Anal. Chem. 2007, 79, 3003-3007
Correspondence
Acoustic Recognition of Counterions in Ion-Exchange Resins Shungo Hirawa, Takashi Masudo, and Tetsuo Okada*
Department of Chemistry, Tokyo Institute of Technology, Meguro-ku, Tokyo 152-8551, Japan
Ion-exchange equilibria are usually evaluated through the determination of ions in the solution phase by an appropriate method such as spectrometry and titration.1,2 In these measurements, a single determination of a selectivity coefficient takes a relatively long time, ranging typically from some tens of minutes to several hours. Such conventional routine has restricted studies on the kinetic or dynamic aspects involved in ion-exchange reactions, and thus, the development of in situ measurements has been required for the design of new materials and the further understanding of ion-exchange phenomena. Kim et al.3-5 reported a laser trapping spectroscopic method for studying ion-exchange processes with dye molecules as fluorescent probes and evaluated their diffusion nature and hydration dynamics in the interior of an ion-exchange resin particle. Although this smart approach provided the information that could not be obtained by classical
chemical analyses, some weak points can also be pointed out; i.e., (1) this in situ method is inapplicable to nonfluorescent ions with plenty of ion-exchange data, (2) spectroscopic probe molecules, e.g., fluorescent dyes, are in general poorly hydrated and therefore strongly adsorbed on ion-exchange resins in water, and (3) the adsorption of such molecular probes may play decisive roles in the entire process, and an ion-exchange mechanism possibly makes a minor contribution. A novel approach that allows in situ evaluation of ion exchange of a wider variety of ions is thus required to interface compiled knowledge with the findings obtained by modern technologies. Various physical forces have been utilized for particle separation, e.g., as external fields in field flow fractionation (FFF).6-11 An important feature common to physical fields is that the forces are functions of the size of a particle. This can be an advantage but simultaneously a disadvantage. Size selectivity of a physical force has actually facilitated the developments of size separation of particles; FFF is a typical example of successful applications of this nature of a physical force. In contrast, size-sorting ability of a physical field often results in undesired separation. Suppose a sample containing particles composed of various materials with various sizes. Particles having different sizes undergo different force in a physical field, even though they are chemically identical. Those with different chemical compositions also experience different force, resulting in very complex separation. The sizesorting ability of the physical field thus causes unnecessary complications, when it is applied to a sample containing particles having various properties. The inability to discriminate a particular property of a sample is thus important to gain information of higher quality from particle separation. We proposed a coupled acoustic-gravity field and showed that the aggregation coordinates of a particle therein is not affected by its size.12-14 Also, we
* To whom correspondence should be addressed. Phone and Fax: +81-35734-2612. E-mail:
[email protected]. (1) New developments in ion exchange: materials, fundamentals, and applications; Proceedings of the International Conference on Ion Exchange, ICIE ’91, Tokyo, Japan, October 2-4, 1991, Abe, M., Kataoka, T., Suzuki, T., Eds.; Kodansha: Tokyo, 1991. (2) Ion exchange processes: Advances and applications; Dyer, A., Hudson, M. J., Williams, P. A., Eds.; Royal Society of Chemistry: Cambridge, 1993. (3) Kim, H.-B.; Hayashi, M.; Nakatani, K.; Kitamura, N.; Sasaki, K.; Hotta, J.; Masuhara, H. Anal. Chem. 1996, 68, 409. (4) Kim, H.-B.; Habuchi, S.; Hayashi, M.; Kitamura, N. Anal. Chem. 1998, 70, 105. (5) Habuchi, S.; Kim, H.-B.; Kitamura, N. Anal. Chem. 2001, 73, 366.
(6) Pasti, L.; Bedani, F.; Contado, C.; Mingozzi, I.; Dondi, F. Anal. Chem. 2004, 76, 6665. (7) Chen, B.; Jiang, H.; Zhu, Y.; Cammers, A.; Selegue, J. P. J. Am. Chem. Soc. 2005, 127, 4166. (8) Watarai, H.; Hideaki Monjushiro, H.; Tsukahara, S.; Suwa, M.; Iiguni, Y. Anal. Sci. 2004, 20, 423. (9) Shintani, T.; Torimura, M.; Sato, H.; Tao, H.; Manabe, T. Anal. Sci. 2005, 21, 57. (10) Gale, B. K.; Caldwell, K. D.; Frazier, A. B. Anal. Chem. 2001, 73, 2345. (11) Masudo, T.; Okada, T. J. Chromatogr., A 2006, 1106, 196. (12) Masudo, T.; Okada, T. Anal. Chem. 2001, 73, 3467. (13) Masudo, T.; Okada, T. Anal. Sci. 2001, 17 (Suppl), i1341. (14) Masudo, T.; Okada, T. Curr. Anal. Chem. 2006, 2, 213.
In a coupled acoustic-gravity field, particles are aggregated at a particular position determined by their acoustic properties, such as density and compressibility. The observation of the aggregation of cation-exchange resin particles in this field has confirmed that the types of countercations strongly affect the behavior of the particles. Detailed analyses have revealed that the different behavior comes not from the density of the resins but from their compressibility as far as H+- and NBu4+-form cationexchange resins are concerned. Since these resins are aggregated at the different positions, we can gain information of counterions from aggregation coordinates. Counterions can thus be recognized through the observation of the aggregation behavior of resin particles in the field without chemical analyses routinely employed in ionexchange studies.
10.1021/ac062030b CCC: $37.00 Published on Web 03/02/2007
© 2007 American Chemical Society
Analytical Chemistry, Vol. 79, No. 7, April 1, 2007 3003
Figure 1. Experimental setup and design of the (1) cell, (2) function generator, (3) power amplifier, (4) oscilloscope, (5) xyz stage, (6) CCD camera with a zoom lens, and (7) personal computer.
presented that this field gives a useful basis to design sizeindependent particle separation.14,15 The coupled acoustic-gravity field should have advantages over other physical fields particularly if we handle particles with various chemical compositions as well as with wide size distribution. Swelling of ion-exchange resins strongly depends on the hydration nature of counterions. Ion-exchange selectivity has been discussed from this standpoint, and the thermodynamic and structural features involved in ion-exchange reactions have been revealed to some extent.16-19 A simple rule is that an ion-exchange resin with a well-hydrated counterion is swollen better than that with a poorly hydrated counterion. For a cation-exchange resin, water contents (or extents of swelling) decrease in the order predictable from the hydration energies of countercations, e.g., in the case of alkali cations, Li+ > Na+ > K+. Swelling of resins results in different densities and compressibilities, which are their essential acoustic properties. Thus, resin particles with different counterions are expected to behave differently in an acoustic field. In this correspondence, preliminary results of the acoustic recognition of counterions in ion-exchange resins are presented, and the usefulness of this method is also discussed. EXPERIMENTAL SECTION The experimental setup is schematically shown in Figure 1. The fused-silica cell was pasted on a 2 cm × 2 cm lead zirconate titanate (PZT) transducer with 500-kHz resonance frequency with epoxy adhesive. The cell wall thickness was 5.56 mm and the channel thickness was 1.50 mm; these thicknesses were equal to the half-wavelengths of 500-kHz ultrasound in silica glass and (15) (16) (17) (18)
Masudo, T.; Okada, T. Anal. Sci. 2004, 20, 753. Okada, T.; Harada, M. Anal. Chem. 2004, 76, 4564. Harada, M.; Okada, T.; Watanabe, I. J. Phys. Chem. B 2002, 106, 34. Toteja, R. S. D.; Jangida, B. L.; Sundaresan, M.; Venkataramani, B. Langmuir 1997, 13, 2980. (19) Nandan, D.; Venkataramani, B.; Gupta, A. R. Langmuir 1993, 9, 1786.
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Analytical Chemistry, Vol. 79, No. 7, April 1, 2007
water, respectively. The node of the standing wave should thus be formed at the center of the channel filled with water. The entire resonance frequency after pasting the cell on the PZT transducer was 480 kHz. The transducer was driven by sinusoidal signals generated by a function generator (model WF1946, NF Electric Co. Ltd.) and amplified by a bipolar high-speed amplifier (model 4015, NF Electric Co. Ltd.). The cell was put on a xyz stage to adjust the vertical and horizontal position. Particle behavior was directly observed by a CCD camera (model CS220, Olympus) with a zoom lens (maximum magnification ×24), installed just beside the cell. Digital data were stored and processed on a personal computer. The coordinates corresponding to the upper and lower ends of the aggregation zone were read out from digital pictures, where one pixel corresponded to 3 µm, and then were averaged. The coordinates reported below are the means of five measurements. Two types of cation-exchange resins having different crosslinking percentages, i.e., Dowex 50WX2 and 50WX4, were used as samples. These resins (H+-forms) were rinsed well with ∼0.5 M HCl solutions and repeatedly rinsed with water until the pH of the filtrates were higher than 4. The H+-form resins were converted into NBu4+ and NEt4+ forms by repeated treatments with a sufficiently large amount of aqueous 0.2-0.4 M NBu4Br and NEt4Br solutions, respectively, and then the resins were rinsed very well with water. Larger cation-exchange selectivity of tetraalkylammonium ions than H+ should confirm the complete replacements of countercations. The ion-exchange capacities of the dried resins were determined by acid-base titration with the H+-form resins; 5.33 ( 0.02 mmol g-1 for H+-form Dowex 50WX2 and 5.30 ( 0.06 mmol g-1 for H+-form Dowex 50WX4. The capacity for NBu4+-form resins was calculated from these values (2.33 mmol g-1 for both). A weighed amount of a dried resin was completely swollen in water. After removing interstitial water by rapid filtration, the swollen resin was weighed. From the weight difference between dried and swollen resins, the number of water molecules adsorbed by an ion-exchange group was calculated. This procedure was repeated at least three times for individual resins. The densities of swollen resins were measured in the following way. An appropriate amount of a dried resin (wR ) FRVR, where FR and VR are the density and volume of the swollen resin, respectively) was put in a glass vessel with a calibrated volume (Vtotal) ) VR + Vw, where VR and Vw are the volumes of the swollen resin and water, respectively). The vessel was filled with water (Vtotal ) VR + Vw, where Vw is the volume of water) and weighed (wtotal ) wR + ww ) FRVR, + FwVw). Rearranging these relations gives
wtotal )
FR - Fw wR + FwVtotal FR
From wtotal-wR plots obtained with a given resin, its FR was determined by curve-fitting. The resins were powdered in a mortar into small particles (typical diameter was 10 µm). Resin particles suspended in water were introduced into the cell with a syringe, and their behavior in the coupled acoustic-gravity field was observed.
Table 1. Properties of Resin Samples resin
density FR/g cm-3
countercation H+
Dowex 50WX4
(
