Diverse Secondary Interactions Between Ions Exchanged Into the Resin Phase and Their Analytical Applications

ANALYTICAL SCIENCES JANUARY 2014, VOL. 30 51 2014 © The Japan Society for Analytical Chemistry

Reviews Diverse Secondary Interactions between Ions Exchanged into the Resin Phase and Their Analytical Applications

Akio YUCHI

Graduate School of Engineering, Nagoya Institute of Technology, Gokiso, Showa, Nagoya 466–8555, Japan

The research activities by the author’s group to elucidate the chemical states of ions within the ion exchange resin phase are summarized. The resin with the higher exchange capacity has the smaller space available for ion exchange, and the higher cross linking degree interferes more with swelling of the resin. As a result, diverse secondary interactions between exchanged ions are observed on the resins of high exchange capacities and high cross linking degrees: the van der Waals contact results in incomplete exchange or enhanced dehydration of ions, hydrogen bond formation between acidic anions, and coadsorption of anions with metal ions. Contribution of the simple ion exchange mechanism to the reactions of iminodiactate-type chelating resins with metal ions in the acidic media is quantitatively discussed. The resulting complexes were successfully applied to preconcentration and separation of anions.

Keywords Ion exchange resin, hydration state, coadsorption, hydrogen bond, preconcentration, separation

(Received August 20, 2013; Accepted October 23, 2013; Published January 10, 2014)

1 Introduction 51 6 Hydration States of Multivalent Cations 2 Background Information 52 and Their Coadsorption with Anions 55 2·1 Resins 7 Reaction of IDA Resin and Gel with 2·2 Ions Excess M(II), M(III), and M(IV) 55 3 Incomplete Exchange by Bulky Ammonium 8 Preconcentration and Chromatography of Ions Due to van der Waals Contact 53 Anions on Chemically Immobilized Zr(IV) 56 4 Enhanced Dehydration of Strongly Hydrated 9 Conclusions 56 Ions Due to van der Waals Contact 53 10 Acknowledgements 57 5 Enhanced Dehydration and Intermolecular 11 References 57 – Hydrogen Bond Formation of H2PO4 on AXRs 54

interaction between ions and exchange groups with 1 Introduction electroneutrality unchanged, as in the case of liquid–liquid ion exchange. The selectivity was conventionally correlated first to The chemistry of ion exchange resins had been intensively the electric charge and second to the radius of the hydrated ion studied since their development in the 1950s, and an elaborate among the ions of the same charge. The formal concentration review was published by Helfferich as early as 1962.1 The of the ions in the resin (3 – 4 mol L–1) is, however, much higher methodology was sophisticated to ion chromatography.2 The than the concentration of the ions in the organic solvents in the primary driving force in these technologies is the electrostatic liquid–liquid ion exchange, so that various secondary interactions are expected between exchanged ions in the resin phase to modify the selectivity. The chemical states of ions within the Akio YUCHI received his Ph.D. degree in 1981 at Nagoya University. He worked at resin phase as products had, however, not been evaluated for a Nagoya Institute of Technology as long time. Research Associate (1981 – 1990), as Recently, a variety of instrumental analyses have been applied Lecturer (1990 – 1992), as Associate to evaluate the chemical states of ions in the resin phase, mainly Professor (1992 – 2003), and as Professor 3–20 (2003 –). His current research interest is by Japanese researchers, and the ion-exchange phenomena the separation and detection of ions based have been discussed at the molecular level. In contrast, we on phase transfer and chemical reactions. prepared resins of low exchange capacities which reduce secondary interactions; simple estimation suggests a larger volume available for ion exchange in such resins. Their performances were compared with those of conventional resins, so as to highlight the diverse secondary interactions shown in E-mail: [email protected] Scheme 1.21–24 In addition, the earlier studies on the reactions of 52 ANALYTICAL SCIENCES JANUARY 2014, VOL. 30

Scheme 1 Schematic illustrations of ions in the resin phase. (a) van der Waals contact between bulky QAs resulting in incomplete exchange (b) Chemical states of Na+ on CXRs of different exchange – capacities (c) Chemical states of H2PO4 on AXRs of different exchange capacities (d) Chemical states of Fe3+ on CXR of high exchange capacity (e) Coadsorption of OH– with Fe3+ on CXR of low exchange capacity (f) Coadsorption of O2– with Fe3+ on CXR of high exchange capacity. The circle connected with the bar indicates CXR or AXR, while the circle with or without the shell denotes the hydrated or naked cation or anion, which occasionally includes even the second hydration shell.

Table 1 Properties of resins and ions

EC Void rvoid r Resin Remark Ion mmol g–1 Å3 Å Å

Cation-exchange resin Alkali cationa

CG-4.6×4 4.55 Not available Amberlyst 252 Li+ 3.4 CG-4×2 3.95 320 4.0 Dowex 50W Na+ 3.8 CG-4×4 4.00 290 4.1 Dowex 50W K+ 4.2 CG-4×8 4.07 260 4.2 Dowex 50W Rb+ 4.3 CG-3.7×8 3.65 290 4.1 Amberlite IR120B Cs+ 4.5 CG-1×2 1.29 1500 7.1 Prepared CG-1 5 0.96 Not available Prepared × Quaternary ammonium ionb CP-4×12 3.91 280 4.0 Amberlite 252 + Chelating resin TMA 3.2 TEA+ 3.9 IP-2 2.10 Not available Amberlite IRC-718 TPA+ 4.4 IG-0.03 0.03 Not available Toyopearl 650M Anion-exchange resin Aniona

AG-3.5×2 3.45 410 4.6 Dowex I F– 4.1 AG-3.5×4 3.48 410 4.6 Dowex I Cl– 4.6 AG-3.5×8 3.46 410 4.6 Dowex I Br– 4.8 – AG-1.5×1 1.45 1350 6.9 Prepared H2PO4 5.3 AG-0.9×2 0.87 2500 8.4 Prepared a. Radius of hydrated ion. b. Radius of naked ion.

chelating resins with metal ions, which inspired the idea of 2 Background Information volume available for ion exchange,25–29 and the analytical applications of the metal complexes with a chelating gel as the composite material for preconcentration and separation of 2·1 Resins anions are also briefly reviewed.30–35 The properties of the resins used are summarized in Table 1. ANALYTICAL SCIENCES JANUARY 2014, VOL. 30 53

Fig. 1 Adsorption isotherms of QAs on CXRs. CXR: (a) CG-1×2, (b) CG-4×2, (c) CG-4×8. QA: ●, TMA+; ▲, TEA+; ■, TPA+. Solid lines show calculated Langmuir isotherms, while dotted horizontal lines show saturated exchange capacities.

The first letter of the abbreviated name indicates whether it is a cationic (C), chelating (I), or anionic (A) resin, the second letter specifies whether it is a gel (G) or porous (P), the first digit shows the approximate exchange capacity (EC), and the second digit the cross-linking degree (CL), if available. The cation exchange resins (CXRs) of low EC were synthesized by polymerization,24 while the anion exchange resins (AXRs) of low EC were derived from the Merrifield resin.22 The volumes available for ions in the resins were estimated as follows: the volume of one functional group associated with a certain ion could be calculated from the exchange capacity and the density of the dried resin, while the volume of the same functional group without ion could be estimated based on the additivity of molar volumes. The difference between these volumes, called void, allows for the space for ions. On the assumption that the entire volume is effectively used, the maximum radius of the spherical species occupying this space in the dry state, rvoid, is estimated as shown in Table 1; all these numbers indicate the radius based on only the additivity without considering the effect of cross-linking. The radius decreases to Fig. 2 Hydration numbers of alkali metal ions on CXRs and in 60% with an increase in capacity by 4 times. Some attempts to water. ◆, nM on CG-1×2; ▲, nM on CG-4×2; ■, nM on CG-4×8; ●, n . M+: Li+, Na+, K+, Rb+, Cs+ in the increasing order of r . experimentally determine the space available for ion exchange M,aq(D) hyd are now in progress in our laboratory.

2·2 Ions The properties of the ions used are also summarized in the mean diameters of micropores in CXRs suspended in water Table 1. The radius of each of the hydrated alkali metal ions (343 Å at divinylbenzene content of 1%; 151 Å at 2%; 58 Å at and anions in water was simply estimated by the sum of the 4%; 30 Å at 8%; 15 Å at 16%) was much larger than twice the ionic radius and twice the van der Waals radius of oxygen ionic radius of QAs (Table 1). A comparison of the ionic radii (2.8 Å) to a precision of 0.1 Å. The ionic radius of quaternary of these ions with the radius of the void of each resin indicates ammonium ion (QA) was calculated from the partial molar that the possible van der Waals contact between bulky QAs at volume at infinite dilution. As a rough image, the radii of high loading into the resin phase rather interferes with the hydrated alkali metal ions and naked QAs cover a comparable quantitative exchange (Scheme 1(a)). range of 3.2 – 4.5 Å, while the radii of hydrated anions are appreciably larger than those of cations. 4 Enhanced Dehydration of Strongly Hydrated Ions Due to van der Waals Contact 3 Incomplete Exchange by Bulky Ammonium Ions Due to van der Waals Contact The hydration number (nM) was determined for alkali metal ions (M+) quantitatively exchanged on the same three CXRs at the The exchange equilibria of K+ on CXRs (CG-1×2, 4×2, 4×8) by relative humidity of 50% and is plotted against the radius of the + + + 21 QAs (TMA , tetramethyl-; TEA , tetraethyl-; TPA , tetrapropyl- hydrated ion (rhyd) in Fig. 2. At this humidity, only strongly 21 ammonium ion) were studied (Fig. 1). Quantitative exchange bound water molecules remained. The nM values of weakly was observed for all QA on CG-1×2 but only for TMA+ on hydrating ions like K+, Rb+, and Cs+ were around 2 on all resins. CG-4×2 and on CG-4×8. The maximum exchange reached 98% These ions are extremely dehydrated (nM compared with the + + + for TEA and 92% for TPA on CG-4×2, and 82% for TEA and hydration number in water, nM,aq(D), determined by diffraction 60% for TPA+ on CG-4×8. Although such phenomena found as or other methods) and may form contact ion-pairs with early as the 1950 s were attributed to the size-exclusion effect, functional groups. In contrast, the nM values of strongly 54 ANALYTICAL SCIENCES JANUARY 2014, VOL. 30

Fig. 3 Effects of exchange capacity (a) and cross linking by DVB (b) on the hydration numbers of anions. (a) Cross linking by 1 – 2% DVB, (b) exchange capacity of 3.5 mmol g–1. Anion: ◆, F–; ●, Cl–; – – ■, Br-; ▲, ClO4 ; ○, H2PO4 .

Fig. 5 Hydration numbers of metal ions on CP-4×12 plotted against the radius of hydrated ion. (●) monovalent ions: Li+, Na+, K+, Rb+, Cs+ (from the left to the right); (■) divalent ions: Be2+, Mg2+, Zn2+, Cd2+, Ca2+, Sr2+, Ba2+; (◆) trivalent ions: Al3+, Ga3+, Sc3+, In3+, Er3+, Nd3+.

6.6 to 5.6, those of Cl– from 3.4 to 2.4, and those of Br– from 2.5 to 1.7 with increases in EC and CL. This also indicates that Fig. 4 Effects of % exchange on selectivity coefficients of phosphate the possible van der Waals contact between hydrated anions at high loading enhances dehydration. on AG-0.9×2 (a) and AG-3.5×8 (b). ■, log K11; □, log K21.

5 Enhanced Dehydration and Intermolecular – hydrating ions decreased with increases in EC and CL: 3.8 on Hydrogen Bond Formation of H2PO4 on AXRs CG-1×2, 3.6 on CG-4×2, and 3.0 on CG-4×8 for Li+; and 3.6 on CG-1×2, 2.8 on CG-4×2, and 2.5 on CG-4×8 for Na+. These The ion-exchange equilibria of Br– on AG-0.9×2 and AG-3.5×8 ions keep their hydration numbers in water and may form by phosphate were analyzed by taking into consideration the separated ion-pairs on the resins of low EC and of low CL, contribution of 1:1 and 1:2 species.23 The selectivity coefficients while the possible van der Waals contact between hydrated ions of 1:1 species (K11) were constant on AG-0.9×2 but increased at at high loading enhances dehydration on the resins of high EC a % exchange higher than 70 on AG-3.5×8 by about unity in and high CL (Scheme 1(b)). logarithmic scale (Fig. 4). This indicates the cooperation – – – The hydration numbers (nX) of singly charged anions (X ) between H2PO4 . The hydration number of H2PO4 on quantitatively exchanged on five AXRs (AG-3.5×8, ×4, ×2, AG-0.9×2 of the largest void was as large as 6.1. The number, AG-1.5×1, AG-0.9×2) at the relative humidity of 50% are shown however, decreased with increases in EC and CL and was as low 22 22 as the function of EC and CL in Fig. 3. Similarly, the nX as 2.4 on AG-3.5×8 as shown in Fig. 3. The infrared spectra – values of weakly hydrating anions were independent of the resin of the H2PO4 -form resins indicate the intermolecular hydrogen – – (e.g., around 0.9 for ClO4 ), while those of F decreased from bonds only at high loading into AG-3.5×8. Thus, high loading ANALYTICAL SCIENCES JANUARY 2014, VOL. 30 55

Fig. 7 Effects of pH on adsorption capacities of Fe3+ (a) and La3+ (b). Aqueous solution, 100 mL containing 0.10 mmol of metal ion and 0.01 mol L–1 KBr; resin, 0.025 g of IP-2 containing 0.05 mmol of IDA groups. Solid curve was calculated using the constants obtained. The other two curves indicate the contribution of [(-LH)3M] and [(-L)(-LH)M], respectively. Horizontal dashed lines correspond to 100, 50, and, if present, 33% occupation of IDA groups, respectively.

Fig. 6 Adsorption capacities of Fe3+ (●) and phosphate (■) on CXRs. CXR: (a) CP-4×12, (b) CG-1×5. Horizontal lines in (a) correspond to 50, 33, and 25% of exchange capacity, while that in (b) to 33%. exchange groups, an increase in adsorption of metal ions accompanied by adsorption of these anions was observed as shown in Fig. 6(a). This indicates coadsorption of anions in spite of the same electric charge with the functional group.24 – of H2PO4 into the resin of high EC and high CL induces Coadsorption of hydroxide was observed even on the resin of formation of the intermolecular hydrogen bonds accompanied the lower EC, CG-1×5, but phosphate simply interfered with the by dehydration so as to avoid the possible van der Waals contact exchange as a masking reagent (Fig. 6(b)). The Mössbauer between hydrated ions (Table 1, Scheme 1(c)). Upon heating, spectra of the Fe3+-OH–-type resins indicated the presence of facile intermolecular dehydration giving diphosphate was two species on CP-4×12 or CG-4.6×4, but one of these species observed only on this resin. prevailed on CG-1×5. The common species was assigned to – [(-S)2Fe-OH] (-S : cation exchange group), while the species found only on the resin of high EC to [(-S)2Fe-O-Fe(S-)2] 6 Hydration States of Multivalent Cations and (Scheme 1(e, f)). The coadsorption in the presence of P(V) was Their Coadsorption with Anions attributed to [(-S)2Fe-(HPO4)-Fe(S-)2]. These two species can exist only on the CXRs of high EC, which guarantees the close The hydration numbers of multivalent ions on CP-4×12 were placement of metal ions to be bridged by anions like O2– or 2– determined and are shown as the function of the radius of HPO4 . 21 hydrated ions in Fig. 5. The nM values of monovalent ions were substantially equal to those on CG-4×8 (Fig. 2) and were independent of the resin type whether gel or porous. The nM 7 Reaction of IDA Resin and Gel with Excess values of divalent ions decreased from 8 to 6, and those of M(II), M(III), and M(IV) trivalent metal ions from 12 to 8 with an increase in rhyd. Because the spaces available for these ions are formally twice or The effect of pH on the reaction of iminodiacetate-type chelating 3+ three times larger than those for monovalent ions and because resin (IP-2, -LH2) with trivalent metal ions (M ) under the the hydration of these ions is much stronger than that of conditions of metal ions in excess against the functional group monovalent ions, the divalent and trivalent metal ions are fully is given in Fig. 7.25 Trivalent iron as well as Al3+, Ga3+, and In3+ hydrated or occasionally hold even a part of the second hydration formed only the 2:1 species according to Eq. (1) as shown in shell in the resin phase and form separated ion pairs with two or Fig. 7(a). three exchange groups (Scheme 1(d)). 3+ + When the increasing amounts of certain anions (P(V), P(III), M + 2(-LH2)  [(-L)(-LH)M] + 3 H (1) P(I), Se(IV), OH–) were present in the exchange of trivalent metal ions (M3+: Fe3+, Al3+, Ga3+, In3+, Sc3+) into CP-4×12 or In contrast, Sc3+, Y3+, and La3+ formed not only the 2:1 species CG-4.6×4 under the conditions of metal ions in excess against but the 3:1 species given by Eq. (2) at the lower pH as shown in 56 ANALYTICAL SCIENCES JANUARY 2014, VOL. 30

Fig. 9 Effects of pH on adsorption of phosphate. Batchwise system – – –4 –1 Fig. 8 Average number of F (◆) and H2PO4 (■) bound to Zr(IV) (○) 10 mL of 10 mol L phosphate solution, 30 μmol of Zr-IG-0.03. – – –4 –1 immobilzed on IG-0.03. pH: 1.7 for F ; 1.7 – 3.0 for H2PO4 . Flow system (●) 0.01 mL of 10 mol L phosphate solution, 3 μmol of Zr-IG-0.03.

Fig. 7(b). + – [(-L)Zr(OH)2] + H + Hn-1A  [(-L)Zr(OH)(Hn-1A)] (7) 3+ + M + 3(-LH2)  [(-LH)3M] + 3 H (2) + – [(-L)Zr(OH)(Hn-1A)] + H + Hn-1A  – As a result, these metal ions are adsorbed in the pH range, [(-L)Zr(Hn-1A)2] (only for F ) (8) which is lower by 1.5 than that expected provided only + – [(-L)(-LH)M] is formed. [(-L)Zr(OH)2] + K + Hn-1A  – + – Similar behaviors were observed for divalent metal ions [(-L)Zr(OH)2(Hn-1A)] ,K (only for F ) (9) (M2+).26 Common metal ions formed only the 1:1 species given by Eq. (3), while Ca2+ formed not only the 1:1 species but the At high pH, on the other hand, the complex showed negligible 2:1 species in the acidic media as given by Eq. (4). affinities for anions, due to the acid dissociation reaction given by Eq. (10).29 2+ + M + (-LH2)  [(-L)M] + 2 H (3) + – – + [(-L)Zr(OH)2] + K + OH  [(-L)Zr(OH)3] ,K (10) 2+ + M + 2(-LH2)  [(-LH)2M] + 2 H (4) Thus, these anions could be adsorbed in the acidic media and The hydration number of 3.3 for [(-L)Ca] indicates that Ca2+ is reversibly desorbed simply by an increase in pH; e.g. Fig. 9 for 32,33 coordinated by terdentate IDA, while that of 5.9 for [(-LH)2Ca] phosphate. The % adsorption of these anions decreased with indicates that Ca2+ simply forms ion pairs with keeping the a decrease in anion concentration, e.g., F– recovery was 98% at hydration sphere.27 10–4, 81% at 10–5, 42% at 10–6 mol L–1. Utilization of the flow The reaction of IP-2 with Zr4+ shows formation of the 2:1 system, however, showed satisfactory recovery even at lower species as given by Eq. (5),28 while that of IG-0.03 shows concentrations. Fluoride in high purity NaCl (24 ng g–1 as NaF formation of the 1:1 species as given by Eq. (6).29 in 99.99% NaCl) and phosphate at sub-ng mL–1 in environmental water were successfully determined in the flow system 4+ + Zr + 2(-LH2)  [(-L)2Zr] + 4 H (5) containing this composite material for separation and preconcentration.32,33 4+ + Zr + (-LH2)  [(-L)Zr(OH)2] + 4 H (6) The same type of gel was also applied to the ligand exchange chromatography of organic anions such as benzoate derivatives The different behaviors are attributed to the distance between in immobilized Zr(IV) affinity chromatography.34 Furthermore, IDA groups, although the difference in polymer matrix (porous the Zr(IV) complex with tetraphenylporphine was successfully polystyrene resin in contrast to polyvinyl gel) may also be a used as a carrier or a mobile anion exchanger in the PVC contributing factor. The reactions of IDA resins having the membrane for potentiometry of citrate.35 lower EC with divalent and trivalent metal ions are now under investigation. 9 Conclusions 8 Preconcentration and Chromatography of Diverse interactions between ions exchanged into the common Anions on Chemically Immobilized Zr(IV) ion exchange resins of higher exchange capacities are indicated by comparing their performance with those of lower exchange Figure 8 shows that the 1:1 complex of Zr(IV) with IG-0.03 has capacities. Solid phases of further low exchange capacities – – – – high affinities for several anions (A : F , H2PO4 , H2AsO4 , eliminate such interactions and emphasize the interaction with – HSeO3 ) in the acidic media by the ligand substitution reactions the polymer matrix in ion chromatography to give high given by Eqs. (7) – (9),30 as in the case of Zr(IV)- selectivities. It is hoped the findings in this review, such as aminopolycarboxylate complexes in aqueous solutions.31 facile condensation of dihydrogen phosphate or coadsorption of ANALYTICAL SCIENCES JANUARY 2014, VOL. 30 57 anions with cations, will promote new applications of common 16. M. Shibukawa, T. Shimasaki, S. Saito, and T. Yarita, Anal. ion-exchange resins. Chem., 2009, 81, 8025. 17. M. Shibukawa, A. Taguchi, Y. Suzuki, K. Saitoh, T. Hiaki, and T. Yarita, Analyst, 2012, 137, 3154. 10 Acknowledgements 18. M. Shibukawa, M. Harada, T. Okada, Y. Ogiyama, T. Shimasaki, Y. Kondo, A. Inoue, and S. Saito, RSC Adv., The author would like to thank all the coworkers involved in this 2012, 2, 8985. research project. This work was supported by a Grant-in-Aid 19. S. Hirawa, T. Masudo, and T. Okada, Anal. Chem., 2007, for Scientific Research from the Ministry of Education, Culture, 79, 3003. and Technology, Japan (No. 23550093). 20. T. Kanazaki, S. Hirawa, M. Harada, and T. Okada, Anal. 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