Anal. Chem. 1982,5 4 , 2378-2379
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In summary, we have demonstrated that resonance ionization mass spectrometry can be used in the analysis of (aqueous) samples of common analytical interest. Matrix and charge exchange effects do not seriously degrade the selectivity of the process, and ionization process is isotopically nonselective.
(9) Inghram, M. G.: Chupka, W. A. Rev. Sci. Instrum. 1953, 24, 518-520. (IO) Martin, W. C.; Zalubas, R.; Hagen. L. "Atomic Energy Levels-The Rare-Earth Elements"; National Bureau of Standards: Washington, DC, 1978: DD 398-403. (11) Keller, R. A,'; Engleman, R., Jr.; Zalewski, E. F. J. Opt. SOC. Am. 1979, 69,738-742. (12) Keller, R. A.; Zalewski, E. F. Appl. Opf. 1980, 19, 3301-3305.
C. M. Miller N. S. Nogar*
ACKNOWLEDGMENT The technical assistance of D. J. Rokop and several discussions with R. A. Keller are gratefully acknowledged.
Group CNC-2, Mail Stop G738 Los Alamos National Laboratory Los Alamos, New Mexico 87545
LITERATURE CITED Whltaker, T. J.; Bushaw, B. A. Chem. Pbys. Left. 1981, 79,506-508. Bushaw, B. A.; Whitaker, T. J. J. Chem. Phys. 1981, 74,6519-6520. Hurst, G. S.;Payne, M. G.; Kramer, S. D.; Young, J. P. Rev. Mod. Phys. 1979, 5 1 , 767-819. Hurst, G. S.;Payne, M. G.; Kramer, S. D.; Chen, C. H. Phys. Today 1980, 3 3 , 24-29. Young, J. P.; Hurst, G. S.; Kramer, S. D.; Payne, M. G. Anal. Chem. 1979, 51, 1050A-1080A. Hurst, G. S. Anal. Chem. 1081, 53, 1448A-1456A. Beekman, D. W.: Callcott, T. A.; Kramer, S.D.; Arakawa, E. T.; Hurst, G. S. I n t . J . Mass Spectrom. Ion Phys. 1980, 34, 89-97. Mayo, S.: Lucatorto, T. B.; Luther, G. G. Anal. Chem. 1982, 5 4 , 553-556.
A. J. Gancarz W. R. Shields Group CNC-7, Mail Stop E514 Los Alamos National Laboratory Los Alamos, New Mexico 87545 RECEIVED for review June 21, 1982. Accepted July 16, 1982. The support of the Department of Energy under the auspices of the Los Alamos National Laboratory is gratefully acknowledged.
Composition Determinations of Liquid Chloroaluminate Molten Salts by Nuclear Magnetic Resonance Spectrometry Sir: Some binary mixtures of aluminum chloride and selected organic chloride salts form melts that are liquid considerably below room temperature. Two types of these mixtures have been studied in recent years: one having alkylpyridinium (I) as the cation (1-3) and another employing a dialkylimidazolium (11)cation (4). These molten salts are 0
-
i,tL R ,
R
I
/N,L;,
R
I1
aprotic, anhydrous ionic liquids that may be useful for electrochemistry, spectroscopy, and synthesis. The melts are prepared by simply mixing AlC13 and the chloride salt of I or 11, resulting in a clear liquid. Many of the physical and chemical properties of the melts depend markedly on composition, i.e., the relative proportions of AlC13 and the organic chloride salt. The composition is usually expressed as the apparent mole fraction of AlC13 (XAQ),although no molecular A1C13 or AlzCls species apparently exist in these melts (5). Important compositiondependent properties of these melts include their acid-base characteristics (4,6, 7) and melting points. For example, melts prepared from I1 (where R = methyl and R' = ethyl) exhibit melting points of 8 "C and -98 OC for XMQ= 0.50 and 0.66, respectively. Clearly it is important to know the composition of a given melt. An approximation of the melt composition may be made from the amount of the ingredients used to prepare the sample. However, impurities in the starting materials and changes in the composition during the melt preparation procedure often result in the need for a more accurate determination of composition. T o date, the most accurate method for composition determination in chloroaluminate melts is by a tedious potentiometric titration of the melt (3-5),where an equivalence point is observed at precisely XAICls = 0.5. We report here a simple, rapid and nondestructive technique for composition determinations in chlo-
roaluminate molten salts having organic cations such as I or 11. We have applied the method to chloroaluminate melts prepared from 1-(1-buty1)pyridinium (BuPy) and l-methyl3-ethylimidazolium (MeEtIm) chlorides.
EXPERIMENTAL SECTION Apparatus. Nuclear magnetic resonance spectra were recorded at 60 MHz on a Varian T-60A or a Hitachi Perkin-Elmer R-24P
spectrometer. Reagents. Chloroaluminatemelts were prepared from purified AlC1, and 1-(1-buty1)pyridinium chloride (6) or l-methyl-3ethylimidazolium chloride (2) as described elsewhere. Procedure. The NMR spectra were obtained from samples contained in 5-mm tubes, which were loaded in dry argon or nitrogen atmosphere gloveboxes. Either an internal standard of chlorotrimethylsilane or an external standard of MezSO was added, and the tubes were securely capped to exclude moisture. The temperature was maintained constant during the NMR measurements (35 f 1 OC for type I melts and 34 f 1 OC for the type I1 melts).
RESULTS AND DISCUSSION Robinson et al. (8)observed that the proton chemical shifts of I (R = 1-butyl) in chloroaluminate molten salts varied as the dielectric of the melt was changed by addition of a solvent. We have performed similar experiments, but we in addition measured proton chemical shifts as a function of XMQin the A1Cl3-BuPy melt and one having a type I1 (MeEtIm) cation. The chemical shifts of the ring protons changed significantly in both types of melts, especially in the compositions where XAlcls< 0.5. Figure 1 shows the composition dependence of the chemical shifts for the two types of melts. Data for the particular proton shift undergoing the largest chemical shift change are shown in each case. For the compositions XAQ < 0.5 the NMR chemical shifts are sensitive indicators of melt composition and may be used to determine the composition of an unknown melt. This is a valuable finding, since the potentiometric methods for determining melt composition that involve measurements at an aluminum electrode are not ap-
0003-2700/82/0354-2378$01.25/0 C 1982 American Chemical Society
Anal. Chem. 1982, 5 4 , 2379-2380 ..
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5.8
U L 0.34 0.38 OA2 0.46 050 0.54 0.58 A IC I
0.62
c
0.66
M OLEFR ACT 10N
Flgure 1. Chemical shift of 6-2 and -6 protons in 1-(1-buty1)pyridinium chloride:AICI, (A)at 35 O C , and C-2 proton in I-methyl-3-ethylimidazoiium:AICI, (0)mixtures at 34 OC as a function of AICi, mole fraction.
plicable for XAICla < 0.5.. Apparently aluminum is oxidized by the melt cation at thlese compositions ( 4 , 6, 7). The changes in chemical shifts may be explained by assuming that each organic cation is closely associated with two anions and is in fast exchange on the NMR time scale. k detailed analysis of the effect of such ion pairing interactions has been made and will be reported separately. A reasonably good fit to the data may be made by a quadratic fit to eq 1,
where h o h d is the measured chemical shift, and a, b, and c are fitted parameters. The principal source of error in the method is in the precision of the NMR chemical shift measurement. A f0.05 ppm precision is readily achieved with most spectrometers, which corresponds to fO.OO1 mole fraction A1C13 in basic melts. The method applies well only to the Xhc13C 0.5 (basic) melts, since the acidic compositions show a less pronounced dependence of chemical shifts on composition. Note that the range of compositions shown for the pyridinium melt is not as extensive as for the imidazalium melt. This is because pyridinium mel1,s are liquid only in the range 0.44
