Parameters Influencing the Electron Spin Resonance Signal Intensity of Metal Ion Complex-Exchanged Resins Doris C. Warren” Department of Chemistry, Houston Baptist University, 7502 Fondren Rd., Houston, Texas 77074
J. M. Fitzgerald’ Department of Chemistry, University of Houston, Houston, Texas 77004
Evaluation of quantitative ESR spectra of first row transition metal ions bound to iorrexchange resins reveals characteristics of a sensitive, measurable ESR signal. The Tanabe-Sugano theory, which qualltatlvely predicts spin relaxation times, facilitates the selection of an lon/ligand comblnation for each d electron configuration. Sensitive combinations of metal ion/ligand/resin for ESR measurements are limited by solution variables and further restricted by additional factors Including nuclear splitting, magnetlc anisotropy and spln-spin interaction. A systematic variation of F e ( I I 1 ) and Co(I1) ligand combinations is reported.
T h e electron spin resonance (ESR) signal intensity is extremely dependent on local paramagnetic ion matrix effects ( I ) . If the spin environment is controlled carefully, the resonance signal intensity is linear with spin concentration ( 2 ) . We have become interested in quantitative measurements of transition metal complex ions exchanged from aqueous solution onto ion-exchange resins ( 3 ) . The ESR signal is strengthened by transfer to a low-dielectric, solid state and this is advantageous for analytical purposes. Both solution conditions (pH, ligand, oxidation state of ion) and resin type (strong or weak acid exchanger, polymeric resin backbone) must be considered when selecting conditions to obtain an intense ESR signal for a particular ion. Relaxation time effects and nuclear spinspin splitting are controlled by the particular ion/ligand combination chosen and influence the ESR signal ( I , $ ) . Relaxation effects may be reflected by an excessively broadened signal. Nuclear spin-spin splitting gives rise to a multiplet signal, where the intensity of a given line is only a fraction of the total intensity. Nuclear spin-spin splitting also increases the minimum detectable number of spins. Effects which result from the use of a solid matrix include magnetic anisotropy ( 1 , 4 ) which decreases signal intensity and alters peak shape and spin-spin interaction which can cause a decrease in signal intensity ( I , 4 ) . Guilbault and Meisel ( 5 ) in an early study of the effect of diverse substances on the ESR spectra of aqueous Mn2+ solutions found that the effects of anions such as C1-, F-, Br-, C2H302-,and SCN- (weak to medium field ligands) are independent of the concentration of Mn2+ and generally insignificant. On the other hand, strong field ligands such as citrate, oxalate, CN-, tartrate, EDTA, and ethylenediamine cause a decrease in the ESR signal when ligand concentrations are equal to or greater than l / m the concentration of Mn(I1). If these ligands are present in concentrations equal t o or greater than the concentration of Mn(II), the resulting complexes exhibit no ESR signal. Therefore, the ligand field surrounding a paramagnetic transition metal ion can profoundly affect the ESR signal shape and intensity. Perturbation theory has been applied ‘Present address, National Health Laboratories, Inc., 1007 Electric Avenue, Vienna, Va. 22180. 1840
ANALYTICAL CHEMISTRY, VOL. 49, NO. 12, OCTOBER 1977
to the spin Hamiltonian ( 4 ) and detailed predictions are available on the relative energies of the spin-orbit coupling and the ligand field. Our investigation was centered on first row transition ions where the ligand field effects predominate. The situation for the second and third row elements is much more complicated (4). Splitting of d electronic energy levels by ligand fields can significantly change the spin-lattice relaxation time of the paramagnetic electron. If relaxation occurs rapidly, the signal broadens to become immeasurable; this is due to the interrelation of the energy of a state and the time spent in that state, given by the Heisenberg uncertainty relationship. The relaxation time (6) is given by Equation 1:
TI=
1 0 4 ( ~ , -~ h2H4T
, ) 4
where A is the spin-orbit coupling constant for a given element, H is the ESR magnetic field strength, and T is the absolute temperature; these are essentially constant for the work reported here. While the relaxation time can be slowed by cooling the sample, precise quantitative analytical results have not been obtained from samples run a t liquid nitrogen temperatures ( 7 ) . Therefore, the difference between the ground state orbital energy, E,, and the first excited level, E2, an energy parameter determined by the ion/ligand combination chosen, is a very important variable. The fourth-power is effect of energy level difference on relaxation time, Ti, considerable. The splitting of d orbitals by ligand fields of varying strengths have been calculated and plotted by Tanabe and Sugano (8). If a qualitative correlation is possible between the inverse peak width, 1/X, of an ESR peak as a function of ligand field strength and Tanabe-Sugano diagrams, the analytical chemist would then have a tool to facilitate the choice of a complex for a particular metal ion. T h e metal ion/ligand combination should have a slow relaxation time and, hence, a strong ESR signal. Other experimental parameters further qualify and narrow the ligand choices for the metal ion. When one considers the numbers of ions, complexing ligands, and resins available, the number of experiments required for every possible combination becomes unrealistically large. In a preliminary investigation, several first row transition metal ions were evaluated; Fe(II1) and Co(I1) were chosen for closer study. The possible combinations for these two ions and various ligands can be separated into three groups: “signal possible”, “signal impossible”, and “marginal”, depending on the predicted relaxation times due to the energy separation between the ground and first excited states (Tanabe-Sugano diagrams). Consider the following examples. For Fe(III), a d5 ion, the Tanabe-Sugano diagram (4,8), shows that E2 is far above E l for both weak and very strong field ligands. Only intermediate strength ligands such as C2OA2-, would lower E2 close to El so as to make the relaxation time rapid enough to significantly broaden the ESR signal into the background noise. Thus, while a number of anionic complexes
Table I. ESR Signal Data for the Complexes of Fe(II1) and Co(I1) Studied Ligand c1FQc,o,zRelativea Fe(II1) signal strengths Relatived Fe(II1) width-’ % Fe(II1) left in filtrate Comments
Relativef Co(I1) signal strength Relative’ Co(I1) width-’ Comments
SCN-
EDTA
CN
12.7b
3.Ob
2.5b
2.8b
1.6b
4.gC
1.0b
3.4
4.7
4.0
1.1
1.2
1.0
1.2
4.5
14.5
0.1
0.1
12.7
9.1
0.8
very narrow singlet
measured g, component only
smalle amount of splitting
very broad singlet
verye broad, small amount of splitting
pronouncede splitting
A
A
1.6
broad singlet
2
1.0
2
1.0
1.5
singlet
singlet broad a Numbers obtained by dividing all peak signal parameters by the CN- area. Polystyrene anex resin, Bio-Rad AG 1-X8. Numbers obtained by dividing all inverse peak widths by the inverse Polystyrene catex resin, Bio-Rad AG 50 W-X8. width value of the EDTA spectrum. e Peak width and area measurements are made from the envelope of the spectrum. Numbers obtained by dividing all peak signal parameters by the F-area. g Signal does not appear above the background Air oxidation prevents measurement (15). 1 Numbers obtained by dividing all inverse peak widths by the inresin signal. verse width value of the F- spectrum. (halides and pseudo-halides) of Fe(II1) are all analytically possible, only one or two of the complexes should give highest intensity. On the other hand, Co(II), a d’ ion, has a Tanabe-Sugano diagram ( 4 , 8) in which the first excited state, Ea, is very close to the ground state, E l , except for a few selected field strengths. Thus only a few ligands will yield Co(I1) complexes with suitable relaxation times. Since the ESR measurement of Co(1I) is virtually impossible in aqueous solution (9, I O ) , any success a t all is worth the effort of binding the complex on a resin. T h e ‘‘signal possible” and “marginal” ligand combinations which should be investigated can be further reduced depending on whether the complex can be prepared in solution (oxidation state stability, formation constants, and p H dependence of these equilibria) and on its ground state multiplicity. When a paramagnetic metal ion complex is bound to an ion-exchange resin, it is known (11)that the primary ligand field will be modified by the field of the ionic binding site of the resin. Therefore the choice of not only ligand, but also resin will affect the ESR signal. We have found the strength of the background signal due t o drying the polymeric resin (from free radicals trapped in the resin backbone) determines the limit-of-detection for the ion studied. T h e upper limit of linearity (ULOL), defined as the highest concentration of analyte which yields the same least-squares slope as lower concentrations on the calibration line, is determined by the interspin interactions of the spins on the beads (3). Magnetic anisotropy can arise when transition metal ions are exchanged onto ion-exchange resins ( I , 4 ) resulting in altered peak shape and less intense spectra. T h e ESR signal parameter, a measure of the area under the first derivative ESR curve, is a function of the recorded peak height and width, instrument gain, and modulation amplitude. The area can be calculated by the method of Brinkman and Freiser (12):
signal parameter
=
3.63 ( X ) ’ Y R G X MA
Where X is the recorded peak width, Y is the peak height, RG is the receiver gain and M A is the modulation amplitude used a t constant power. The value of the constant associated with the calculation of peak area depends on peak shape (6);
for our work we normalized all axeas t o account for the constant. T h e ESR signal parameter, in arbitrary units of cm3/(receiver gain units) (Gauss), is linear with spin concentration when conditions are carefully controlled (12).
EXPERIMENTAL Apparatus. A Varian E-3 ESR spectrometer fitted with an E4531 multipurpose microwave cavity was used for all measurements. Conventional low-dielectric 4.20 mm (id.) X 178 mm quartz sample tubes were used. Resin quantities were selected so as to completely fill the portion of the ESR tube located in the ESR cavity. The iron concentrations of the filtrates of each resin sample were measured using a Perkin-Elmer Model 305 atomic absorption spectrometer. Reagents. Stock solutions (lo-’ M) of CoC12in distilled water and FeNH,(SO& in 0.1 M HC1 were prepared by weight from reagent grade chemicals. Stock solutions (0.1 M) were prepared from each of the following reagent grade complexing agents: KCN, Na2C?04,KCNS, NaF, EDTA (pH adjusted to 10 with NaOH), and 8-hydroxyquinoline-&sulfonic acid (Q-) (pH adjusted to 9 with NaOH). An 8 M stock solution of HCl(12) was also prepared. Two polystyrene based ion-exchange resins were used: (1) Bio-Rad, Analytical Grade AG1-X8 (200-400 mesh) anex resin in the chloride form (3.2 mequiv/g, dry resin) and (2) Bio-Rad, Analytical Grade AG 5OW-XB (200-400 mesh) catex resin in the hydrogen form (5.1 mequiv/g, dry basis). The resins were obtained from Bio-Rad Laboratories, 32nd and Griffin Ave., Richmond, Calif. 94804. Procedures. Instrument. The ESR instrument and cell, filled with resin sample, are tuned according t o standard procedures (2,5, 7,9). After tuning, the instrument, is placed in the “operate” mode; the microwave power raised to the maximum obtainable with the sample in question (this depensds on the amount of water bound to the resin); the detector current brought to 300 p A by turning the iris screw; and the frequency error adjusted to zero. The field is then set to center the spectrum (3400 G) with the scan range at *500 G. An appropriate modulation amplitude (4, 6) for the particular sample (10 G in .all of these experiments) is chosen. The receiver gain is selected to obtain the maximum pen displacement on the chart paper. The time constant used depends on the receiver gain and varies from 0.1 to 3 s. The minimum time constant is used which yields a spectrum with little or no noise. A 4 min scan time is used. Sample Preparation. Solutions (1 X M) of the paramagnetic complex ions were prepared by volumetric dilution of the stock solutions with the appropriate complexing agent; then ANALYTICAL CHEMISTRY, VOL. 49, NO. 1:2, OCTOBER 1977
1841
200 mL of each Fe(II1) solution were equilibrated with 1.000g of the appropriate oven-dried resin and 100 mL of each Co(I1) solution were equilibrated with 1.000 g of air-dried anex resin. After stirring vigorously for 15 min, the resins were filtered off;
the Fe(II1) samples oven-dried and the Co(I1) samples air-dried. The Fe(II1) remaining in each of the supernatant solutions was assayed by atomic absorption. The ESR spectra were then obtained at 20 mW [Co(II) samples] and 200 mW [Fe(III) samples]. The samples for the calibration curve with the Fe(III)-Cl- complex were prepared by volumetric dilution of the stock solution with 8 M HC1 (13) and equilibration of 200 mL of each concentration with 1.000 g of air-dried anex resin. The resin samples were filtered, air-dried, and then oven-dried at 110 "C for 10 min. The ESR spectra were then obtained at 200-mW power.
RESULTS AND DISCUSSION T h e ESR integrated signal intensities were calculated for M Fe(II1) with Cl-, F-,Q-, C 2 0 4 " SCN-, ~, the complexes of EDTA, and C N - and ~ normalized relative to that for CN- as shown in Table I. T h e 1 / X value was calculated for each ligand and normalized relative to the value of EDTA (Table I). T h e 1/X data correlates with those predicted by Tanabe-Sugano theory for the weak field and medium field cases. That is, the relaxation time should be slower and hence signal strength larger for weak field ligands; while the relaxation time is faster and, therefore, the signal strength smaller for medium field ligands (due t o broadening of the signal into the background noise). The F- and SCN- ESR signal parameters are somewhat less intense than expected due to the amount of Fe(II1) left in the filtrate after equilibration. The Q- ESR signal exhibits magnetic anisotropy. T h e ESR signal parameter is calculated from the strong g, peak only; the signal parameter of the smaller 811 component is not included. A smaller signal parameter than predicted results from this approximation. In the case of the strong field ligands, EDTA and CN~.,both 1/X and ESR signal strength are less than predicted from Tanabe-Sugano theory because the signals are broadened due to nuclear splitting. From the data in Table I, i t can be concluded that t o obtain maximum ESR signal intensity for an Fe(II1) ESR experiment, one would choose the C1- complex. A least-squares data reduction of a calibration line (12) for the C1- complex of Fe(II1) exchanged on strong anex resin over the concentration range from 5 x to 5 X M (original solution concentration) resulted in a coefficient of linearity of 0.9954 and a relative standard error of the estimate of 0.37%. In the case of the Co(I1) system, ESR data for lo-' M Co(I1) complexes on the polystyrene resins were obtained in a similar manner t o those for Fe(II1); normalized data are summarized in Table I. T h e results are in reasonable accord with Tan-
1842
A N A L Y T I C A L CHEMISTRY, VOL. 49, NO. 12, OCTOBER 1977
abeSugano theory for Co(I1). Two Tanabe-Sugano diagrams must be consulted because C1 and F-complexes are tetrahedral and should therefore be correlated to the energy diagram for that symmetry (14); while the two strong field complexes are octahedral. Measurements were found t o be complicated by air oxidation of Co(I1) to diamagnetic Co(II1) in strong ligand fields (15). Co(I1) is thus less amenable t o routine ESR quantitation than is Fe(II1) (9). Thus one can conclude that several experimental factors must be considered while selecting the ion/ligand/resin combination to be used for a quantitative ESR experiment. Criteria which give an ESR signal of maximum intensity have been identified. The ideal system will be one which gives an ESR signal that (1) has no nuclear splitting; (2) has a slow relaxation time; (3) is isotropic; and (4)has no spin-spin interaction. T h e first two criteria may be ascribed to the ligand chosen while the last two are inherent in a solid matrix system. It has been shown also that correlation of ESR inverse peak width data with Tanabe-Sugano diagrams can qualitatively aid the chemist in choosing a ligand which will give the most intense ESR signal for a particular ion when the ionlligand combination is exchanged onto ion-exchange resins.
ACKNOWLEDGMENT The assistance of Reagan Davis in collecting portions of the experimental data is acknowledged.
LITERATURE CITED (1) P. B. Ayscough, "Electron Spin Resonance in Chemistry", Methuen & Company Ltd., London, 1967, pp 29, 60, 108-113, 170. (2) G. G. Guilbault and G. J. Lubrano. Anal. Lett., 1, 725 (1968). (3) D. C. Warren and J. M. Fitzgerald, Anal. Chem., 49, 250 (1977). (4) J. E. Wertz and J. R. Bolton, Electron Spin Resonance", McGraw-Hill, New York, N.Y., 1972, Chapters 3, 7, 11, and Appendix D. (5) G. G. Guilbault and T. Meisel. Anal. Chem., 41, 1100 (1969). (6) C. P. Poole, "Electron Spin Resonance", Interscience, New York, N.Y., 1967, pp 18-22, 171. (7) E. S. Moyer and G . G. Guilbault, Anal. Chim. Acta, 52, 281 (1970). (8) Y. Tanabe and S. Sugano, J . Phys. SOC. Jpn., 9 , 753, 766 (1954). (9) G. G .Guilbault and E. S. Moyer, Anal. Chem., 42, 441 (1970). (10) E. S. Moyer and W. J. McCarthy, Anal. Chlm. Acta, 48, 79 (1969). (11) R. J. Faber and M. T. Rogers, J . Am. Chem. Soc., 81, 1849 (1959). (12) W . J. Brinkrnan and H. Freiser, Anal. Lett., 4, 513 (1971). (13) J. S.Fritz and G. H. Schenk, "Quantitative Analytical Chemistry", Allyn and Bacon, Inc., Boston, Mass., 1974, pp 429-432. (14) F. A. Cotton and G. Wilkinson, "Advanced Inorganic Chemistry", Interscience, New York, N.Y., 1966, pp 673-675. (15) J. J. Lingane, "Analytical Chemistry of Selected Metallic Elements", Reinhold, New York, N.Y., 1966, Chapter 9.
RECEILTD for review July 18,1975. Resubmitted May 23,1977. Accepted July 27, 1977. Financial support of Grant E-384 from the Robert A. Welch Foundation is gratefully acknowledged. The National Science Foundation and the Benjamin Clayton Foundation provided funds for the ESR spectrometer.
