Table 111. pK. for Benzoic and Hydroxybenzoic Acids
Acid Benzoic p-Hydroxybenzoic m-Hydroxybenzoic o-Hydroxybenzoic
PK 4.20 4.48 4.08 2.97
the ion exchange column before either of these compounds. Other factors such as molecular configuration, adsorption, solubility, etc., besides p H probably have some effect on the separation. Other acids having pK, values greater than 3 should be separated by this technique. ACKNOWLEDGMENT
their separation is much more difficult. The separation of 0.8 mg. of p-hydroxybenzoic acid from 78.4 mg. m-hydroxybenzoic acid is a severe test for this separation. If more nearly equal quantities were ion exchanged, the separation would be much greater. Although benzoic acid has a pK, value between that of p-hydroxybenzoic and m-hydroxybenzoic, it emerges from
The aiithors thank T. E. Majewski arid G. C. AIattson for preparation of the lmminnteci snlicvlanilides.
(3) Cats, H., Onrust, H., Chem. W e e k blad 54,456 (1958). (4) Cooke, A. C., New Zealand J . S c i 1 , 412 (1958). (5) Halver, C. V. D., J . Assoc. OBc. Agr. Chemists 43,593 (1960). ( 6 ) Joux, J. L., Ann. Fals. Fraudes 50, 205 (1957). (7) Lederer, M., Australian J . Sci. 1 1 , 208 (1949).
(8) Lemaire, H., Schramm, C. H., Cahn, A., J . Pharm. Sci. 50,831 (1961). ( 9 ) Logie, D., Analyst 82,563 (1957). (10) Marvel, C. S., Rands, R. D., Jr., J . Am. Chem. SOC.72,2642 (1950). (11) Mitchell, L. C., J . Assoc. O j i c . A g r . Chernzsts 40,592 (1957). (12) Plapp, E. W.,Casida, J. E 4 ~ ~ 1 , . CHEM30,1622 (1958). ( 1 3 ) Shelley, R. S . , Uniberger, C. J., ~
LITERATURE CITED (
S., Williams, R. J. R., Tiselius, A., Acta Chem. Scand. 6 ,
1) Alm, R.
826 (1952). (2) Busch, H., Hurlbert, R. B., Potter, V. R., J . Biol. Chem. 196,717 (1952).
Zbid., 31,593 (1959).
(14) Skelly, N. E.,Ibzd., 33, 271.(1961). (15) Thommes, G. A., Lernmger, E.,
Ibid., 30,1361 (1958).
RECEIVED for review March 18, 1963. hccepted July 10, 1963.
Quantitative Analysis of Aspirin, Phenacetin, and Caffeine Mixtures by Nuclear Magnetic Resonance Spectrometry DONALD P. HOLLIS Spectroscopy Applications laboratories, Instrument Division, Varian Associates, 6 1 7 Hansen Way, Palo Alto, Calif.
b A procedure is described utilizing NMR spectrometry in the rapid quantitative analysis of mixtures of aspirin, phenacetin, and caffeine. Several known mixtures have been analyzed as well as some commercial preparations. The average deviations from the correct results were: aspirin, 1.1 %; phenacetin, 2.2%; and caffeine, 3.2%. The time required for the analysis is about 20 minutes.
C
has been dpvoted to the problem of developing suitable methods for the routine quantitative analysis of commercial analgesic preparations containing aspirin, phenacetin, and caffeine (APC). Procedures have been published utilizing separation by extraction, partition chromatography, and ultraviolet, visible, and infrared spectrophotometry. Conners cites a number of references on this subject (1). The method of the National Formulary (2) is of the extraction type but is not suitable for routine quality control work because of the time required to complete the analysis. Of the other procedures the infrared method of Parke et al. (3) seems to be most advantageous in terms of accuracy and speed. ONSIDERABLE EFFORT
1682
ANALYTICAL CHEMISTRY
Nuclear magnetic resonance spectrometry (NMR) can also be used very conveniently to analyze APC mixtures. The speed and accuracy of the S M R method to be described is about the same as that of other spectrometric methods, but it has the advantages of being more direct in the sense that a separate analytical peak of known origin is present for each component and that no calibration curves are required since the absorptivities for the protons giving rise t o the various analytical peaks are constant and unaffected by solvent or solute interactions. I n common with other spectrometric methods, no separation of the components is required for NMR analysis. The basic principle which permits the use of magnetic resonance absorption as a quantitative measure of a particular substance is that the signal strength is proportional to the number of magnetic nuclei. The dependence of the signal intensity on the relaxation times TI and Tz,the intensity of the driving radio frequency field, and the field sweep rate have been discussed in the literature (6, 6). I t should be remembered that the signal strength is also inversely proportional to the absolute temperature. In the present work the fact that the instrumental conditions chosen for the analyses are such that the
integrated intensities of the peaks employed are proportional to the number of protons present was demonstrated by the accuracy with which the composition of known standard mixtures could be determined. EXPERIMENTAL
Apparatus. Spectra were obtained a t 60 mc. per second using the Varian A-60 analytical KMR spectrometer, and in one case a t 100 mc. per second using the T’arinn HR-100 NMR spectrometer. Procedure. Weigh accurately into an A-60 NMR sample tube about 60 mg. of the carefully powdered sample. Using a micropipet, introduce exactly 0.500 ml. of CDClJ into the tube. It is important to avoid exceeding the solubility of aspirin which is least soluble of the three components. Cap the sample tube and shake, and gently warm the sample to effect complete solution of the APC. Binder materials such as starch and lactose will not dissolve but they do not interfere with the analysis and can be left in the sample tube. The sample is placed in the -4-60 spectrometer and the YAIR spectrum is obtained, the instrument gain, RF field, and sweep rate being adjusted to give a convenient integral presentation. The integral of each peak of interest should be run several times and the average
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value taken. To obtain an absolute integral calibration for a particular set of instrument settings, a standard sample containing a known concentration of one of the components, or for that matter a known concentration of any proton-containing compound, is substituted for the sample to be analyzed and its integral is taken. In this work a sample containing 50 mg. of caffeine per ml. of solution served as a standard.
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RESULTS AND DISCUSSION
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