic phase (one-half that of the amine). In this section, we analyze the effect of pH on acid extraction. The effect of pH on the extraction of acetic acid by various extractants (Figure 4) is demonstrated in an article published by Reisinger and King.29 In their analysis, the authors mention that the extraction sensitivity (or the capacity drop-off) is at about pH ) 5 when the acid is extracted by TOPO (trioctylphosphine oxide). This was also the pH of the first drop-off when the acid was extracted by a quaternary amine salt, whereas the drop-offs when the acid was extracted by secondary and tertiary amines were between 6 and 7. The authors did not explain the first results but pointed out that the values of the drop-offs when the acid was extracted by secondary and tertiary amines were similar to those found by Tung and King30 for lactic and succinic acid extraction with the same extractants. They suggested that the 1:1 acid/base complexation constant is

Figure 4. Extraction of acetic acid at 25 °C by various extractants in 1-octanol diluent. Initial concentrations: 0.45 mol/L aqueous acetic acid, 0.3 mol/kg extractant in 1-octanol (Reisinger and King28).

directly proportional to the aqueous ionization constant of the extracted acid. This suggestion does not seem to explain the fact that the pH values of the drop-off are similar for acids with different pKa values. We suggest a different explanation. The TOPO and quaternary ammonium salt (chloride or acetate) act as weaker bases than the acetate ion. Therefore, the acid is mainly extracted through H-bond interactions to form TOPO‚‚‚HAc, R4N+‚‚‚Cl-‚‚‚HAc, and/or R4N+‚‚‚Ac-‚‚‚HAc, so that the acid in the organic phase is in its undissociated form. As the pH is increased, the undissociated acetic acid becomes neutralized, and the extraction drop-

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Figure 5. pH dependence of propionic acid extraction by 0.5 mol/ kg amine in kerosene.

off occurs at a pH about equal to the pKa of acetic acid (which is 4.75). At the same time, in the case of acetic acid extraction by secondary and tertiary amines, the acid is extracted mainly through the formation of the ion pair R3NH+‚‚‚Ac-, because the extractant acts as a stronger base than the anion of the acetic acid. In these cases, the protonated amine is the species neutralized upon base addition. The drop-off in this mechanism is determined by the pKa of the protonated amine. That is the reason for the similar pH values of the drop-offs for the extractions of acetic, lactic, and succinic acids by the same secondary or tertiary amine-based extractant, although the pKa values of the acids are different. This explanation is supported by the results reported in our previous article1 (Figure 5). Figure 5 presents the pH dependence of propionic acid extraction by Primene JMT in kerosene (pHhn(HCl) ≈ 7), which is a much stronger base than the propionate anion (pKaA ) 4.88). As a result, at a pH of about 4, essentially all of the amine is protonated to form ion pairs and to reach stoichiometric extraction. Additional propionic acid is extracted through H-bonding to the ion pair or through solvation in the relatively polar medium, i.e., R3NH+ Pro-‚‚‚HPro. As the pH is increased, the stronger acid in the organic phase, namely, the undissociated propionic acid, is neutralized first, followed by the weaker acid, the protonated amine. Therefore, the extraction curve shows two drop-offs: one at about the pKa of the propionic acid to reach the stoichiometric level of the ion pair and another at about the pKaB value. A similar result was observed in the extraction of acetic acid by the quaternary amine salt. In contrast to the primary amine, the basicity of trioctylamine in kerosene solution is lower than that of the propionate anion in aqueous solution. As a result, less than half of the amine is expected to be protonated when equilibrated with propionic acid/sodium propionate aqueous solution at pH > 3.4 (see Figure 2). Yet, as shown in Figure 5, the drop-off in the extraction (pH ≈ 4.5) is at about the pKaA, indicating that propionic acid is extracted through H-bond formation or through solvation. We suggest the same explanation as for the extraction of acetic acid by TOPO (Figure 4). 4. Summary This article further investigates the extraction of carboxylic acids by amine-based extractants, according to the theory suggested in our previous article.1 The

extraction mechanism strongly depends on the acidbase properties of the extractant and of the extracted acid. Ion-pair formation is the main extraction mechanism for systems in which the basicity of the amine is higher than that of the anion of the extracted acid, whereas H-bonding of an undissociated acid becomes important only for the extraction of weak acids by weak bases. One can predict the acid form in the organic phase by the difference between the apparent basicity of the extractant upon extraction of HCl, pHhn(HCl,D), and the pKa of the extracted acid. As expected, the IR results showed that, in systems where this difference is close to 0, the dissociated acid and the undissociated acid are both present in the organic phase. In systems where the difference is negative, the main acid form in the organic phase is the undissociated one, and where the difference is positive, the acid is mainly dissociated. The apparent basicity of the extractant, pHhn, was also determined when half-loaded with various monocarboxylic acids instead of with hydrochloric acid. Although the basicity sequences of the extractants are similar for the various acids, the apparent basicity was found to be very sensitive to the properties of the anion. In most cases, the pHhn value decreases in the order extractant loaded with trichloroacetic acid . propionic acid > hydrochloric acid > acetic acid ≈ lactic acid. Although, compared with low-molecular-weight amines in water, the system of trichloroacetic acid extraction with JMT has higher steric hindrance and lower polarity, the measured apparent basicity of JMT is similar. This high basicity is explained by the high acidity of trichloroacetic acid and by its low charge density, which reduces the energy lost when the ion leaves the aqueous phase. On the other hand, the high pHhn values for the amine loaded with propionic acid are explained by the high hydrophobicity of the propionate ion, i.e., by the low repulsion of the ion pairs from the organic phase. The values for the mass-action constant of H-bond formation, KH, were calculated for cases in which the IR spectrum showed mainly undissociated acid in the organic phase. The weaker the acid and the higher the hydrophobicity, the higher the KH. The KH values found in the present work are low compared with the pKa’s of the acids. Thus, according to this theory, the pKa value is the main factor that determines the sensitivity of the extraction to pH. This conclusion was confirmed by the results. Literature Cited (1) 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. (2) Yabanaver, V. M.; Wang, D. I. C. Extractive Fermentation for Lactic Acid Production. Biotechnol. Bioeng. 1991, 37, 1095. (3) Yang, S. T.; White, S. A.; Haso, S. T. Extraction of Carboxylic Acids with Tertiary and Quaternary Amines: Effect of pH. Ind. Eng. Chem. Res. 1991, 30, 1335. (4) Tung, L. A. Recovery of Carboxylic Acids at pH Greater than pKa. Ph.D. Thesis, Lawrence Berkeley Laboratory, University of California, Berkeley, CA, 1993. (5) Tamada, J. A. Extraction of Carboxylic Acids by Amine Extractant. Ph.D. Thesis, Lawrence Berkeley Laboratory, University of California, Berkeley, CA, 1989. (6) Kirsch, T.; Ziegenfuss, H.; Maurer, G. Distribution of Citric, Acetic, and Oxalic Acid between Water and Organic Solutions of Tri-n-octylamine. Fluid Phase Equilib. 1997, 129, 235. (7) Vogel, A. I. Vogel’s Textbook of Quantitative Inorganic Analysis, Including Elementary Instrumental Analysis, 4th ed.; Longman Group Limited: London, pp 889, 1978.

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Received for review July 5, 2001 Revised manuscript received October 31, 2002 Accepted November 3, 2002 IE010578X