Synergistic Pseudo-Hydroxide Extraction - ACS Publications

Synergistic Pseudo-Hydroxide Extraction: Synergism and Anion Selectivity in Sodium. Extraction Using a Crown Ether and a Series of. Weak Lipophilic Ac...
0 downloads 0 Views 109KB Size
Download PDF
Anal. Chem. 2003, 75, 405-412

Synergistic Pseudo-Hydroxide Extraction: Synergism and Anion Selectivity in Sodium Extraction Using a Crown Ether and a Series of Weak Lipophilic Acids Tatiana G. Levitskaia,† Peter V. Bonnesen, C. Kevin Chambliss,‡ and Bruce A. Moyer*

Chemical Sciences Division, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, Tennessee 37830-6119

The nature of the weak lipophilic acid used in synergistic combination with a model crown ether cation host was shown to have a strong effect on the strength and selectivity of sodium hydroxide separation from alkaline aqueous salt solutions. Sodium ion-pair extraction employing only cis-syn-cis-dicyclohexano-18-crown-6 (1) in nitrobenzene (NB) was correlated with the standard Gibbs energy (∆Gp°) of anion partitioning into NB and was notably weak and nonselective for the hydroxide ion, in accord with Hofmeister bias. The Hofmeister order can be selectively overcome for NaOH by utilization of acid-base chemistry coupled with complexation of sodium ion in the NB phase. Upon addition of a lipophilic organic acid into the solution of 1 in NB, sodium extraction was selectively enhanced due to the initiation of an exchange reaction between the aqueous sodium ion and the ionizable proton of the organic acid. A series of weak lipophilic hydroxy acids (HA) including fluorinated alcohols and phenols was tested. The resulting synergistic pseudo-hydroxide extraction correlates with the pKa of the employed HA; the most acidic cation exchangers provide the greatest synergism. The synergistic factor obtained using a fluorinated benzyl alcohol 7 was as high as 256. Ion-pair extraction of neutral sodium salts was not changed or only mildly enhanced by addition of HA into the NB solution of 1. This enhancement was explained by hydrogen bonding of HA with the anion as related to the hardness of the anion and the acidity of HA. In comparison with the synergism observed for NaOH, this enhancement was weak and unable to overcome the Hofmeister effect. Examination of extraction selectivity revealed that the combination of 1 and 7 preferentially extracted NaOH over all other sodium salts, including the normally preferred nitrate and perchlorate salts. Quantitative recovery of NaOH from the NB phase was demonstrated via hydrolysis of the organic acid upon a single contact of the loaded solvent with water. Anion recognition and transport in liquid-liquid separation systems is an area of growing interest in supramolecular science.1 Among the many inorganic anions of importance, the hydroxide ion is a common matrix ion from which one often desires to 10.1021/ac0259212 CCC: $25.00 Published on Web 01/03/2003

© 2003 American Chemical Society

selectively separate analytes. However, very limited effort has been directed toward selective separation of the hydroxide ion itself. Table 1 summarizes eight cases whereby host-guest principles could be utilized to effect hydroxide separation via ion-pair extraction (cases 1-5) or cation exchange (cases 6-8).2 Cases 1-5 propose direct extraction of the hydroxide ion together with a cation, which will here be taken as sodium. Separation of NaOH by ion-pair extraction (cases 1-5) represents a particular challenge, because both ions have a small thermochemical radius and high affinity for water. Hence, a large energy barrier of ion partitioning into the organic phase must be overcome.3 It follows that high selectivity of OH- extraction from aqueous salt matrixes would be difficult to achieve due to the Hofmeister effect,4 in which large charge-diffuse anions are preferentially extracted.5 Approaches to NaOH separation based on a cation-exchange principle (cases 6-8) are especially suited for the hydroxide ion, because they avoid the Hofmeister effect by resorting to Bro¨nsted acidbase chemistry. The simplest example, case 6, has already been demonstrated.6,7 This so-called pseudo-hydroxide extraction process employs a weak lipophilic organic hydroxy acid (HA) (e.g., phenol or fluorinated alcohol) dissolved in a water-immiscible * Corresponding author. Tel: 1-865-574-6718. Fax: 1-865-574-4939. E-mail: [email protected]. † Present address: Radiochemical Science and Engineering Group, Pacific Northwest National Laboratory, P.O. Box 999; MSIN P7-22, Richland, WA 99352. ‡ Present address: Department of Chemistry, Baylor University, Waco, TX 76798. (1) Bianchi, A., Bowman-James, K., Garcia-Espana, E., Eds.; Supramolecular Chemistry of Anions; Wiley-VCH: New York, 1997. (2) Moyer, B. A.; Bonnesen, P. V.; Chambliss, C. K.; Haverlock, T. J.; Marchand, A. P.; Chong, H.-S.; McKim, A. S.; Krishnudu, K.; Ravikumar, K. S.; Kumar, V. S.; Takhi, M. In Nuclear Site Remediation: First Accomplishments of the Environmental Science Program; Eller, P. G., Heineman, W. R., Eds.; ACS Symposium Series 778; American Chemical Society: Washington, DC, 2001; pp 114-132. (3) Marcus, Y. Ion Properties; Marcel Dekker: New York, 1997. (4) Hofmeister, F. Arch. Exp. Pathol. Pharmakol. 1888, 24, 247-269. (5) Moyer, B. A.; Bonnesen, P. V. In Supramolecular Chemistry of Anions; Bianchi, A., Bowman-James, K., Garcia-Espana, E., Eds.; Wiley-VCH: New York, 1997; pp 377-416. (6) Grinstead, R. R. U.S. Patent 3,598,547, August 10, 1971. Grinstead, R. R. U.S. Patent 3,598,548, August 10, 1971. (7) Chambliss, C. K.; Haverlock, T. J.; Bonnesen, P. V.; Engle, N. L.; Moyer, B. A. Environ. Sci. Technol. 2002, 36, 1861-1867. Moyer, B. A.; Chambliss, C. K.; Bonnesen, P. V.; Keever, T. J. Solvent and Process for Recovery of Hydroxide from Aqueous Mixtures. U.S. Patent 6,322, 702, November 27, 2001.

Analytical Chemistry, Vol. 75, No. 3, February 1, 2003 405

Table 1. Eight Approaches to Recognition and Extraction of Sodium Hydroxide Using Host-Guest Chemistry case

name

1 2

no host cation host

3 4 5 6

anion host dual hosts ion-pair host pseudo-hydroxide extraction synergistic pseudohydroxide extraction ditopic pseudohydroxide extraction

7 8

description a polar solvent (e.g., 1-octanol) is used to solvate both cation and anion a cation host (e.g., crown ether) binds the cation; anion remains primarily solvated an anion host binds the anion; cation remains primarily solvated cation host and anion host are used in synergistic combination a single host binds both the cation and anion a lipophilic weak organic acid is employed to effect cation exchange; no hosts are used a cation host is used in synergistic combination with a lipophilic weak organic acid a weakly acidic functionality is incorporated into the cation host

Chart 1. Structural Formulas of Investigated Compounds

diluent to extract a hydroxide equivalent by exchange of its ionizable proton for an alkali metal cation at elevated pH. On contact with water, the organic-phase sodium salt releases its cation via hydrolysis, thereby producing an aqueous solution of the alkali metal hydroxide and regenerating the organic acid. Cation-exchange processes involving alkali metal cations are often hindered by poor cation solvation in typical water-immiscible diluents. An effective cation host lowers the net barrier to cation extraction via complexation, synergistically enhancing the alkali metal extraction at a given pH (case 7). An added benefit of the host molecule entails control over the cation selectivity. Previously, we reported for the first time synergistic pseudohydroxide extraction (case 7) based on the cooperative effect of a proton-ionizable fluorinated alcohol and novel cage-annulated crown ethers in nitrobenzene (NB).8 It was demonstrated that synergism is selective for hydroxide and depends on the structure of the cation host. The present work extends the investigation by examining the effect of the organic hydroxy acid on the synergism and selectivity for NaOH extraction in the presence of a representative cation host. To function efficiently in pseudo-hydroxide extraction, HA must meet three principal requirements. Namely, it must be acidic enough to efficiently deprotonate on contact with an aqueous solution of elevated pH; it must have sufficient conjugate basicity so that it can be regenerated via hydrolysis at the regeneration step; and it must possess sufficiently high lipophilicity to prevent its loss into the aqueous phase. Accordingly, six different lipophilic hydroxy acids were tested, with pKa values ranging from 20 for 1-octanol (2) to 8.8 for 4-alkyl-R,R-bis(8) Levitskaia, T. G.; Moyer, B. A.; Bonnesen, P. V.; Marchand, A. P.; Krishnudu, K.; Chen, Z.; Huang, Z.; Kruger, H. G.; McKim, A. S. J. Am. Chem. Soc. 2001, 123, 12099-12100.

406 Analytical Chemistry, Vol. 75, No. 3, February 1, 2003

Table 2. pKa Values of Studied Hydroxy Acids hydroxy acid

pKa

ref source

2 4, 5 6 7

20 12.5 9.9 8.8

9 10a 11 10b

a Estimated from the group contribution method. b The pK value a for 7 is assumed to be the same as for R,R-bis(trifluoromethyl)benzyl 10 alcohol presuming that 4-alkylation has only slight effect on the acidity.

(trifluoromethyl)benzyl alcohols 7 (Chart 1 and Table 2).9-11 Based on lipophilicity parameters12 and previous data,7 all hydroxy acids examined, with the exception of 7a, are considered to have sufficient lipophilicity to afford negligible partitioning to the aqueous phase. The commercially available crown ether cis-syncis-dicyclohexano-18-crown-6 (1) served as a model lipophilic cation host. NB was chosen as the diluent, both because of its high dielectric constant ( ) 34.8),13 which suppresses ion-pairing effects in the organic phase, and because of its lack of hydrogen bond donicity,14 which minimizes effects of specific interactions (9) Kreshkov, A. P.; Aldarova, N. Sh.; Tanganov, B. B.; Slavgorodskaya, M. V. Zh. Fis. Khim. 1970, 44 (1), 241-243. (10) Chang, I. S.; Price, J. T.; Tomlinson, A. J.; Willis, C. J. Can. J. Chem. 1972, 50 (4), 512-520. (11) Bolton, P. D.; Hall, F. M.; Kudrynski, J. Aust. J. Chem. 1972, 25, 75-80. (12) Hansch, C.; Leo, J. Substituent Constants for Correlation Analysis in Chemistry and Biology; Wiley-Interscience: New York, 1979. (13) Marcus, Y. In Principles and Practices of Solvent Extraction; Rydberg, J., Musikas, C., Choppin, G., Eds.; Marcel Dekker: New York, 1992; pp 2170. (14) Marcus, Y. Chem. Soc. Rev. 1993, 409-416.

Figure 1. Dependence of the standard molar Gibbs energy of anion partitioning between water and NB on reciprocal thermochemical anion radii. Solid symbols denote ∆Gp°(X-) taken from ref 15. Open symbol corresponds to the working estimate of ∆Gp°(OH-) used in this report. Anion radii were taken from ref 3.

with anions. Reported Gibbs energies of anion partitioning ∆Gp°(X-) from water to NB15 plotted in Figure 1 exhibit a strong Hofmeister bias. The plot also affords an empirical estimate of the unavailable value of ∆Gp°(OH-), as discussed in the Appendix. Distribution experiments were performed by contacting aqueous phases containing a single salt NaX (where X- ) ClO4-, NO3-, Br-, Cl-, F-, or OH-) with a NB phase containing crown ether 1 alone (case 2 in Table 1), HA used alone (case 6), and a combination of 1 with HA (case 7). Extraction by NB with no extractant (case 1) or with HA used alone (case 6) was expected to be negligible, owing to the highly positive ∆Gp° values for Na+ and X-.3 Complexation of the anions by weak hydroxy acids is well documented and represents a potential competitive effect that could influence selectivity.16 Thus, it was of additional interest whether the combination of 1 and the hydroxy acids in the NB phase would result in synergistic enhancement of NaX ion-pair extraction under neutral conditions and how this would compare with synergistic pseudo-hydroxide extraction. Finally, recovery of the NaOH as an aqueous solution and regeneration of the cation exchanger were demonstrated. EXPERIMENTAL SECTION General Information. All chemicals obtained from commercial suppliers were reagent grade or higher and used as received unless otherwise specified. The sodium salts (Aldrich, 99.99%) were dried at 110 °C for at least 48 h before use. Sodium hydroxide solutions were prepared from a 50% stock NaOH solution with degassed water and stored in sealed Teflon FEP bottles. Aqueous solutions were prepared using distilled water deionized to 18 MΩ‚cm with a Barnstead Nanopure water (15) Makrlik, E. Pol. J. Chem. 1997, 396-399. (16) (a) Odashima, K.; Ito, T.; Tohda, K.; Umezawa, Y. Chem. Pharm. Bull. 1998, 46(8), 1248-1253. (b) Mashkovsky, A. A.; Nabiullin, A. A.; Odinokov, S. E. J. Chem. Soc., Faraday Trans. 1 1987, 83, 1879-1883. (c) Ryall, R. R.; Strobel, H. A.; Symons, M. C. R. J. Phys. Chem. 1977, 81 (3), 253-256. (d) Kenjo, T.; Brown, S.; Held, E.; Diamond, R. M. J. Phys. Chem. 1972, 76, 1775-1783. (e) Green, R. D.; Martin, J. S.; Cassie, W. B. McG.; Hyne, J. B. Can. J. Chem. 1969, 47, 1639-1648. (f) Rulinda, J. B.; Zeegers-Huyskens, Th. Spectrosc. Lett. 1979, 12(1), 33-43. (g) Barcza, L.; Pope, M. T. J. Phys. Chem. 1973, 77, 1795-1796. (h) Turner, D. J.; Diamond, R. M. J. Phys. Chem. 1968, 72, 2831-2837.

Figure 2. Dependence of log DNa,1 on the standard molar Gibbs energy of anion partitioning between water and NB. An open symbol corresponds to the loading-corrected value of DNa,1 for perchlorate calculated based on the amount of free extractant in the organic phase. Conditions: 1 M aqueous sodium salt, 0.02 M 1 in NB, 1:1 phase ratio, and 25 °C. Experimental uncertainty is approximately 3-5% unless otherwise indicated by error bars.

purification system. ACS grade nitrobenzene (Aldrich) was purified by washing with 0.1 M NaOH and H2O followed by distillation. (Caution: Nitrobenzene is a toxic chemical that should only be handled in a well-functioning chemical fume hood.) The cis-syncis isomer of dicyclohexano-18-crown-6 (Aldrich) was separated from the cis-anti-cis isomer by selective crystallization from heptane. 1,1,1-Trifluoro-3-(4-tert-octylphenoxy)-2-propanol (4), 1,1,1trifluoro-3-(3,5-di-tert-butylphenoxy)-2-propanol (5), 4-methyl-R,Rbis(trifluoromethyl)benzyl alcohol (7a), and 4-octyl-R,R-bis(trifluoromethyl)benzyl alcohol (7b) were prepared as described elsewhere.8,17-19 Liquid-Liquid Distribution Experiments. Equal volumes (0.4 mL each) of organic phase (solution of 0.02 M 1 alone or in combination with 0.04 M HA in NB) and aqueous phase (1 M aqueous solution of NaX, where X- ) ClO4-, NO3-, Br-, Cl-, F-, or OH-, spiked with 22Na tracer) were equilibrated in 2-mL polyethylene cryogenic vials by repeated inversion on a GlassCol laboratory rotator in a constant-temperature air box at 25 ( 0.2 °C for 1 h. Subsequently, the samples were centrifuged for 3 min. To determine the Na+ ion distribution ratio (DNa) [Na+]org/ [Na+]aq), a fraction of each phase was removed, and the activity of 22Na was measured by γ-radiometric techniques.7 Duplicate experiments were performed for every data point. Experimental precision was approximately 3-5%, unless otherwise indicated by error bars in Figure 2, as estimated from a combination of replicate determinations, volumetric error, and counting precision. The minimum detectable extraction of Na+ here corresponds to DNa ) 5 × 10-5. Control experiments are tabulated in the Supporting Information. Extraction of NaX into neat NB (case 1 in Table 1) was feeble and only detected for NaClO4, which gave a DNa value of 9((1) × (17) Duchemin, C. R.; Engle, N. L.; Bonnesen, P. V.; Haverlock, T. J.; Delmau, L. H.; Moyer, B. A. Solvent Extr. Ion Exch. 2001, 19, 1037-1058. (18) Liu, K.-T.; Kuo, M.-Y.; Shu, C.-F. J. Am. Chem. Soc. 1982, 104, 211-215. (19) Haverlock, T. J.; Bonnesen, P. V.; Moyer, B. A. Solvent Extr. Ion Exch., submitted for publication.

Analytical Chemistry, Vol. 75, No. 3, February 1, 2003

407

Table 3. Sodium Ion Distribution Ratios (DNa) from Aqueous Solutions to NB Phases Containing 1 Alone or in Combination with Hydroxy Acids 2-7 at 25 °Ca aqueous NaX

DNa,1

1+2

NaClO4 NaNO3 NaBr NaCl NaF NaOH

2.02 × 10-2 9.14 × 10-3 3.93 × 10-3 6.2((0.1) × 10-4 6((1) × 10-5 1.4((0.3) × 10-4

2.00 × 10-2 9.60 × 10-3 3.74 × 10-3 7.8((0.1) × 10-4 9((1) × 10-5 1.7((0.3) × 10-4

DNa,1+HA obtained using a combination of 1 and hydroxy acids HA 1+3 1+4 1+5 1+6 2.02 × 10-2 1.02 × 10-2 4.60 × 10-3 1.13 × 10-3 7((1) × 10-5 c

1.95 × 10-2 1.08 × 10-2 5.70 × 10-3 1.34 × 10-3 1.2 × 10-4 5.22 × 10-3

2.08 × 10-2 1.04 × 10-2 5.66 × 10-3 1.38 × 10-3 1.3((0.3) × 10-4 5.21 × 10-3

2.03 × 10-2 1.12 × 10-2 5.65 × 10-3 1.63 × 10-3 2.9 × 10-4 1.10 × 10-2

1 + 7b 2.02 × 10-2 1.13 × 10-2 5.23 × 10-3 1.27 × 10-3 5.4((0.1) × 10-4 3.61 × 10-2

a Aqueous phase: 1 M NaX. Organic phase: 0.02 M 1 in NB or 0.02 M 1 and 0.04 M HA in NB. Phase ratio was unity. Experimental standard deviation is 3-5% unless otherwise indicated in the table. Each value shown represents an average of duplicates. b Values of DNa,1+HA were obtained using 7a for NaX (where X- ) ClO4-, NO3-, Br-, Cl-, or F-) and 7b for NaOH. c Values of DNa,1+3 were not determined due to precipitate formation in the extraction system.

10-5. Likewise, addition of HA to NB also effected little or no detectable extraction (DNa e 26 × 10-5). All of the control values were negligible relative to the corresponding DNa,1+HA values. Values of DNa,1+HA for NaOH using 1 in combination with alcohols 3 and 7a were not obtained, owing to the formation of solid phases upon extraction. To demonstrate regeneration of the hydroxy acids and recovery of NaOH following extraction, a stripping experiment was conducted. Initial extractions were performed from 1, 3, or 5 M aqueous NaOH solutions spiked with 22Na tracer using a NB phase containing 0.02 M 1 and 0.04 M 5 or 6. The NB phases from the extraction steps were subsequently stripped with water at an organic-to-aqueous phase ratio of 1:3 and DNa was determined. Parallel experiments were performed without 22Na tracer, and the concentration of the base in the stripping solution was determined by titration with standardized HCl solution.

and [Na‚1+]org ) [X-]org. The corresponding equilibrium constant can then be expressed as

RESULTS AND DISCUSSION NaX Extraction by Crown Ether. To quantify the extraction of the sodium salts by 1 used alone (case 2 in Table 1), equal volumes of an aqueous phase containing 1 M NaX were equilibrated with a NB phase containing 0.02 M 1. The sodium distribution ratio was found to depend strongly on the nature of the anion in accord with Hofmeister bias in favor of large chargediffuse anions (Table 3). Sodium hydroxide and fluoride accordingly exhibited the weakest extraction among all sodium salts studied. The extraction of the sodium salts by 1 may be directly related to the standard Gibbs energies of anion partitioning ∆Gp°(X-). Application of the neutral extractant promotes ion-pair extraction, and the high polarity of the NB diluent makes it reasonable to assume complete dissociation of the ionic species in the organic phase.20 The extraction process can be described by the wellknown equilibrium for alkali metal salt extraction by many neutral crown ethers in polar diluents:20

The extraction constant can be expressed in terms of the standard Gibbs energy of ion partitioning (∆Gp°) between water and NB (eq 4), where Kf is the formation constant of Na‚1+ complex in

1(org) + Na

+

(aq) +

X

(aq) h

Na‚1

+

(org) +

X

(org)

(1)

Since NaX is the only electrolyte in the system, [Na+]aq ) [X-]aq (20) Moyer, B. A. In Molecular Recognition: Receptors for Cationic Guests; Gokel, G. W., Ed.; Comprehensive Supramolecular Chemistry; Atwood, J. L.; Davies, J. E. D.; MacNicol, D. D.; Vo¨gtle, F.; Lehn, J.-M., Eds.; Pergamon: Oxford, 1996; Vol. 1; pp 377-416.

408

Analytical Chemistry, Vol. 75, No. 3, February 1, 2003

Kex( ) [Na‚1+]org2y(,org2/{[1]orgy1[Na+]aq2y(,aq2} (2)

where brackets denote molarity, y( is the mean molar activity coefficient for aqueous or organic ionic species, and y1 is the molar activity coefficient of crown ether in the organic phase. The sodium distribution ratio DNa,1 ([Na‚1+]org/[Na+]aq) may then be written as

log DNa,1 ) 1 1 1 log Kex( + log(y1y(,aq2/y(,org2) + log[1]org (3) 2 2 2

log Kex( ) log Kf - [∆Gp°(Na+) + ∆Gp°(X-)]/2.303RT (4)

the water-saturated NB phase. Combination of eqs 3 and 4 results in

log DNa,1 ) 1 {log Kf - [∆Gp°(Na+) + ∆Gp°(X-)]/2.303RT} + 2 1 1 log(y1y(,aq2/y(,org2) + log[1]org (5) 2 2 It may be seen that when loading is low (i.e., [1]org = 0.02) and the logarithm of the ratio of activity coefficients [log(y1y(,aq2/ y(,org2)] is effectively constant among the extraction systems studied, the sodium distribution between phases depends only on the anion partitioning from water into NB. Under such conditions, eq 5 can be rewritten as

1 log DNa,1 = const - ∆Gp°(X-)/2.303RT 2

(6)

Table 4. Derived Values of the Formation Constant log Kf for Homogeneous Complexation of Na+ Ion in NB by 1 anion

DNa,1a

y(,aqb

y(,orgc

(y(,aq/ y(,org)2

∆Gp°(X-) kJ mol-1d

log Kfe

ClO4NO3BrClFOH-

2.01 × 10-2 9.14 × 10-3 3.93 × 10-3 6.21 × 10-4 5.51 × 10-5 1.35 × 10-4

0.667 0.560 0.672 0.668 0.572 0.667

0.346 0.489 0.625 0.830 0.946 0.917

3.72 1.31 1.16 0.65 0.37 0.53

-2.2 16.3 21.5 29.8 37.6 36.0

6.62 6.67 6.70 6.20 6.54

a Taken from Table 3. b Calculated using the Pitzer treatment.22 Calculated using the Debye-Hu ¨ ckel treatment.23 d Values for all anions except OH- taken from ref 15, which employs the tetraphenylarsonium tetraphenylborate (TATB) extrathermodynamic convention. The value of ∆Gp°(OH-) is an estimate given in the Appendix. e Calculated using eq 5. The value of ∆G °(Na+) was taken as 34.2.21 p

c

Although the conditions for linearity following eq 6 were not fully met (see below), a reasonable plot of log DNa,1 versus ∆Gp°(X-)/2.303RT was obtained, as shown in Figure 2. In the extraction of sodium perchlorate, high loading of crown ether was observed, which resulted in a corresponding decrease in DNa,1. A loading-corrected value of DNa,1 for perchlorate (open symbol) was calculated based on the estimated concentration of free extractant in the organic phase. A linear plot with the slope of -0.56 ( 0.04 was obtained, close to the value (-0.5) predicted by eq 6. Agreement of the observed extraction behavior with eq 6 suggests that the extraction mechanism described by eq 1 is predominant for all sodium salts, including sodium hydroxide. Anion selectivity is thus controlled by solvation and is independent of the host and its sodium complex in this system.20 Moreover, the good agreement observed for the NaOH distribution datum with the plotted data for other sodium salts supports the estimated ∆Gp°(OH-) value of 36 ( 1 kJ mol-1 (see Appendix). To further validate the internal consistency of the above treatment of the extraction experiment, the formation constant of Na‚1+ complex in the water-saturated NB (log Kf) was derived via eq 5 using distribution data obtained for each sodium-anion pair except perchlorate. For this calculation, values of ∆Gp°(Na+)21 and ∆Gp°(X-)15 were taken from the literature, and the values of the mean activity coefficients y(,aq and y(,org were estimated using Pitzer22 and Debye-Hu¨ckel23 treatments, respectively; the value of the activity coefficient of 1, y1, was assumed to be equal to unity in accord with the low concentration of 1. The results are shown in Table 4, demonstrating good internal consistency among the values of log Kf corresponding to the different anions studied. An average value of 6.7 ( 0.4 was obtained for nitrate, bromide, and chloride, considered to be the most reliable data. The uncertainty of this value reflects the uncertainty of ∆Gp°(X-), taken as (2 kJ mol-1.3 It may be noted that even fluoride and hydroxide, having very low distribution ratios, gave comparable values of log Kf. The value 6.7, however, is in only fair agreement with the value (21) Rais, J. Collect. Czech. Chem. Commun. 1971, 36, 3253-3262. (22) Pitzer, K. S. In Activity Coefficients in Electrolyte Solutions; Pitzer, K. S., Ed.; CRC Press: Boca Raton, FL, 1991; Chapter 3. (23) Robinson, R. A.; Stokes, R. H. Electrolyte Solutions, Academic Press: New York, 1955.

of 7.8 reported by Makrlik24 for a mixture of isomers of dicyclohexano-18-crown-6. NaX Extraction by Hydroxy Acids. In accord with the unfavorable cation partitioning into NB, negligible extraction of Na+ was observed by HA used alone. Distribution experiments were performed for each sodium salt using a NB solution of 0.04 M HA. The presence of a lipophilic weak hydroxy acid in the NB phase introduces a potential cation-exchange mechanism exclusively for hydroxide ion (case 6). When 1-octanol, which solvates both cations and anions well, is used as the diluent, pseudohydroxide extraction by 4-7 is strong and appears to follow the simple equilibrium7,19,25

HA(org) + Na+(aq) + OH-(aq) h A-(org) + Na+(org) + H2O(aq) (7)

In NB, however, only the most acidic hydroxy acids, 3,5-ditert-butylphenol (6) and fluorinated benzyl alcohols 7a and 7b, extracted sodium ion from aqueous NaOH solution, though barely detectably [6((5) × 10-5 for 6 and 7((1) × 10-5 for 7a]. Under neutral conditions (X- * OH-), interaction of HA with any of the anions was not strong enough to permit salt extraction in the absence of the cation host. No detectable extraction of NaX (where X- ) NO3-, Br-, Cl-, or F-) was found. Values of DNa observed for NaClO4 fell within the small range (8-14) × 10-5, essentially no change from that observed for NB alone (9 × 10-5). Values of DNa for NB alone and of DNa,HA are reported in Table S-1 in the Supporting Information. NaX Extraction by a Combination of Crown Ether and Hydroxy Acid. As demonstrated by distribution experiments described above using neutral host 1 alone (case 2), only marginal quantities of NaOH can be extracted via an ion-pair extraction mechanism. In our previous work,8 synergistic sodium pseudohydroxide extraction (case 7) was demonstrated using a protonionizable fluorinated alcohol 5 and cage-annulated crown ethers in NB. For the present study, hydroxy acids in a wide pKa range were chosen (Chart 1 and Table 2). It should be noted that reported pKa values refer to homogeneous aqueous solution and are useful here only to indicate relative HA acidity. To investigate the effect of hydroxy acid structure on sodium ion extraction by 1, the NB phases containing combinations of 1 and HA at 0.02 and 0.04 M concentrations, respectively, were tested (Table 3). Figure 3 compares the distribution behavior of Na+ ion observed for each combination of 1 and HA with the corresponding Na+ distribution using 1 alone. A data point falls on the diagonal line when sodium distribution values are the same with and without HA. Data points above the diagonal line indicate synergistic enhancement of the sodium extraction by the cooperative effect of the hydroxy acid and the crown ether. Extraction of Na+ with anions other than OH- ion exhibited moderate to no enhancement upon addition of HA into the extraction solvent, whereas significant enhancement of NaOH extraction occurred, presumably due to initiation of a biphasic acid-base reaction. The weak synergism and exhibition of the Hofmeister order in (24) Makrlik, E.; Vanura, P. J. Radioanal. Nucl. Chem. 1997, 223 (1-2), 229231. (25) Maya, L.; Moyer, B. A.; Lance, M. J. Appl. Spectrosc., in press.

Analytical Chemistry, Vol. 75, No. 3, February 1, 2003

409

ion concentration in the NB phase (0.0345 M). This value exceeds the organic concentration of the cation host 1 of 0.02 M by 73% and presumably indicates conversion of over 86% of 7b to its sodium salt. By contrast, the control 7b in NB gave negligible levels of sodium in the NB phase (0.000 26 M). It could be reasoned that the extracted sodium may exist both as a complex with 1 and as a second species not involving 1, as in Na‚1+ and A-‚Na+‚A-, for example. Some synergism may be expected for the extraction of the neutral NaX salts by combinations of 1 and HA based upon a hydrogen bond interaction between HA and the small anions, as described by the simplest model equilibrium given in eq 9. This

nHA(org) + 1(org) + Na+(aq) + X-(aq) h X‚(HA)n-(org) + Na‚1+(org) (9)

Figure 3. Comparison of Na+ ion distribution results obtained using crown ether 1 with and without hydroxy acids 2-7 in NB at 25 °C. Values of DNa,1 and DNa,1+HA for ClO4- and OH- taken from Table 3 were corrected for the feeble extraction by NB or by HA alone in NB. Aqueous phase: 1 M NaX (where X- ) ClO4-, NO3-, Br-, Cl-, F-, or OH-). Organic phase: 0.02 M 1 in NB or 0.02 M 1 + 0.04 M HA in NB. The phase ratio was 1:1.

extraction of all sodium salts other than NaOH supports the hypothesized cation-exchange process for NaOH, which in the simplest possible model may be expressed as:8

HA(org) + 1(org) + Na+(aq) + OH-(aq) h A-(org) + Na‚1+(org) + H2O(aq) (8) Accordingly, the synergism is expected to arise from the simultaneous cooperative action of 1 and HA. This model predicts that the synergistic enhancement depends on the properties of the hydroxy acid. The expected trend of 7 > 6 > 5 ∼ 4 > 2 in accord with the acidity of HA (Table 2) is clearly demonstrated. Synergistic factors (S ) DNa,1+HA/(DNa,1 + DNa,HA) ≈ DNa,1+HA/ DNa,1) for NaOH were 256, 78, 37, 37, and 1.2 for 7b, 6, 5, 4, and 2, respectively. The least acidic compound in the series, 1-octanol, exhibited only marginal change with respect to NaOH extraction by 1 alone, and therefore, no cation-exchange mechanism is anticipated in this case. More acidic fluorinated alcohols 4 and 5 and phenol 6 showed significant synergistic enhancement, indicating partial deprotonation of HA upon contact with 1 M NaOH. Extractions of sodium ion using 4 and 5 were nearly equal in strength, implying that the number and position of lipophilic alkyl groups on the benzene ring have little effect on the functionality of the remote hydroxy group. The greatest synergism was observed for the combination of 1 with the most acidic fluorinated benzyl alcohol 7b. The correlation between synergism of pseudohydroxide extraction and acidity of HA is consistent with our previous results on sodium pseudo-hydroxide extraction using solutions of hydroxy acids in 1-octanol.7 An interesting observation is that NaOH extraction by the combination of 1 and 7b resulted in an unexpectedly high sodium 410

Analytical Chemistry, Vol. 75, No. 3, February 1, 2003

interaction lowers the energy barrier of anion partitioning into the NB phase via an ion-pair extraction mechanism (variant of case 2). The equilibrium process in eq 9 predicts that synergistic enhancement of the NaX extraction depends on the stability of the anionic solvate formed in the NB phase and, consequently, on the hardness of the anion and on the hydrogen bond donor strength of HA. Numerous studies have demonstrated hydrogen bond formation between various halide anions and protogenic compounds, such as phenols, alcohols, and water. The charge density of an anion determines its strength as a proton acceptor, and the general trend F- > Cl- > Br- > I- is usually observed. Large charge-diffuse anions, such as picrate and perchlorate, have very small receptivity to the hydrogen bonding and interact relatively weakly with hydrogen bond donors.16b,g As may be seen in Figure 3 and Table 3, our results are consistent with the above observations. Combination of HA with 1 resulted in essentially no effect on the extraction of NaClO4. Extraction of the smaller anions was enhanced in the expected order F- > Cl- > Br- > NO3-. Thus, the synergism observed in ion-pair extraction by simultaneous utilization of cation host and HA in the organic phase can be described as anti-Hofmeister or favoring small hydrophilic anions. However, it is not strong enough to reverse the overall extraction dependence on anion size. Thus, F- remains the most weakly extracted anion, and the Hofmeister selectivity is manifested overall. Correlation between the acidity of the hydroxy acids and their ability to interact with anions has been previously described in the literature,16a,c,e the solvation power of the most acidic compounds being the strongest. In the experiments described herein, the same trend was observed and was most pronounced with fluoride. NaF extraction was synergistically enhanced by combined application of 1 and HA in the order of increasing HA acidity: 7a > 6 > 5 ) 4 > 3 > 2. Synergized pseudo-hydroxide extraction followed the same order, but the magnitude of the enhancement was much greater. Under neutral conditions, the highest synergistic factor observed for the most powerful combination (i.e., F- and 7a) was only 9, whereas the hydroxy acid 7b enhanced NaOH extraction by 1 by a factor of 256. Extraction selectivity for NaOH over NaX distribution may be evaluated by examination of the data in Table 3 (see also Figure S-1 in Supporting Information). Hofmeister bias dominates in ionpair extraction of NaOH by 1 alone or its combination with the

Table 5. Recovery of Extracted Sodium Hydroxide into Watera extractants 1+5 1+6

[NaOH]aq,extr, Mb

[Na+]org,extr, Mc

[Na+]aq,strip, Md

[OH-]aq,strip, Me

[OH-]strip/ [Na+]strip

strip NaOH, %

1 3 5 1 3 5

5.47 × 10-3 1.12 × 10-2 1.30 × 10-2 1.05 × 10-2 1.36 × 10-2 1.61 × 10-2

1.80 × 10-3 3.51 × 10-3 4.28 × 10-3 3.42 × 10-3 4.43 × 10-3 5.24 × 10-3

1.75 × 10-3 3.75 × 10-3 4.32 × 10-3 3.48 × 10-3 4.38 × 10-3 5.16 × 10-3

0.97 1.07 1.01 1.02 0.99 0.99

96 100 100 99 97 96

a All contacts performed at 25 °C. b Initial aqueous-phase NaOH molarity in the extraction step. c Sodium ion concentration in the NB phase determined by 22Na tracer analysis after equilibration at unit organic-to-aqueous (O/A) phase volume ratio in the extraction step. d Sodium ion concentration in the aqueous phase determined by 22Na tracer analysis after equilibration at an O/A ratio of 1/3 (volume of organic phase/volume of aqueous phase) in the stripping step. e Hydroxide ion concentration in the aqueous phase determined by acid-base titration after equilibration at an O/A ratio of 1/3 (volume of organic phase/volume of aqueous phase) in the stripping step.

least acidic hydroxy acid 1-octanol. When the cation-exchange mechanism is “turned on”, OH- selectivity over the F- anion greatly improves. Interestingly, the OH-/F- selectivity is fairly insensitive to the acidity of HA for 5-7, reflecting similar responses of the synergism in pseudo-hydroxide and ion-pair extractions to the acidity of the hydroxy acid. All cation exchangers exhibit OH- selectivity over Cl- anions. It may be noted for the combination of 1 and the most acidic hydroxy acid 7b that the synergistic pseudo-hydroxide extraction exceeded ion-pair extraction of nitrate, the chief anion in the majority of nuclear wastes, and even perchlorate. Regeneration of Cation Exchanger and NaOH Solution. Efficient regeneration of the extractant and recovery of sodium hydroxide by stripping with water was previously demonstrated for pseudo-hydroxide extraction.7,26 Likewise, for synergistic pseudo-hydroxide extraction, it is important to demonstrate whether the extracted hydroxide equivalents are recoverable as NaOH. Accordingly, a NB solution of 0.02 M 1 and 0.04 M 5 or 6 was equilibrated with an equal volume of aqueous 1, 3, or 5 M NaOH solution. In the stripping solution, Na+ ion and total base concentrations were determined by 22Na counting and titration techniques, respectively. The total base concentration corresponds to the concentration of hydroxide in the strip, provided that partitioning of alkoxide or phenoxide sodium salts to the stripping solution is negligible.7 As shown in Table 5, quantitative recovery of sodium hydroxide and, thus, recovery of the cation exchanger and 1 was demonstrated. Excellent agreement was observed between Na+ and OH- ion concentrations in the stripping phase. CONCLUSIONS This work demonstrates the cooperativity between the acidbase chemistry of weak hydroxy acids and cation recognition and how it may be exploited for the selective separation of NaOH from inorganic aqueous solutions. Used separately, a lipophilic hydroxy acid and a sodium ion host extract marginal if any NaOH. In the combined system, the cation host lowers the large energy barrier of sodium partitioning into the NB phase, synergizing cation exchange. The reverse reaction affords recovery of NaOH upon (26) Haverlock, T. J.; Bonnesen, P. V.; Brown, G. M.; Chambliss, C. K.; Levitskaia, T. G.; Moyer, B. A. In Proc. Int. Solvent Extr. Conf. (ISEC ’02); Cape Town, South Africa, March 17-21, 2002; Sole K. C., Cole, P. M., Preston, J. S., Robinson, D. J., Eds.; Chris van Rensburg Publications: Melville, South Africa, 2002, Vol. 1, pp 396-401.

stripping with water, whereby the organic cation exchanger is regenerated to its protonated form via hydrolysis. Weakly acidic lipophilic fluorinated alcohols and phenols were shown to be suitable candidates for synergistic pseudo-hydroxide extraction, as they afford efficient extraction and stripping of NaOH. Ionpair extraction of neutral sodium salts was not changed (NaClO4) or was at best moderately enhanced by addition of HA into the NB solution of 1. Selectivity for NaOH over other sodium salts including highly extractable nitrate and perchlorate is a significant finding, with potential analytical applications or even development of industrial separation processes. Although the experiments described herein have focused on sequential extraction and stripping in liquid-liquid systems, it is obvious that membrane transport or chromatographic variations of the same concept could be readily devised. Intensive programs in the U.S. Department of Energy complex to find solutions to the difficult problems related to treatment and disposal of alkaline high-level nuclear wastes stored in underground tanks at several sites27 have recently prompted investigations of the separation of sodium hydroxide from complex aqueous salt media.2,7 ACKNOWLEDGMENT This research was sponsored by the Environmental Management Science Program of the Offices of Science and Environmental Management, U.S. Department of Energy, under Contract DEAC05-00OR22725 with Oak Ridge National Laboratory, managed by UT-Battelle, LLC. The participation of T.G.L. was made possible by an appointment to the Oak Ridge National Laboratory Postgraduate Program administered by the Oak Ridge Associated Universities. SUPPORTING INFORMATION AVAILABLE Table giving sodium ion distribution ratios from aqueous solutions to NB with or without HA. Figure illustrating the (27) Bunker, B.; Virden, J.; Kuhn, B.; Quinn, R. Encylcopedia of Energy Technology and the Environment; Wiley: New York, 1995; pp 2023-2032. Gephart, R. E.; Lundgren, R. E. Hanford Tank Clean up: A Guide to Understanding the Technical Issues. Report PNL-10773, Pacific Northwest National Laboratory, Richland, WA, 1995. Nuclear Wastes: Technologies for Separations and Transmutation; National Research Council, National Academy Press: Washington, DC, 1996. Science and Technology for Disposal of Radioactive Tank Wastes; Schultz, W. W., Lombardo, N. J., Eds.; Plenum Press: New York, 1998. Nuclear Site Remediation: First Accomplishments of the Environmental Science Program; Eller, P. G., Heineman, W. R., Eds.; ACS Symposium Series 778; American Chemical Society: Washington, DC, 2001.

Analytical Chemistry, Vol. 75, No. 3, February 1, 2003

411

selectivity of hydroxide over other anions. This material is available free of charge via the Internet at http://pubs.acs.org. APPENDIX Estimation of the Standard Molar Gibbs Energy of Hydroxide Ion Partitioning between Water and Nitrobezene. For the purposes of rationalizing selectivity behavior, an estimate of the standard Gibbs energy of partitioning of hydroxide ion from water to NB was obtained. Figure 1 shows the dependence of the standard molar Gibbs energy of anion partitioning from water to NB ∆Gp°(X-) (TATB convention) versus the reciprocal thermochemical radius3 of the anion 1/r. The value for ∆Gp°(OH-), given by the open symbol in the plot, is unavailable in the literature and was estimated using an empirical exponential fitting. It is obvious that anion partitioning into NB becomes highly unfavorable as the anion size decreases. A nearly linear bias is observed for Cl- and larger anions, but ∆Gp°(X-) levels off for small anions such as F- and OH-. It is worth further discussion of the anion partitioning behavior shown in Figure 1, both to justify the estimate for ∆Gp°(OH-) and to shed light on anion selectivity in general. The proposed explanation of the leveling-off effect is extensive hydration of small charge-dense anions in the NB phase. As reported in earlier studies, the amount of water coextracted with R4N+X- salts into NB was highly dependent on the nature of the anion and correlated with the Hofmeister lyotropic series.28 The hydration ratio, defined as the number of moles of excess solubilized water per mole of R4N+X- in water-saturated NB, increased from 0.4 for ClO4- to 4.2 for F-. Although data are lacking for OH-, it would (28) Arnett, E. M.; Chawla, B.; Hornung, N. J. J. Solution Chem. 1977, 6 (12), 781-818.

412

Analytical Chemistry, Vol. 75, No. 3, February 1, 2003

be expected to have a hydration ratio similar to that of F-. This implies that during the partitioning process large hydrophobic anions nearly completely lose their first hydration shell, which is substituted by the solvation shell in the organic solvent. The smaller anions, being more strongly hydrated than solvated by common water-immiscible solvents, partition in their hydrated form, and the actual radius of the small anion transported into the NB phase becomes r + ∆r, where ∆r is the thickness of the hydration shell. For Cl- and smaller univalent anions, r , ∆r, and thus, ∆r effectively compensates for the reduced anion radius and determines the work done for the cavity formation in the solvent in order to accommodate an anion. The Gibbs energy of anion partitioning thus becomes almost insensitive to the anion size and levels off when plotted versus 1/r. As demonstrated previously, hydration of an anion in the water-saturated diluent accounts for the difference between the Gibbs energy of anion partitioning and the Gibbs energy of anion transfer.29 The latter refers to the hypothetical extraction system wherein mutual saturation between water and organic diluent does not occur. Legitimacy of the estimated ∆Gp°(OH-) value for NB was further demonstrated in this work by liquid-liquid distribution experiments of sodium salts employing a cation host; namely, the linear relationship in Figure 2 is expected and was observed.

Received for review July 8, 2002. Accepted December 3, 2002. AC0259212 (29) Ivanov, I. M.; Zaitsev, V. P. Anion exchange, hydration, and solvation of anions in nonaqueous solutions. Proc. Int. Solvent Extr. Conf. (ISEC ’80); 1980; Paper 80-54.