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.