© Copyright 2001 by the American Chemical Society
VOLUME 105, NUMBER 17, MAY 3, 2001
LETTERS Separation of Transition Metal Ions in an Inhomogeneous Magnetic Field M. Fujiwara,* D. Kodoi, W. Duan, and Y. Tanimoto* Graduate School of Science, Hiroshima UniVersity, Kagamiyama, Higashi-Hiroshima 739-8526, Japan ReceiVed: September 29, 2000; In Final Form: February 2, 2001
The magnetic separation was investigated for Fe3+, Co2+, Ni2+, Cu2+, Zn2+, Ag+, and Cd2+ ions in an inhomogeneous field of 410 kOe2 cm-1. By application of the field to the solutions spotted on silica gel, the Fe3+, Co2+, Ni2+, and Cu2+ ions were attracted toward the field center, but the Zn2+, Ag+, and Cd2+ ions stayed at the spot origin. The movements depended upon the susceptibilities of the metal ions. The findings demonstrate that the use of a magnetic field has potential to separate ions in chemical and biological areas.
Ions with different charges are separated in an electric field, but what happens to ions with different susceptibilities in a magnetic field? The electric force acting on divalent ions in a 1 V cm-1 field is ∼106 times as large as the magnetic force on paramagnetic ions (susceptibility χ ) 2000 × 10-6 cm3 mol-1) in a 400 kOe2 cm-1 field. The thermal energy of a solvent atmosphere is much greater than the magnetic energy of ions themselves. Therefore, it is believed that ions are diffused by the thermal fluctuation but are not separated by the magnetic force in solution. This letter reports the separation of transition metal ions in a magnetic field. The movements are described in detail for Cu2+ (χ ) 1500 × 10-6 cm3 mol-1)1 and Ag+ (χ ) -24 × 10-6 cm3 mol-1)2 ions in solution spotted at a point on silica gel. By application of a 410 kOe2 cm-1 field, the Cu2+ ions were drawn toward the field center, but the Ag+ ions were not affected. The 410 kOe2 cm-1 field was applied with a superconducting magnet (Oxford Spectromag 1,000). The field direction was horizontal. Cu(NO3)2‚3H2O (Wako, 99.9%) and AgNO3 (Wako, 99.8%) were dissolved in deionized water to prepare Cu2+ (1.0 mol dm-3) and Ag+ (1.0 mol dm-3) solutions, respectively. Silica gel (Wako, 75-150 µm, 27 g) was laid as a support in a glass vessel (390 × 40 × 10 mm) with deionized water (36 cm3). The Cu2+ (0.050 cm3) and Ag+ (0.050 cm3) solutions were mixed thoroughly and spotted on the silica gel. The glass vessel was placed in the magnet bore (374 × φ 50.4 mm) so that the
spot point was 100 mm from the field center, allowed to stand at 295 K for 14 h, and then removed from the magnet. The Cu2+ ions were colored blue ([Cu(NH3)4]2+) by a spray of ammonia solution (Kanto, 28.0-30.0%), and the Ag+ ions gray (Ag metal) by irradiation of an ultrahigh-pressure mercury lamp (Ushio UI-501C, 500 W). The movements of the Cu2+ and Ag+ ions were recorded with a camera (Nikon Nikomat FTN) on photographs, which were then scanned (Epson GT-5,000ART) into a personal computer (Apple 7,600/132). The color-density profiles of the Cu2+ and Ag+ ions were analyzed with an imageprocessing program (NIH Image 1.55). The magnetic field was inhomogeneous. In Figure 1 is shown the dependence of the intensity [H(z)] and intensity × gradient [H(z)∂H(z)/∂z] upon the distance (z) from the field center. The intensity reaches 80 kOe at the field center (z ) 0 mm), and the intensity × gradient is 410 kOe2 cm-1 at 65 mm from the field center (z ) (65 mm). The separation of the Cu2+ (χ ) 1500 × 10-6 cm3 mol-1)1 and Ag+ (χ ) -24 × 10-6 cm3 mol-1)2 ions in the magnetic field is shown in Figure 2. The solutions containing the Cu2+ and Ag+ ions were mixed and spotted on the support at 100 mm from the field center (z′ ) z + 100 mm ) 0 mm). The thermal diffusion of the solvent was suppressed by contact with the support surface. On application of the field, the Cu2+ ions moved by ∼48 mm toward the field center (z′ ∼ 48 mm), but the Ag+ ions stayed at the spot origin (z′ ∼ 0 mm).
10.1021/jp003562d CCC: $20.00 © 2001 American Chemical Society Published on Web 04/05/2001
3344 J. Phys. Chem. B, Vol. 105, No. 17, 2001
Letters
Figure 1. Intensity and intensity × gradient profiles for the inhomogeneous magnetic field. Figure 3. Distributions of Cu2+ and Ag+ ions on silica gel. Cu2+ and Ag+ ion solutions were mixed and spotted at a 0 mm point which was 100 mm from the field center.
It is clear that the larger mobilities (Fe3+ > Co2+ > Ni2+ > Cu2+) correspond to the larger susceptibilities (Fe3+ > Co2+ > Ni2+ > Cu2+) for the paramagnetic ions. The small mobilities undetected (Zn2+, Ag+, Cd2+) are considered to result from the small susceptibilities for the diamagnetic ions. The magnetic separation of transition metal ions is explained qualitatively by the difference of susceptibilities. Suppose a mole number n of ions with molar susceptibility χ are placed at position z in a magnetic field of intensity H(z) and gradient ∂H(z)/∂z (see Figure 1). The magnetic energy E(z) for the ions is expressed as
E(z) ) -(n/2)χH(z)2
(1)
and the magnetic force F(z) acting on the ions is given as
F(z) ) -∂E(z)/∂z ) nχH(z)∂H(z)/∂z
Figure 2. Separation of Cu2+ and Ag+ ions on silica gel. Ion solutions were spotted at a 0 mm point that was 100 mm from the field center. (a) Cu2+ and Ag+ ion solutions were mixed and spotted; (b) Cu2+ ion solution was spotted; (c) Ag+ ion solution was spotted.
In Figure 3 are shown the distributions of the Cu2+ and Ag+ ions in the magnetic field, measured along the distance z′ from the spot point of the solutions containing the Cu2+ and Ag+ ions. When the whole area is divided by a line of z′ ) 10 mm, the purity of the Cu2+ ions is ∼90 mol % in the region of z′ > 10 mm, and the purity of the Ag+ ions ∼90 mol % in the region of z′ < 10 mm. It should be noted that the separation of the different metal ions is good. The separation was also tested for Fe3+ (χ ) 15 000 × 10-6 cm3 mol-1),1 Co2+ (χ ) 9500 × 10-6 cm3 mol-1),1 Ni2+ (χ ) 4200 × 10-6 cm3 mol-1),1 Zn2+ (χ ) -10 × 10-6 cm3 mol-1),2 and Cd2+ (χ ) -22 × 10-6 cm3 mol-1)2 ions in the 410 kOe2 cm-1 field. The Fe3+, Co2+, and Ni2+ ions moved by ∼100, ∼80, and ∼70 mm, respectively, toward the field center, but the Zn2+ and Cd2+ ions did not leave the spot origin.
(2)
If the ions are paramagnetic (χ > 0), the magnetic energy E(z) is minimized and they are stabilized at the maximum point of the field intensity, i.e., the magnetic force F(z) acts and they are attracted in the direction where the field intensity increases. If the ions are diamagnetic (χ < 0), they are stable outside the field, i.e., they are repulsive to the field. The susceptibilities differ among transition metal ions, and the mobilities differ among them. The ions with larger susceptibility move into larger distance. The movements are discussed now on the model of a drift motion of a single paramagnetic ion. For an ion of hydrodynamic radius R moving in a medium of viscosity η, the drift velocity V(z) is related to the magnetic force F(z) by
V(z) ) F(z)/f
(3)
where f is the frictional coefficient given as
f ) 6πRη
(4)
If a single Cu2+ ion (χ ) 1500 × 10-6 cm3 mol-1, R ) 2.0 Å) is assumed to move in water (η ) 1.0 mPa s) under the 410 kOe2 cm-1 field, the drift velocity V(z) is estimated to be ∼9.7 × 10-6 mm h-1. The calculated value is surprisingly smaller in comparison to the observed value of ∼48 mm/14 h, i.e., ∼3.4
Letters mm h-1. The discrepancy denies that a paramagnetic ion moves solely in water. For the movements of paramagnetic ions, another possibility is discussed in terms of a whole flow of the solution composed of ions and solvent molecules like convection. Since the ions and solvent molecules collide frequently with each other in the solution, the magnetic force on the paramagnetic ions might be conveyed to the surrounding ions and solvent molecules. It might be necessary to consider that all of the ions and solvent molecules move together for the dense solution of the paramagnetic ions by a gradient magnetic field. The magnetic-field-induced convection was reported for bulk solutions in a Cu2+/Cu-Zn2+/Zn redox reaction.3,4 The Zn wire was placed on chromatography paper with the Cu2+ solution (0.5 mol dm-3). After a 410 kOe2 cm-1 field was applied along the Zn wire for 1 h, the Cu2+ ions were attracted by >50 mm to the field center and the Cu dendrites were formed near there. It was suggested that the solution containing the Cu2+ ions moved to the stronger field inducing the convection, which forced the portion rich in the Zn2+ ions to go in the opposite direction. The role of silica gel is discussed finally. If the ions have different adsorption activities on the surface of silica gel, the less-adsorbed ions might move farther during some ion motion or solution flow than the strongly-adsorbed ions. Since the
J. Phys. Chem. B, Vol. 105, No. 17, 2001 3345 paramagnetic and diamagnetic ions in a droplet spotted on water are not separated without silica gel even by exposure to a magnetic field, it might be important to think of the possibility of adsorption-desorption equilibrium on the surface. In any case, the mechanisms remain unresolved at present and must wait further clarification for the motion of paramagnetic ions and the role of silica gel. The separation in a magnetic field is known for paramagnetic particles (R > 1 µm) but has not been reported for ions. The results presented here demonstrate that transition metal ions move to be separated by some distances depending upon the susceptibilities in a magnetic field. Since the susceptibilities range widely for transition metal ions and complex ions, the application of a magnetic field has potential to be used for the separation of them in chemical and biological areas. References and Notes (1) Ko¨nig, E. In Landolt-Bo¨ rnstein; Hellwege, K.-H., Hellwege, A. M., Eds.; Springer-Verlag: Berlin, 1966; New Series, Vol. II/2, Chapter 2. (2) Ko¨nig, E.; Ko¨nig, G. in Landolt-Bo¨ rnstein; Hellwege, K.-H., Hellwege, A. M., Eds.; Springer-Verlag: Berlin, 1984; New Series, Vol. II/12a, Chapter 1.1.6. (3) Tanimoto, Y.; Yano, H.; Watanabe, S.; Katsuki, A.; Duan, W.; Fujiwara, M. Bull. Chem. Soc. Jpn. 2000, 73, 867. (4) Duan, W.; Fujiwara, M.; Tanimoto, Y. Bull. Chem. Soc. Jpn. 2000, 73, 2461.
