Ind. Eng. Chem. Res. 2003, 42, 1301-1307
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Selectivity in Monocarboxylic Acids Extraction from Their Mixture Solutions Using an Amine-Based Extractant: Effect of pH Riki Canari and Aharon M. Eyal* Casali Institute of Applied Chemistry The Hebrew University of Jerusalem, 91904 Jerusalem, Israel
Extraction selectivity and its dependence on pH were studied in binary mixtures of lactic, propionic, dichloroacetic, trichloroacetic, and hydrochloric acids using extractants containing Primene JMT, tris(2-ethylhexyl)amine, or tri-n-octylamine. The results are explained by extending the theory developed in previous publications of this series (Ind. Eng. Chem. Res. 2003, 42, 1285-1292 and 1293-1300). Three main cases were found: (1) In cases where the extractant is a weaker base than both anions of the extracted acids, the acids are extracted mainly by H-bonding or solvation interactions. The weaker and more hydrophobic acid is preferably extracted, and the selectivity increases with increasing pH. (2) In cases where the extractant is a stronger base than both anions of the extracted acids, both acids are extracted by ion-pair interactions. The strong acid is preferably extracted, and the selectivity increases with increasing pH. However, in some cases, the hydrophobicity of the anion is most important. (3) The third case is when the extractant is a stronger base than one acid’s anion, but a weaker base than the other’s. At elevated pH, the strongest acid in the system is preferably neutralized. That acid might be the undissociated one or the protonated amine bound to the stronger acid. Therefore, the selectivity can reverse direction with increasing pH. 1. Introduction Fermentation processes have recently become of great importance in the manufacture of food, feed, and industrial products. These processes are known to have many advantages. Fermentation uses low-cost renewable resources (mainly cereals) as raw material. Furthermore, it is environmentally friendly and is characterized by low temperatures and pressures, as well as high yields. Therefore, many compounds are produced by fermentation, including commodities such as carboxylic acids, amino acids, and ethanol, as well as highvalue products such as penicillin, vitamins, and enzymes. One drawback of this technology is that products are formed in fermentation broth solutions characterized by large amounts of various impurities. In the manufacture of carboxylic acids such as citric and lactic acids, the most difficult impurities to separate from the product acid are contaminating carboxylic acids, produced by the microorganisms or introduced with the raw materials. Fermentation broths from the manufacture of carboxylic acid are also characterized by relatively low product concentrations and, in some cases (e.g., lactic acid), by relatively high pH’s because most carboxylic acid producing microorganisms are sensitive to low pH. Separation and concentration stages (referred to in industry as “downstream processing”) can contribute significantly to overall production costs. To make the fermentation route more attractive, separation science is faced with the need to create low-cost technologies to selectively separate product carboxylic acids from highly contaminated fermentation liquors of relatively high pH. Unfortunately, scientific understanding of the effect of pH on the selectivity of extracting carboxylic acids is limited when compared to the needs of industry. Fur* To whom correspondence should be addressed. Tel.: 9722-6585843. Fax: 972-2-6584533. E-mail:
[email protected].
thermore, only a few publications have dealt with the effect of pH on acids extraction1-4 even from single-acid solutions. In our previous articles,1,5 we analyzed the effect of pH on the extraction of monocarboxylic acids by means of amine-based extractants. We divided the extraction mechanisms into two main categories: (1) H-bonding and solvation and (2) ion-pair formation. For relatively weak bases, extraction is controlled by Hbonding or solvation and is mainly determined by the concentration of the undissociated acid. Extraction is therefore strongly dependent on the pKa of the acid. Ionpair formation is the dominant mechanism when the amine extractant is more basic than the anion of the extracted acid pKhn > pKa (where pHnh is the pH of halfneutralization as determined by Grinstead’s method6,7). In those cases, the extraction strongly depends on the pKa of the extractant. In the work presented in another article,8 we studied the effect of pH on the extraction of dicarboxylic acids. Results presented in the literature4,9-12 were also explained in that work. Very few publications have dealt with extraction selectivity in systems containing mixtures of acids and amine-based extractants. Jagirdar and Sharma13 studied the selectivity in the extraction of carboxylic acids by tri-n-octylamine in xylene (and in 2-ethylhexanol). Selectivity values for the stronger acid in extraction from mixtures of acetic and monochloroacetic acids were found to be in the range of 23.2-37.8; from mixtures of monochloroacetic and dichloroacetic acids, 4-6.7; from mixtures of dichloroacetic and trichloroacetic acids, 15.3-16.8; from mixtures of formic and oxalic acids, 9.2-9.5, and from mixtures of glycolic and oxalic acids, 2.3-4.8. Kirsch and Maurer14-16 studied the distribution of citric, acetic, and oxalic acids between their binary aqueous mixtures and an organic solution of tri-noctylamine in three diluents. They found that, when there is an excess of the carboxylic group over the amine in the feed, the stronger acid is preferably extracted.
10.1021/ie020898w CCC: $25.00 © 2003 American Chemical Society Published on Web 03/08/2003
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Moreover, using a model that they developed for singleacid extractions, they calculated the extractions of citric and oxalic acids and found the results to be in reasonable agreement with the experimental values. However, when acetic acid is one of the extracted acids, the experimental extraction results are higher than predicted. They assumed that the higher extraction is a result of mixed complexes. Much information is available on the effect of pH on acid extraction by organic solvents. Thus, adding a base (e.g., NaOH) to an aqueous solution containing acids preferably neutralizes the stronger one. As a result, when this solution is put into contact with a polar organic solvent, such as alkanol, the weaker acid is preferably extracted. The process is much more complicated in cases of acid-extracting system containing an amine in the organic phase. In this case, the acids can be neutralized in both phases, the mineral base in the aqueous phase and the amine in the organic one. Only one publication was found reporting extraction selectivity and its dependence on pH in mixtures of acids using amine extractants. Malmary et al.17 studied the extraction of tartaric and malic acids from their aqueous solution at several initial pH values. The extractants were tributyl phosphate (TBP) in dodecane and triisooctylamine in 1-octanol. They found that the weaker extractant (TBP) selectively extracted the weaker acid (malic acid) and that the amine-based extractant preferably extracted the stronger acid (tartaric acid). Their explanation was that TBP binds the undissociated acid, causing the weaker acid to be extracted selectively. For the case of amine-based extractants, they suggested that ion exchange take place, so that the stronger acid is preferably extracted. As mentioned above, most of the reported studies have examined the extraction of single carboxylic acids from solutions of high proton activity. Thus, the main limiting parameter is the amount of amine, which causes the acids to compete in their interactions with it. We suggest that systems of high pH be viewed as being modified by the addition of some strong mineral base that neutralizes acidic groups in both the aqueous and organic phases (the extracted acids and the protonated amine). In these cases, the amount of available proton becomes the limiting parameter. Thus, the anions of the acids and the amine compete with each other for their interactions with the available protons, and in addition, the acids compete for their interactions with the amine. Various experimental procedures can be used to test extraction selectivity as a function of pH. We chose to add acids to the organic phase, which we then put into contact with aqueous solutions of increasing mineral base concentration. This procedure best fits our approach, which views the components of the extraction system as bases (the mineral base, the amine, and the anions of the acids) competing for interactions with the available protons. The objective of the present work is to analyze the effect of pH on the selectivity of extracting monoprotic acids from their mixture solutions. As already mentioned, the added strong mineral base neutralizes acidic groups in both the aqueous and organic phases. Yet, because we are interested in analyzing the acid species formed in the organic phase and because the main acidic group in systems where pH > pKa, is the protonated amine, we focus on analyzing the neutralization reac-
Table 1. Acidities of the Extracted Acids (pKa’s)18 and the Protonated Amine (pHhn(octanol,HCl))5
a
acid
pKa1 or pHhn
lactic propionic dichloroacetic trichloroacetic hydrochloric19 protonated TEHA in 1-octanol protonated TOA in kerosene protonated JMT in 1-octanol
3.8 4.9 1.5 0.7 -7 1.8 3.2, 5.5a 7.3
pHhn(octanol,dichloroacetic acid).
Table 2. Extraction Conditions
organic-phase composition
initial concentrations initial in the organic phase added O/A (acid I, acid II, amine) NaOH (g/g) (mol/kg) (mmol)
propionic and lactic acids 5/4.2 and TEHA in 1-octanol di- and trichloroacetic acids 4/3.6 and TOA in kerosene propionic and lactic acids 5/4.2 and JMT in 1-octanol propionic acid and HCl 20/30 and TOA in kerosene
0.40, 0.40, 0.41
0-4.2
0.44, 0.44, 0.44
0-2.7
0.41, 0.41, 0.44
0-4.2
0.45,0.42,0.39
0-16
tions in the organic phase. The most important parameter in choosing the examined systems was the acidbase properties (where pKaI and pKaII represent the pKa’s of the two acids and pHhn represents the acidity of the conjugate amine in the selected diluent). We consider four cases: (1) extraction of two acids by a very weak base, where pHhn < pKaI < pKaII; (2) extraction of two acids by a moderately strong base, where pKaI < pKaII < pHhn; (3) extraction of two acids by a very strong base, where pKaI < pKaII < pHhn; and (4) extraction of two acids by a moderately strong base, where pKaI < pHhn < pKaII 2. Experimental Section Materials. The amines used were a C18-C22 primary amine, Primene JMT (Rohm & Haas, technical grade), the branched tertiary amine tris(2-ethylhexyl)amine (TEHA; Hoechst, technical grade), and tri-n-octylamine (TOA; Hoechst, technical grade). The diluents used were 1-octanol (Merck, 99%) and the low-aromatics kerosene Parasol (Paz). The acids used were hydrochloric acid (Frutarom, analytical grade, ∼32%), propionic acid (BDH, 99%), dichloroacetic acid (Fluka, 99%), trichloroacetic acid (BDH, 99%), and lactic acid (Merck, 90%, extra pure). Table 1 presents the acidities of the extracted acids and the pH’s of half-neutralization of the amines in given diluents, pHhn. (In cases where two values are presented, the first is for the amine loaded with hydrochloridic acid and the second is for the amine with dichloroacetic acid.) Experimental Methods. The extractants (the amines in the diluents) were loaded with the acids, and samples from the resulting solutions were equilibrated with aqueous solutions containing increasing amounts of NaOH. Table 2 lists the organic-to-aqueous phase ratios (O/A, w/w), the initial concentrations of the acids and the amine, and the amounts of NaOH added in each experiment. At equilibrium, the pH of the aqueous phase was determined using a Cole-Parmer pH meter (8350-95,-97) with a Reagecon model GCFC-Hg combination pH electrode. The acid concentration in the aqueous phase was determined (in duplicate) in most
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Figure 1. Extraction of lactic and propionic acids from their mixture solution by 0.5 mol/kg TEHA in 1-octanol.
experiments by HPLC (at 60 °C; the column was Polyspher OA KC (Merck), the mobile phase was 0.005 N H2SO4, and the flow rate was 0.4 mL/min). The acid concentration in the organic phase was calculated by material balance. In the fourth experiment, the HCl concentration in the aqueous phase was determined by AgNO3 titration and that of the propionic acid by proton balance. The pH values at which the extractions were most strongly affected by changes in pH (denoted here as “drop-offs”) were calculated from the torsion points of the extraction vs pH curves (these points correspond to the pH values where the first derivative curves reach minima). The proton concentrations in the organic phase were determined by NaOH titration and compared with those calculated on the basis of the HPLC results. In most contacts, there was a deviation of less than 5% between these two values. In very few contacts, the deviation was up to 10%. (The accuracy test could not be used for propionic and HCl extraction by TOA in kerosene.) The selectivity values (to acid I compared with acid II) were calculated as follows: ([acid I]org[acid II]aq)/ ([acid I]aq[acid II]org), where the concentrations of the acids in the aqueous phase are the sums of the concentrations of their dissociated and undissociated forms. 3. Results and Discussion Propionic and Lactic Acid Extraction by TEHA in 1-Octanol. Figure 1 presents the pH dependence of propionic and lactic acid extraction from their mixture solution and the selectivity to propionic acid. The extractant was a weak-amine-based extractant, TEHA in 1-octanol. The extraction curves for both acids show high loadings in the low pH range and low loadings in the high pH range. The drop-offs were found to be pH ≈ 4 and pH ≈ 5 for lactic and propionic acid, respectively. The selectivity to propionic acid increases from ∼8 to ∼30 as the pH was increased. In this system, the extractant is a weaker base (pHhn ≈ 2) than the anions of both acids (pKa,lactic acid ) 3.78 and pKa,propionic acid ) 4.87). Therefore, the extraction mechanism is dominated by H-bonding or solvation, and the two acids present in the organic phase are mostly in their undissociated forms. Hence, the propionic acid is preferably extracted because of its higher hydrophobicity. The added stronger base (NaOH) preferably neutralizes first the strongest acid in the system, which is the undissociated lactic acid. The neutralized acid is
Figure 2. Extraction of dichloro and trichloroacetic acids from their mixture solution by 0.45 mol/kg TOA in kerosene.
transferred into the aqueous phase, causing the selectivity to propionic acid to increase with increasing pH. The drop-offs for lactic and propionic acids are at about their pKa values because the added base neutralizes the undissociated acids. Trichloroacetic and Dichloroacetic Acid Extraction by TOA in Kerosene. Figure 2 presents the pH dependence of trichloroacetic and dichloroacetic acid extraction from their mixture solution by a moderately strong amine-based extractant, TOA in kerosene. It also shows the total acid concentration in the organic phase and the selectivity (to trichloroacetic acid) as functions of the pH. The curve for the total acid concentration shows two drop-offs. The first is in the above-stoichiometric-extraction region (where the acid concentration in the organic phase is higher than that of the amine) in the pH range of 1.5-3, and the second is in the pH range of ∼7-8. In addition, this figure shows that the selectivity to trichloroacetic acid increases with increasing pH. The difference between the loading of dichloroacetic acid in the above-stoichiometric range and that in the stoichiometric range is greater than the difference for trichloroacetic acid. Thus, although both acids in the above-stoichiometric range are bound by H-bonding and by solvation to the ion-pair species, the main additional acid is dichloroacetic acid. Yet, when a mineral base is added to the system (upon pH elevation), the base first reacts with the strongest acid in the system, which is the undissociated trichloroacetic acid. The neutralized trichloroacetic acid is expected to transfer into the aqueous phase. If this were, in fact, the mechanism, one would expect the selectivity to trichloroacetic acid to decrease with increasing pH, which is not the case. Our explanation is that, at this stage, an ion-exchange reaction takes place between the trichloroacetate in the aqueous phase and the dichloroacetate bound to the amine in ion-pair form, R3NH+‚‚‚CHCl2COO-, resulting in a slight increase in the selectivity to trichloroacetic acid. The pHhn values for the protonated amines in R3NH+‚‚‚CCl3COO- and R3NH+‚‚‚CHCl2COO- in the single-acid systems were determined1,5 to be 6.5 and 5.5, respectively. As explained in a previous publication of this series,5 the apparent acidity of a given protonated amine, R3NH+, is not necessarily a constant value. It depends on the properties (mainly acidity and hydrophobicity) of the coresponding counteranion. In the
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Figure 3. Extraction of propionic and lactic acids from their single-acid solutions by 0.5 mol/kg JMT in 1-octanol.
present system, the extractant is a stronger base than the anions of the two acids, so that it is protonated while the anions are not. Therefore, when the amine concentration in the organic phase is higher than (or similar to) the combined concentration of the two acids, the two acid species present in the system are the protonated amines. According to their pHhn values, the protonated amine bound to the dichloroacetate, R3NH+‚‚‚CCl2COO-, acts as the stronger acid and reacts first with the added strong base, NaOH. Therefore, the selectivity to trichloroacetic acid increases with increasing pH. Propionic and Lactic Acid Extraction by JMT in 1-Octanol. Figure 3 presents the pH dependences of the extractions of lactic and propionic acids from their single-acid solutions by JMT in 1-octanol. The extraction curves show high loadings in the lower pH range, followed by a drop-off at pH ≈ 7.5. JMT in 1-octanol is a strong base (pHhn(octanol,HCL) ≈ 7.3) compared to the anions of the two acids. Therefore, the acid present in the extraction systems (when the amine concentration in the organic phase is greater than that of the acids) is the protonated JMT, which is paired with the anions of the acids R3NH+‚‚‚Prop- or R3NH+‚‚‚Lact-. As explained in a previous paper of this series,5 the drop-offs in such cases represent the neutralization of the protonated amine by the strongest base (NaOH). Note that, judging from the extractions from single-acid solutions (as suggested by many authors), one would conclude no significant selectivity at pH > 5. Figure 4 presents the pH dependence of lactic and propionic acid extraction from their mixture solution by JMT in 1-octanol. Contrary to expectations based on the results for the corresponding single-acid systems, through most of the pH range, extraction is selective to propionic acid. Unlike the case of extraction from a mixture of dichloroacetic and trichloroacetic acids, the selectivity is to the weaker acid. In that regard, the extraction selectivity obtained using the strong amine here is similar to that obtained using the very weak amine (TEHA) for similar solutions. This figure shows the total acid concentration in the organic phase and the selectivity to propionic acid as functions of the pH. The total acid concentration curve is divided into three parts. The first part (from the lowest pH to point a) represents the above-stoichiometric extraction. The second part (from point a to point b) represents the stoichiometric extraction where the total acid concentration is the same as the concentration of the amine, and the third part, above pH ≈ 7.4, is where
Figure 4. Extraction of lactic and propionic acids from their mixture solution by 0.5 mol/kg JMT in 1-octanol.
the combined acid concentration is lower than that of the amine. The lactic acid curve also contains these three parts. The drop-offs for each of the curves (that for the total acids and that for lactic acid) were found to occur at pH ≈ 4.5 and pH ≈ 7.5. The propionic acid curve has a low slope in the loading curve up to pH ≈ 7.4 and a sharp decrease above this pH. The value of the selectivity (to propionic acid) at the lowest pH is about 1 and increases with the pH (this dependence also shows the three sections). These results can be explained as follows: The main additional acid extracted in the above-stoichiometric extraction (through H-bonding or solvation interactions) is the lactic acid. This is indicated by the lactic acid extraction curve, which shows an increase in the abovestoichiometric part, whereas the propionic acid extraction curve does not. (A previous article20 suggests that weak and hydrophobic acids are preferably extracted in the above-stoichiometric range, which seems to conflict with the results presented here. Yet, the concentration of undissociated propionic acid in the aqueous phase in the relevant pH range was so low that essentially the only acid in the system to be extracted was the lactic acid.) The acids present in the abovestoichiometric range are, therefore, the undissociated lactic acid and the protonated amine. The added strong base (NaOH) first neutralizes the strongest acid in the system, the undissociated lactic acid. As a result, the drop-off in the overall extraction curve is at the lactic acid pKa. Starting at point b, the main species in the organic phase are the protonated amines paired with the two anions, i.e., R3NH+‚‚‚Prop and R3NH+‚‚‚Lact. The selectivity in this range is to propionic acid, and it increases with increasing pH. Thus, although lactic acid is a stronger acid than propionic acid, the aminepropionate pair is more stable in the organic phase than amine-lactate because of the higher hydrophobicity of the propionate ion. Propionic and Hydrochloric Acid Extraction by TOA in Kerosene. Figure 5 presents the pH dependences of the extractions of propionic and hydrochloric acids from their single-acid solutions by a moderately strong amine-based extractant, TOA in kerosene. The extraction curve for HCl shows a high loading in the low pH range, which decreases sharply at a pH of about 3.0. The extraction of propionic acid decreases sharply
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4.87). An interesting observation is that, at low pH, even though free amine molecules are present, propionic acid is preferably bonded to the protonated amine-chloride ion-pair. Tamada and King12 described the same phenomenon. They found that the association constant values for the formation of a 1:1 trioctylamine/propionic acid species is -0.13, whereas, for the 1:2 species R3N‚‚‚HProp‚‚‚HProp, it is 0.1. Hence, propionic acid preferably forms H-bonds with an ion-pair rather than forming an ion-pair with the moderately strong amine. Above pH ≈ 3, NaOH preferably neutralizes the strongest acid in the system, which is the protonated amine. As a result, the selectivity to propionic acid increases. 4. Summary and Conclusions Figure 5. Extraction of propionic and hydrochloric acids from their single-acid solutions by 0.5 mol/kg TOA in kerosene.
Figure 6. Extraction of propionic and hydrochloric acids from their mixture solution by 0.5 mol/kg TOA in kerosene.
at a pH of about 5. In a previous article,5 we concluded that propionic acid extraction by TOA in kerosene is controlled mainly by H-bonding or solvation and is determined by the concentration of undissociated propionic acid. Therefore, the extraction is strongly dependent on the pKa of propionic acid, which is 4.87. In contrast, the dominant mechanism for the extraction of HCl by this extractant is ion-pair formation. Therefore, the pH of the drop-off represents the pKa of the protonated amine paired with the chloride ion. Judging from the extractions from single-acid solutions, one would expect a very high selectivity to propionic acid above pH ≈ 3. Figure 6 presents the pH dependence for the extraction of propionic and hydrochloric acid from their mixture solution by TOA in kerosene. In addition, it presents the total acid concentration in the organic phase and the selectivity to propionic acid. The extraction curve for HCl exhibits behavior very similar to that of its single-acid solution, but the pH of the drop-off is lower. The loading of HCl reaches a concentration of about 0.4 mol/kg, which is very close to the concentration of the amine. The extraction curve for propionic acid resembles that for HCl in the lower pH range (from pH ) 1 to pH ) 3.5). The results can be explained as follows: At low pH, propionic acid forms the above-stoichiometric species R3NH+‚‚‚Cl-‚‚‚HProp. The main acids in the organic phase are, therefore, the protonated amine R3NH+‚‚‚Cl(pKa ≈ 3) and the undissociated propionic acid (pKa )
The previous publications in this series5,7 explained the various possible extraction mechanisms, as determined by the properties of the extractant and those of the extracted acid. Through these mechanisms, the dependence of extraction on the pH in single-acid systems was analyzed. The present article summarizes a study of the selectivity in extraction from a solution containing a mixture of monoprotic acids and its dependence on pH. The analysis of single-acid systems was extended to explain these results. On the basis of that analysis, the extraction systems can be divided into three classes in terms of extraction selectivity and its dependence on pH: Systems in Which the Two Acids Are Extracted Mainly by H-Bonding or Solvation Interactions. In cases where the extractant is a weaker base than the anions of the two acids, the extraction occurs mainly through H-bonding or solvation interactions, and the acids are present in the organic phase in their undissociated forms. In such cases, weaker and more hydrophobic acids are preferably extracted. As pH increases (equivalent to the addition of a strong mineral base) the stronger acid in the system is preferably neutralized; thus, the selectivity to the weaker extracted acid increases. The strongest effect is observed around the pKa of the stronger acid. In systems where the extraction is controlled mainly by H-bonding or by solvation, the extraction of one acid can be enhanced by the presence of another acid that increases the polarity and the protic properties of the organic phase. This synergistic effect can decrease as the pH is increased, because this extraction mechanism is dependent on the concentration of the undissociated acid. The first experiments discussed in this work, where propionic and lactic acid are extracted by TEHA, belong to this category. The results show that propionic acid (the weaker acid) is preferably extracted in the lower pH range, and the selectivity is increased at higher pH, particularly where the pH is higher than the pKa of lactic acid. Malmary et al.17 studied the selectivity and the effect of pH for the extraction of tartaric and malic acids using a very weak extractant, tributyl phosphate (TBP) in dodecane. Their results agree with our explanation. They found that the weaker acid (malic acid) was selectively extracted and that the selectivity increased with pH. H-bonding and/or solvation interactions are also the mechanisms for the added acids in the above-stoichiometric range. In such cases, as in the cases of Hbonding-controlled substoichiometric extraction dis-
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cussed above, weak and hydrophobic acids are selectively extracted over stronger and less hydrophobic ones. This selectivity can be seen in the experiment where dichloroacetic and trichloroacetic acids are extracted by TOA, in which the main acid in addition to the ion-pair species in the above-stoichiometric range is dichloroacetic acid. Systems in Which Both Acids Are Extracted Mainly by Ion-Pair Interactions. In cases where both acids are extracted via ion-pair formation, the acidic species in the organic phases are the conjugate acid of the amine, R3NH+, bound to the anions of the extracted acids. Increasing the pH is equivalent to adding to the system a strong base that neutralizes these acidic species. The selectivity, thus, depends on the acidity of the protonated amines. This acidity was found to be very sensitive to the properties of the anion,1,5 such as bacisity, hydrophobicity, electron density, and ability to form H-bonds. Other important parameters are the amine concentration and diluent properties. In many cases, the stronger acid forms a stronger interaction with the amine and is selectively extracted. In such cases, the protonated amine bound to the anion of the stronger acid, R3NH+‚‚‚-X, acts as a weaker acid than the protonated amine bound to the weaker acid, R3NH+‚‚‚-A. Thus, as the pH increases, the stronger acid in the system, R3NH+‚‚‚-A, is preferably neutralized, and the selectivity to HX increases. This is the case for the extraction of dichloroacetic and trichloroacetic acids with TOA at Z < 1. Jagirdar and Sharma13 examined the extraction of carboxylic acids from mixtures (acetic and monochloroacetic acids, monochloroacetic and dichloroacetic acids, dichloroacetic and trichloroacetic acids, formic and oxalic acids, and glycolic and oxalic acids) by tri-noctylamine in xylene or in 2-ethyl-hexanol. Most of those systems belong to the present case: the extractant is a stronger base than both anions of the acids. In agreement with our results, they found that the selectivity was for the stronger acid. The results of Kirsch and Maurer14-16 show the same tendency. They studied the distribution of citric, acetic, and oxalic acids between their binary aqueous mixtures and tri-n-octylamine in three diluents. In all systems, the stronger acid was selectively extracted. These two studies examined the extraction of carboxylic acids from their solutions of high proton activity (no addition of mineral base). Thus, the main limiting parameter was the amount of amine, and the effect of pH on the selectivity was not examined. Malmary et al.17 studied the selectivity and the effect of pH in the extraction of tartaric and malic acids from their aqueous solution using a moderately strong base extractant, triisooctylamine in 1-octanol. In agreement with our results, they found that the stronger acid (tartaric acid) was selectively extracted and that the selectivity increased with pH. However, the acidity of the extracted acid, as measured in the aqueous phase (pKa), is not the only parameter affecting the selectivity in this kind of interaction. The experiment described in this work in which lactic and propionic acids were extracted with JMT demonstrates that the hydrophobicity of the anion is a very important parameter. Propionic acid was preferably extracted although it is the weaker acid. In systems where the extraction is mainly controlled by ion-pair interactions, the acids compete for interactions with the same amine. The extraction of one acid
is decreased by the presence of the other acid in the organic phase. This antagonistic effect can be seen in the above-discussed cases. Systems in Which One Acid Is Extracted Mainly through H-Bonding or Solvation Interactions While the Other Is Extracted through Ion-Pair Formation. Typically, such systems are formed when the extractant is a stronger base than the anion of one extracted acid (HX), but a weaker base than the anion of the other acid (HA). At elevated pH, the strongest acid is preferably neutralized. This acid might be the undissociated one or the protonated amine bound to the stronger acid. Therefore, the selectivity might reverse direction with increasing pH, as in the case of the hydrochloric and propionic acid extraction by TOA discussed here. In such cases, a synergistic extraction can be found. On one hand, the ion-pair interaction R3NH+‚‚‚-X is enhanced by the presence of the undissociated acid in the organic phase, HA, which acts as a polar diluent. On the other hand, the ion-pair species forms bonding sites for the undissociated acid, HA. This is the case in the results of Kirsch and Maurer,14-16 where acetic acid was extracted together with citric or oxalic acid by trin-octylamine in three diluents. The distribution ratios of the acids were higher than predicted from the singleacid systems. 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) Reisinger, H.; King, C. J. Extraction and Sorption of Acetic Acid at pH above pKa To Form Calcium Magnesium Acetate. Ind. Eng. Chem. Res. 1995, 34, 845. (3) Yang, S. T.; White, S. A.; Hsu, S. T. Extraction of Carboxylic Acids with Tertiary and Quaternary Amines: Effect of pH. Ind. Eng. Chem. Res. 1991, 30, 1335. (4) Tung, A. T.; King, C. J. Sorption and Extraction of Lactic and Succinic Acid at pH > pKa1. 1. Factors Governing Equilibria. Ind. Eng. Chem. Res. 1994, 33, 3217. (5) Canari, R.; Eyal, A. M. Extraction of Carboxylic Acids by Amine-Based Extractants: Apparent Extractant Basicity According to the pH of Half-Neutralization. Ind. Eng. Chem. Res. 2003, 42, 1285-1292. (6) Grinstead, R. R. Base Strengths of Amine in Liquid-Liquid Extraction Systems. In Proceedings of the International Solvent Extraction Conference (ISEC); Dyrssen, D., Liljenzin, J. O., Rydberg, J., Eds.; North Holland Publishing Company: Amsterdam, 1966; p 427. (7) Grinstead, R. R.; Davis, J. C. Base Strengths of AmineAmine Hydrochloride Systems in Toluene. J. Phys. Chem. 1968, 72 (5), 1630. (8) Canari, R.; Eyal, A. M. Effect of pH on Dicarboxylic Acids Extraction by Amine-Based Extractants. Ind. Eng. Chem. Res. 2003, 42, 1293-1300. (9) Gusakova, G. V.; Denisov, G. S.; Smolyanskii, A. L.; Shraiber, V. M. Hydrogen Bonding and Proton Transfer in Carboxylic Acid-Amine Systems. Spectroscopic Investigation of the Equilibrium Between a Molecular Complex and Ion Pair. J. Gen. Chem. USSR 1989, 56, 531 (English Translation). (10) Kirsch, T.; Maurer, G. Distribution of Oxalic Acid between Water and Organic Solution of Tri-n-octylamine. Ind. Eng. Chem. Res. 1996, 35, 1722-1735. (11) Kuwano, Y.; Kusano, K.; Takahashi, T.; Kondo, K.; Nakashio, F. Extraction Equilibria of Lower Carboxylic Acids with Long-Chain Alkylamine. Soc. Chem. Eng. Jpn. 1982, 8 (4), 404. (12) Tamada, J. A.; King, C. J. Extraction of Carboxylic Acids with Amine Extractants. 2. Chemical Interaction and Interpretation of Data. Ind. Eng. Chem. Res. 1990, 29, 1327.
Ind. Eng. Chem. Res., Vol. 42, No. 7, 2003 1307 (13) Jagirdar, G. C.; Sharma, M. M. Recovery and Separation of Mixtures of Organic Acids from Dilute Aqueous Solutions. J. Sep. Process Technol. 1980, 12, 40. (14) Kirsch, T.; Maurer, G. Distribution of Binary Mixtures of Citric, Acetic and Oxalic Acid Between Water and Organic Solutions of Tri-n-octylamine. Part II: Organic Solvent Methylisobutylketone. Fluid Phase Equilib. 1998, 142, 215. (15) Kirsch, T.; Maurer, G. Distribution of Binary Mixtures of Citric, Acetic and Oxalic Acid Between Water and Organic Solutions of Tri-n-octylamine. Part I: Organic Solvent Toluene. Fluid Phase Equilib. 1997, 131, 213. (16) Kirsch, T.; Maurer, G. Distribution of Binary Mixtures of Citric, Acetic and Oxalic Acid Between Water and Organic Solutions of Tri-n-octylamine. Part III: Organic Solvent Chloroform. Fluid Phase Equilib. 1998, 146, 297. (17) Malmary, G.; Vezier, A.; Robert, A.; Mourgues, J.; Conte, T.; Molinier, J. Recovery of Tartaric and Malic Acids from Dilute
Aqueous Effluents by Solvent Extraction. J. Chem. Technol. Biotechnol. 1994, 60, 67. (18) Handbook of Chemistry and Physics, 73rd ed.; Lide, D. R., Ed.; CRC Press: Boca Raton, FL, 1992-1993. (19) Perrin, D. D. Ionisation Constants of Inorganic Acids and Bases in Aqueous Solution, 2d ed.; Pergamon Press: New York, 1982. (20) Eyal, A. M.; Arbel-Hadd, M.; Hadi, S.; Canari, R.; Haringman, A.; Hazan, B. Extraction of Acids, Water and Hydrophilic Molecules by Amine and Amine Salts. In Proceedings of the International Solvent Extraction Conference (ISEC’93); Logsdail, D. H., Slater, M. J., Eds.; Elsevier Science: New York, 1993; Vol. 2, p 723.
Received for review November 1, 2002 Revised manuscript received November 3, 2002 Accepted November 3, 2002 IE020898W
