At the beginning of the titration, the phenol is converted to the phenolate ion. Because the 3-MS solvent has little tendency to form hydrogen bonds, it does not compete with the association of the phenolate anion with the remaining phenol by hydrogen bonding. This increases the apparent acid strength of the free phenol while at the same time forming the more weakly acidic phenol-phenolate complex. Thus at the midpoint of the titration, there is an inflection for the completion of the free phenol titration and the beginning of the acid-anion complex titration. Behavior of this type is useful in studying the degree of hindering of certain acids. The titration of formic acid also shows definite evidence of complex formation as can be seen in Figure 5(b). The curve for the titration of an amine oxide in 3-MS with perchloric acid is shown in Figure 3(b). Again two inflections are obtained for a monofunctional compound, with the first inflection corresponding to one-half theory and the final inflection occurring at the theoretical end point. The titration curve indicates a remarkable difference in the basicities of the two species being titrated. The half neutralization potential (HNP) of the first specie indicates a base almost as strong as guanidine (pK, 13.6). The HNP of the second
base is close to that of pyridine whose pK, of 5.2 is approximately that expected for the amine oxide. A full explanation has not yet been developed for this behavior, but it may involve the formation of a base-cation complex (BHBf) in a manner analogous to the acid-anion complex formed in the titration of phenols. The double inflection for the amine oxides is a function of the hydrogen bonding capability of the titration solvent and also the ion association tendency of the anion of the acid titrant. Further studies are being made and will be the subject of another report. At their present cost, moderate purity, and limited availability, it is not expected that the sulfolanes will become widely used as solvents for routine titrations. However, because of the very large potential span available, their moderately high dielectric constant, and low hydrogen bonding capability, the sulfolanes will be useful for special applications and theoretical studies. RECEIVED for review July 27, 1967. Accepted September 14, 1967. Presented in part at the 20th Annual Summer Symposium on Analytical Chemistry, Claremont, Calif., June 1967.
High Resolution Proton Magnetic Resonance of Liquids Adsorbed QTP a Pyrogenic Silica J. H. Pickett and L. B. Rogers Department of Chemistry, Purdue University, Lafayette, Ind. 47907
NUCLEARMAGNETIC RESONANCE STUDIES of molecules adsorbed on solid surfaces have usually necessitated the use of broad-line or spin-echo techniques because of short spinspin relaxation times. For example, water adsorbed on silica has been studied by Zimmerman et al. ( I ) , by Woessner ( 2 , 3 ) , and by Kvlividze (4, and benzene adsorbed on silica has been studied by Woessner (5). Relaxation times reported by these authors would imply line widths in the range of 500 Hz. The only study of adsorbates on silica in which high resolution NMR spectra were observed is that reported by Karagounis (6), who found that mesitylene adsorbed on silica gave sharp resonance lines (twice the line width of the liquid) at a two-layer surface coverage and considerably broader lines as the coverage was increased to thirty layers. However, Karagounis gave no details about the type of silica used. Recent work in this laboratory has shown that high resolution spectra can be obtained for a variety of organic molecules adsorbed on a pyrogenic silica, but that other types of silica do not give similar results. EXPERIMENTAL
Apparatus. Spectra were taken at ambient temperature with a Varian A-60A NMR spectrometer. Reagents. The following silica adsorbents were studied : Cab-0-Si1 M-5 (7) and Aerosil 2491-380 (8), which are extremely fine, nonporous, pyrogenic (fumed) silicas; Quso F 22 (P), a low porosity, microfine, precipitated silica; and Davison Grade 81 (IO), a high porosity, precipitated silica gel, N o drying or other pretreatment was necessary. Procedure. To prepare the samples which had low values of 0 (0 = calculated number of adsorbed layers), the silica 1872
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was packed in a standard 5-mm 0.d. NMR sample tube and adsorbates were added as vapors or liquids, the same results being obtained in either case. However, for higher 6 (6 > 10) preparations, it was necessary to add liquid to a known weight of silica in a larger container, mix thoroughly, and allow several hours for equilibration before the mixture was added to a sample tube. Chemical shifts were measured relative to tetramethylsilane, which was added directly to the sample to give an internal reference. Line widths are reported as the full width at half-maximum height (FWHM). RESULTS
Preliminary studies with mesitylene adsorbed on two different types of silica at low surface coverage (0 = 2) showed broad lines (FWHM = 100 Hz) for Davison 81 silica, but narrow lines (FWHM = 4 Hz) for Cab-0-Si1 M-5. As more mesitylene was added, the lines got much broader (100 Hz) on Cab-0-Si1 and only slightly narrower (90 Hz) on Davison 81. Similar behavior was observed for benzene, acetone, and acetaldehyde adsorbed on these two silicas. (1) J. R. Zirnmerman, B. G. Holmes, and J. A. Lasater, J. Phys. Chem., 60, 1157, (1956); 62, 1157 (1958). (2) D. E. Woessner and J. R. Zimmerman,J . Phys. Chem., 67,1590 (1963). (3) D.E.Woessner, J. Chem. Phys., 39, 2783 (1963). (4) V. 1. Kvlhidze, Dokl. Akad. Nazik SSSR, 157, 158 (1964). (5) D. E. Woessner, J. Phys. Chem., 70, 1217 (1966). (6) G. Karagounis, Nature, 201, 604 (1964). (7) Cabot Gorp., Oxides Division, Boston, Mass. (8) Degussa, Inc.,Pigments Div., Kearney, N. J. (9) Philadelphia Quartz Co., Philadelphia, Pa. (10)Davison Chemical Co., Baltimore, Md.
120
100
80 c
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5 60 ? 40
20
0
10
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40
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Number of Layers Figure 1. Line width us. calculated number of surface layers for adsorbates on Cab-0-Si1 M-5 A . Cyclohexane B. Water
Detailed studies of line width as a function of coverage were then made for cyclohexane, water, and ethanol adsorbed on Cab-0-Si1 M-5. Data for cyclohexane are shown in Figure 1, where it can be seen that the lines were narrow for 0 < 20, significantly broader for 20 < 0 < 54, and narrow again for 0 > 54. Because cyclohexane has twelve equivalent hydrogens per molecule, it was possible to extend measurements down to 0 = 0.25, at which coverage the line width was only 6 Hz. For comparison purposes, the line width of pure liquid cyclohexane was 2 Hz. Figure 1 also shows data for water adsorbed on Cab-0-Sil; in this case, the
inability to achieve narrow lines at low coverage was probably due to exchange of the water hydrogens with hydrogens from hydroxyl groups on the silica surface. With adsorbed ethanol (absolute), the methyl and methylene hydrogen line widths followed a curve similar to that for cyclohexane, while the hydroxyl hydrogen line width followed the curve for water, Chemical shifts in the adsorbed state (on Cab-0-Si1 at low 0) were the same (within 0.1 ppm) as for the neat liquids, and no large changes in integrated intensities were observed. Of particular interest was the presence in the spectra of certain molecules of fine structure due to spin-spin coupling. Nearly complete resolution of ethyl group fine structure was observed for ethanol and 3-pentanone adsorbed on Cab-0-Sil M-5, although for molecules with smaller coupling constants and more complex spin-spin interactions, the resolution of fine structure was poorer. As illustrations, the spectra of 3-pentanone and allyl alcohol are shown in Figure 2, along with liquid spectra for comparison. With molecules such as pyridine and aniline, good resolution could not be achieved. This is in agreement with the observation of Karagounis (6) on adsorbed pyridine. He concluded that a change in the spin-spin interaction occurred upon adsorption. Actually, spin-spin does occur in the adsorbed state (on Cab-0-Sil), as illustrated above, but may not be resolvable if the coupling constants are too small. This is because line widths of molecules adsorbed on Cab-0-Si1 are greater than the corresponding line widths in the liquid state. Surprisingly, no fine structure was observed for any compounds adsorbed on the Aerosil silica, whose physical properties (high purity, low porosity, low moisture content, and extremely fine particle size) are reportedly similar to those of Cab-0-Sil. Neither could fine structure be obtained for molecules adsorbed on Quso F 22. Line widths for adsorbates on these two silica reagents were about 20 Hz (FWHM) at low 0 and increased to a maximum of about 100 Hz at higher 6 values. By contrast, line widths for molecules adsorbed on Davison 81 silica gel were nearly 300 Hz at low coverage, and decreased monotonically a5 0 was increased.
Figure 2. NMR spectra of 3-pentanone (upper left) and allyl alcohol (upper right) adsorbed on Cab-0-Si1 M-5 at 10-layer coverage. Below each of these spectra is the spectrum of the pure liquid VOL. 39, NO. 14, DECEMBER 1967
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When ethanol was adsorbed on this silica, only one broad peak was observed at 0 = 0.5, two peaks were visible at 0 = 2.0, and all three peaks appeared at 0 = 6. Apparently, the silica used by Karagounis was Cab-0-Si1 or a similar product, DISCUSSION
The narrow resonance lines observed for adsorbates on Cab-0-Si1 M-5 at low surface coverages are unusual and are probably best explained by postulating that the molecules are in a highly mobile state in which local magnetic fields are averaged. The broadening at higher coverages (0 = 20 to 54) has been explained by Karagounis (6), who assumed that polarization and orientation of the first few layers by the surface causes further layers to be built up with a lattice-like order. Such an ordered structure would cause line broadening because of the nonaveraging of local magnetic fields. Above about 50 layers the ordered structure apparently breaks down, and a more mobile, gel-like structure is formed. For example, a mixture of five parts (by weight) of cyclohexane and one part Cab-0-Si1 has the consistency of a stiff paste, but its cyclohexane resonance line is no wider than that of the pure liquid. The ability to obtain NMR spectra of molecules at fractional monolayer coverages is of considerable interest to those studying adsorption processes on silica surfaces. Woessner (5), using spin-echo techniques, found that a 3/4-statistical monolayer of benzene adsorbed on a high purity precipitated silica gel had, near room temperature, a spin-spin relaxation time of about three msec, which would correspond to a halfwidth on the order of 300 Hz. Contrast this with a value
of 4 Hz (obtained in this laboratory with a high resolution instrument) for the same concentration of benzene adsorbed on Cab-0-Si1 M-5. Clearly, it would be desirable to know what properties of pyrogenic silicas make them different from other silicas with respect to NMR behavior. Studies are now in progress to determine which structural and/or compositional factors influence the line widths of substances adsorbed on pyrogenic and precipitated silicas. These results may promote the usefulness of NMR for the characterization of silica surfaces and for the study of molecules in the adsorbed state. I n addition, they should facilitate the quantitative collection and direct NMR identification of fractions from gas chromatography along the lines proposed by Amy e f a!. ( I I ) , who used liquid impregnated supports to trap fractions for mass spectrometric analysis. ACKNOWLEDGMENT
The authors thank J. W. Amy, W. E. Baitinger, and J. R. Barnes for their assistance with the preliminary measurements and for their continuing guidance and encouragement. RECEIVED for review July 26, 1967. Accepted September 13, 1967. Work supported by a National Science Foundation Traineeship (to JHP) and by U.S. Atomic Energy Commission Contract AT(11-1)-1222. (11) J. W. Amy, E. M. Chait, W. E. Baitinger, and F. W. McLafferty, ANAL.CHEM.,37, 1265 (1965).
aratbn of ) by Anion Exchange Fletcher L. Moore Analytical Chemistry Division,Oak Ridge National Laboratory, Oak Ridge, Tenn.
ONEOF THE MOST arduous tasks in transplutonium chemistry is the separation of berkelium from cerium (1-5). Methods based on liquid-liquid extraction of berkelium(1V) with di(2-ethylhexy1)orthophosphoric acid (3) or 2-thenoyltrifluoroacetone (5) are highly selective but do not separate berkelium from cerium because of their very similar oxidation potentials. Two recent papers describe the separation of berkelium(II1) from cerium(II1) by extraction chromatography using di(2ethylhexy1)orthophosphoric acid in hydrochloric acid (6) and nitric acid (7) systems. Most interestingly, during recent research studies the author discovered a simple, sharp, quantitative separation of berke(1) S. G. Thompson, B. €3. Cunningham, and 6. T. Seaborg, J. Am. Chem. Sac., 72,2798 (1950). (2) G. H. Higgins, “The Radiochemistry of the Transcurium Elements,” NAS-NS-3031 (1960). (3) D. F. Peppard, S . W. Moline, and G. W. Mason, J. Inorg. Nucl. Chem., 4, 344 (1957). (4) F. L. Moore and W. T. Mullins, ANAL.CHEM., 37,687 (1965). ( 5 ) F. L. Moore, Ibid., 38, 1872 (1966). (6) J. Kooi and R. Boden, Radiochim. Acta, 3, 226 (1964). (7) F. L. Moore and A. Jurriaanse, ANAL.CHEM., 39,733 (1967).
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lium(1V) from cerium(1V) in nitric acid solution. These elements had not been separated previously when present in the tetravalent oxidation states, EXPERIMENTAL Apparatus. An internal sample methane proportional counter was used for alpha and beta counting at voltage settings of 2900 and 4300, respectively. A NaI well-type scintillation counter, 1.75 X 2 inches, was used for gamma counting. A glass tube, 5-mm i.d. and 150 mm in length was drawn to a tip at one end. A small glass wool plug was inserted in the tube to retain the resin. Reagents. Dowex 1-x4 (100-200 mesh) strong base anion exchange resin, available from Bio-Rad Laboratories, Richmond, Calif., in the chloride form, was converted to the solunitrate form by washing several times with 5M “Os tion. Column Preparation. By use of a medicine dropper, the resin slurry was added to the glass tube to a height of 50 mm. Fifty milligrams of lead dioxide were carefully mixed into the resin bed with a platinum or stainless steel wire, being careful to eliminate air bubbles. The column should not be allowed
