1930
GEORGE R. HERTEL8x11HERBERT M. CLARK
T’ol. 65
PARAMAGNETIC RESONANCE BEHAVIOR OF TETRACHLOROFERRATE 10s IX ISOPROPYL ETHER1 BY GEORGER. HERTEL~ AND HERBERT M. CLARK Department of Chemistry, Rensselaer Polytechnic Institute, Troy, New York Received Decrmber 16, 1960
The iron complex extracted into isopropyl ether from aqueous solutions of FeCl, in 6 M HC1 was investigated by means of electron-paramagnetic-resonance (e.p.r.). Spectra of anhydrous FeC18 and the extracted complex are similar and consist of Lorentz-shaped lines centered a t g = 2.013, indicating a tetrahedral configuration for FeC14-. The marked differences between these spectra and that of FeC13.6H20is taken as evidence that the water in ether extracts containing the hydrated tetrachloroferric acid is not closely associated with the iron. The variation of the e.p.r. line width with ethereal iron concentration is interpreted as evidence of a varying relaxation phenomenon attributed to an association of the tetrachloroferric acid a t concentrations greater than about 0.005 M .
Introduction The extraction of ferric chloride from hydrochloric acid solution by isopropyl ether has been the subject of several investigat,ions. With respect to the extractable species, it is generally accepted that the iron is extracted as tetrachloroferric acid, HFeC14.3 On the basis of phase distribution, spectral and electrolysis studies3S4there is evidence that the acid is hydrated and that it is the proton rather than the tetrachloroferrate ion which is hydrated. In addition, the well known dependence of the distribution ratio on iron concentration has been explained in terms of an activity or selfsalting out effect in the aqueous layer6and in terms of polymerization6 or ion-association‘‘.8or clusteringg of the extracted iron complex. These explanations of the deviations from the Nernst distribution law have been based on phase distribution, spectral, isopiestic, conductance and e.m.f. studies of the extracted species. I n this paper an account is giv.m of an investigation in which the iron complex extracted by isopropyl ether was studied by means of an electron-paramagnetic-resonance (e.p.r.) spectrometer. The paramagnetic resonance behavior of iron salts has been reported previously by several investigators for salts in the solid statelOtlland in aqueous ~olution.l~-~~ (1) Part of a thesis submitted to Rensselaer Polytechnic Institute by George R. Hertel in partial fulfillment of the requirements for the degree of Master of Science. (2) Experimental work reported herein was done a t Knolls Atomic Power Laboratory, operated by the General Electric Company for the U. S. Atomic Energy Commission. (3) H. L. Friedman, J . Am. Chem. SOC.,74, 5 (1952). (4) A. H. Laurene, D. E. Campbell, S. E. Wiberley and H. M . Clark J . Phys. (‘hem., 60, 901 (1956). (5) N. E. Nachtrieb and R. E. Fryxell, J . Am. Chem. Soc., 70, 3552 (1948). (6) R. J. Myers and D. E. Rletzler, zbid., 73, 3772 (1950). f7) D. E . Campbell, H. >I. Clark and W. H. Bauer, J . Phys. Chem., 62, 506 (1958). (8) J. Saldick, ibid., 60, 500 (1956). (9) N. H. Nachtrieb and R. E. Fryxell, J . Am. Chem. Soc., 74, 897 (1952). (10) D. M. S. Bagguley, B. Bleaney, J. H. E. Griffiths, R. P. Penrose and B. I. Plumpton, Proc. Phys. Soc. (London), 61, 542, 551 (1948). (11) B. Bleaney and K. W. H. Stevens, Repts. Proor. i n Phys., 16, 108 (1953). (12) B. R. McGarvey, J . Phys. Chem., 61, 1232 (1957). (13) N. S. Garif’yanov, Soviet Phys. J E T P , 10, 1101 (1960). (14) V. I. Avvakumov, N. 5. Garif’yanov, B. M. Koayrev and P C. Tishkov, ibid., 10, 1110 (1960).
Experimental A Varian Model V-4500 Spectrometer and a Varian Model V-4007 Six-Inch Electromagnet System were used. The magnetic field was linearly variable from zero to over 6OOO gauss. In the Varian equipment a microwave bridge circuit is employed in order to attain high sensitivity and a favorable signal to noise ratio. An AFC (automatic frequency control) circuit “locks” the klystron to the fixed resonant frequency of the cavity, which is located in the air gap of the magnet and makee up one arm of the bridge circuit. The bridge is balanced under conditions of no resonance. As the magnetic field is varied through resonance conditions, the sample absorbs energy causing the bridge to unbalance, thus producing a signal a t the crystal detector in the bridge. The microwave power level is held constant throughout any one scan. A Numar Model M-2 precision gaussmeter was used t o measure field strength. The cavity resonance frequency (approximately 9500 Mc .) was measured by zero-beating the klystron signal with a harmonic of a frequency delivered by a Hewlett-Packard Model 450A transfer oscillator. Oscillator frequencies were measured by a Hewlett-Packard Model 624B electronic counter standardized by zero-beating with radio station WWV, Washington, D. C. The Varian Model V4500-30 rectangular microwave cavity was used for the investigations made at room temperature. Low temperature experiments were made using a specially constructed rectangular c a - d y capable of being cooled to the temperature of liquid nitrogen. Quartz sample tubes were used to avoid interfering spectra caused by the impurities found in other materials. Line widths, defined to be the separation of the peaks of the derivative curve, were taken from spectra recorded a t a rate of approximately 25 gauss/minute and a response time of 3 seconds. Checks were made a t different settings to ensure that no line broadening effects were present. Line shapes were determined using the method described by Bloembergen, Purcell and Pound.16 The ratio of maximum positive to maximum negative slopes of the derivative IS 2.2 for a line having Gaussian shape and 4 for a line having Lorentzian shape. Experiments were conducted on solid anhydrous ferric chloride, anhydrous ferric chloride in isopropyl ether, solid hydrated ferric chloride, and the ether phase obtained by the isopropyl ether extraction of 0.2 M ferric chloride from 6 M hydrochloric acid solution. Anhydrous samples were prepared in a dry argon atmosphere. Two samples of anhydrous ferric chloride and two 50X aliquots of a saturated solution of anhydrous ferric chloride in anhydrous isopropyl ether (purified by shaking with an acidified ferrous sulfate solution, dried over calcium chloride and distilled over calcium hydride in a closed system) were sealed in sample tubes. Ether extraction and dilutions were made following the procedure outlined by Campbell.’ Two 100-ml. samples of approximately 0.2 M ferric chloride solutions in 6 M hydrochloric acid were shaken with equal volumes of purified, dry isopropyl ether. The samples were allowed to equilibrate for 48 hours before the phases were separated (15) N. Bloemhergen, E. 78, 679 (1948).
M. Purcell
and R. V. Pound, Phys. Ret-.,
XOV., 1961
PA4R.4MAGNETIC
RESOXANCE O F TETRdCHLOROFERRATE
I O N IK ISOPROPYL
ETHER 1931
The ether phases were analyzed for iron by a procedure based on a colorimetric method developed by L a ~ 1 e r . l ~ Ddutions of the ether phases were made with purified ISOpropyl ether which had been equilibrated over an equal volume of 6 31 hydrochloric acid.
Results Spectra of anhydrous FeCI3 in anhydrous isopropyl ether, solid anhydrous FeCI3, hydrated Feel3, nncl iropropyl ether extracts of the iron complex were obtained a t 25 and -196’. The spectra did not change with temperature. Careful search failed to reveal fine or hyperfine structure w e n at sweep amplitudes of less than 0.5 gauss, a sweep rate of 5 gauss/min., and a response time of 3 seconds. The line width of anhydrous Feel3 was found to be 370 f 3 gauss. For hydrated Feel3 a weak, extremely broad, asymmetric line (>1200 gauss wide) centered about g = 2 was observed. There was an indication of a second, very weak resonance at a very low field (Fig. 1). The variations in line width, AH, of the ether extracts as the concentration of iron was decreased by dilution are illustrated graphically in Fig. 2 . Line shapes were Lorentzian in character, with one imporiant exception. The center peak of the complex spectrum of hydrated ferric chloride had a definite Gaussian shape. G-factor measurements for all spectra gave a value slightly higher (approx. 2.013 f 0.006) than the free electron value of 2.0023. There appeared to be no significant deviations from this value in any of the spectra.
I
6000
Fig. 1.-E.p.r.
I
I
4000
I
I
2000
I
I 0
H O(gauss). spectra of FeC13.6Hz0 (solid curve) and anhydrous Fee13 (dotted curve).
400
?.-.
2 & 300 v
2 200
I I I I 1 Discussion 10-4 10-3 10-2 10-1 The fact that spectra taken at different temperaFe concn., moles/l. tures were identical is a strong indication that the Fig. 2.-Dependence of e.p.r. line width on Fe(II1) spin-lattice relaxation time of the ferric ion in this concentration in the isopropyl ether extract of FeC13 from system is long in comparison to the spin-spin 6 M HC1. relaxation time. As Van Vleckl7 has shown, the shorter of the two relaxation times controls the shape and width of the resonance line observed width and shape of the line and the spin-lattice for ether extracts having iron concentrations in the time is strongly temperature dependent, whereas range of 10-1 to 10+ M are attributed to such the spin-spin time is not. The line shape is also polymerization. As the concentration is decreased, consistent with the supposition that spin-spin the clusters begin to break up, the dipole-dipole relaxation is the dominant type in this system. interactions become more important and the line In addition, the Lorentzian shape indicates strong width increases. With further dilution, the monoexchange interactions between iron atoms. Gar- meric iron complex becomes the predominate specstens and Iiiebsonlq have found exchange narrowing ies, the line-broadening dipole interactions are responsible for the decrease in the line width of the weakened, the spin-spin relaxation time is lengthmanganous spectrum of highly concentrated aque- ened considerably and the line width decreases ous solutions (> 6 111 1lnCl2). The evidence in rapidly as shown in Fig. 2. The proximity of the observed g-value of thq this work is that exchange narrowing is a dominant factor at concentrations as low as 0.02 M Fe(II1). ferric complex in the ether solutions to the free However, according to Van Vleck,2o exchange electron value of 2.0023 indicates that the five narrowing does not occur unless similar para- unpaired d electrons are relatively undisturbed magnetic ions are precessing about parallel axes. by other ions. It is also a definite indication that It is assumed that conditions favorable for exchange none of the 3d orbitals is being used as a bonding interaction are provided by a polymerization or orbital in the FeCL- complex, for if any were, clustering of the tetrachloroferric acid. The the special case of the half-filled shell would be destroyed and the spectrum altered. This is (16) H 111 Lanler, MIT-1110, p 33, hfarch 10, 1953 considered to be strong evidence that the FeC1,(17) J I€ Van X’leck, Phys R e & ,57, 426 (1940). (18) P \T 4nderqon and P R Welss, Rezs Wod P h y s , 25, 269 ion has a tetrahedral sp3configuration. (19%) The similarity of the spectra of anhydrous ferric (19) 3% .4 Garstens and S 1% Llebson, J. Chen. Phys., 20, 1647 chloride and of the ferric complex in the ether ex(19i2) tracts and the distinct dissimilarities between these (20) J I{ \ a n Tlech, P h V s Rez , 74, 1168 (1948)
1932
GERALDs. GOLDEN .4ND HERBERT AT. CLlRK
spectra and tjhespectrum of hydrated ferric chloride indicate that water is not closely connected to the tetrachloroferrate anion; ie., the anion resembles the anhydrous salt much more than the hydrated ferric chloride. Acknowledgment.-The authors wish to express their gratitude to Dr. Norval J. Hawkins for his
Tol. 63
instructive discussions concerning magnetic resonance phenomena. The iron analyses were performed by Carolyn J. McCoy and David A. Del Grosso. Certain materials used in this research were made available a t Rensselaer Polytechnic Institute by the U. S. Atomic Energy Commission under Contract No. AT(30-1)-1663.
THE EXTRACTION OF FERRIC BROMIDE BY DIETHYL ETHER1 BYGERALD S. GOLDENAND HERBERT M. CLARK Department of Chemistry, Rensselaer Polytechnic Institute, Troy, N e w York Received December 8.9, 1960
The evtraction of Fe(II1) fron aqueous acidic bromide solutions into diethyl ether a t 24.9" was studied as a function of the concentration of hydrobromic acid, iron and salting agents. Included in the study was an investigation of the extraction of hydrobromic acid into diethyl ether. Iron extracts as a strong acid, HFeBra, which forms ion clusters in the organic phase a t high concentrations. The amount of water accompanying the extracting species into the ether decreases with an increase of the ionic strength of the aqueous phase. When the molar ratio of water to hydrogen ion in the ether phase reaches four, two ether phases form. With further increase in aqueous salt concentration, the ratio remains four in the lighter ether phase but continues to decrease approaching unity in the heavier phase.
Although many investigators have studied the solvent extraction of iron(JI1) as tetrachloroferric acid by various organic solvents, relatively have studied the extraction of tetrabromoferric acid. I n this paper the results of an investigation of the extraction of Fe(II1) from acidic bromide solution by diethyl ether are described. The investigation included a study of the extraction of hydrobromic acid since it not only co-extracts with tetrabromoferric acid, but also largely controls the relative volumes of the equilibrated phases Experimental Materials.-Reagent grade diethyl ether was treated to remove peroxides, dried over CaClz and distilled over calcium hydride. The fraction boiling a t 34.8 f 0.2" was collected. Reagent grade 48% hydrobromic acid was distilled in a light-protected Pyrex column. The constant boiling fraction was colkcted and stored in low-actinic glass vessels. Ferric bromide stock solution (1 M ) was prepared by dissolving C.P.ferric oxide in an excess of concentrated hydrobromic acid and stored in low-actinic glassware. Analysis showed less than 0.05% Fe(I1). -411 other materials were reagent grade. Procedure.-Equal volumes of an aqueous solution of the desired romposition and of diethyl ether were mixed thoroughly in stoppered, low-actinic glass cylinders and allowed to equilibrate with intermittent shaking in a water-bath a t a temperature of 24.9 rt 0.1" for a t least 48 hours. This equilibration time was necessary in order to ensure that the Fmall amount of non-extractable Fe(I1) (approximately 0.1% of the total iron) formed in the ether phase migrated to the aqueous phase. Conductance measurements for ether extracts wwe made with a dipping-type cell having a cell constant of 0.100 cm.-l. Absorption spectra were obtained with either a Beckman Model B spectrophotometer or a Perkin-Elmer Spectracord, Model 4000. Methods of Analysis.-In the absence of iron, the HBr concentration in the aqueous phase wae determined by t i t r e tion with standard sodium hydroxide and that in the ether (1) Abstracted from a thesis presented by Gerald S. Golden to Rensselaer Polytecbnio Institute in partial fulfillment of the reqriirements of the Ph.D. degree. This work was supported by the U. S. Atomic Energy Commission, Contract No. AT(30-1)-1663. (2) I. Wada and R. Ishi, Sci. Papers Inst. Phys. Chem. Research (Tokyo), 24, 135 (1934). (3) W. A. E. McBryde and J. H. Yoe, Anal. Chem., 20, 1094 (1948). (4) R. Bock, H. Kusche and E. Book, Z . anal. Chem.. 136, 167 (1953). (5) H. G. Richter, S.M. Thesis. Mass. Inst. of Tech., 1950.
phase by the Volhard method. The latter method was used also for the determination of bromide when iron was present. In the ether phase, where all the iron was present as Fe(III), iron analyses were performed on the same aliquot used for the determination of bromide. After removal of the AgBr precipitate, the solution was acidified with HC1, the iron reduced by Zimmermann-Reinhardt procedure and titrated with Ce(1V) to the ferroin end-point. In the aqueous phase, the total iron concentration was determined by precipitation with ammonia, then dissolution in HCl and titration with ceric sulfate after a stannous chloride reduction. Foy very low iron concentrations aluminum was used as a carrier. The concentration of Fe(I1) was found by adding an excess of AgNOa to an aliquot of the aqueous phase and titrating immediately with ceric sulfate. The aqueous Fe( 111) concentration was found by difference. In the ether phase the hydrogen ion concentration was calculated as the difference between the bromide ion concentration and three times the ferric ion concentration. Water in the ether phase was determined by the Karl Fischer method. In the presence of iron, the Laurenee modification of the latter method was used.
Results and Discussion Extraction of HBr.-The distribution of HBr is shown in Fig. 1. The data of Chalkely and Williams' a t 13' are included for comparison. Above 4 M initial aqueous HBr concentration the solubility of diethyl ether in the aqueous phase increases rapidly with resulting increase in the volume of the aqueous phase and decrease in the volume of the ether phase. Above 6.7 M HBr only one liquid phase remains after mixing. Over the two-liquid-phase range of HBr concentration the equilibrium concentration of HzO in the ether phase decreases with increasing initial HBr concentration from 0.444 41 for water-saturated ether to 0.332 M at 3.01 M HRr and to 0.227 M a t 6.17 M HBr. As the water in the aqueous phase becomes insufficient to fully solvate the proton, a t the higher concentrations of HBr, water is withdrawn from the ether phase, the solubility of ether in the aqueous phase increasps, and conditions become favorable for solvation of (6) A. H. Laurene, Anal. Chem.. 24, 1496 (1952). (7) D. E. Chalkely and R. J. P. WilliarnP, J . Chem. Soc., 1920
(1955).
