Quantitative Analysis by Phosphorus.31 Nuclear Magnetic Resonance Spectrometry James G : Colson and David H ; Marr Hooker Chemical Corporation, Research Center, Grand Island, N . Y . 14307 ALTHOUGHTHERE HAVE BEEN numerous publications using 31P nuclear magnetic resonance (NMR) spectrometry in the structural elucidation of phosphorus-containing molecules (Z-3), its application as a quantitative tool has been quite restricted (4-7). We have found the method to be extremely useful as an analytical technique in specific instances. In those cases where direct comparison is possible, the 31PNMR approach is much more rapid, more informative, and just as accurate as other methods. The usefulness of 31PNMR as a tool for quantitative analysis depends on the well known fact that the area under the peaks of an NMR spectrum is proportional to the relative number of 31Pnuclei producing it, providing certain criteria are met. These necessary conditions for lH and l9F nuclei have been amply discussed in the literature (8-10) and in general also apply to the 31Pnucleus. The major difference in the 31P system is its low sensitivity relative to the hydrogen nucleus. The lowest detectable concentration of any component is about 1.0% of content with a precision of 1 4 . 0 z , considerably higher than normally found in lH analyses. Thus, the higher power levels necessary to obtain adequate S/N ratios in the 31Psystem increase the likelihood of saturation. In addition, the low S/N ratio requires replicate measurements to give reproducible results. However, with care, the technique is quite accurate and reliable. We have applied 31P NMR quantitative analysis to three separate systems : the ortho and condensed phosphates which have been previously studied (2, I I ) and for which a number of chemical methods including ion exchange and paper chromatography exist ; the analysis of tris(hydroxymethy1)phosphine (THP), handled only with difficulty by other methods; and, finally, a system of mixed thiophosphates for which, to our knowledge, no adequate analytical procedure exists. EXPERIMENTAL Apparatus. All spectra were obtained on a Varian Associates HA-100 NMR spectrometer operating at 40.5 MHz in a field of 23,490 gauss with output to a Varian G-14 strip chart recorder. (1) M. M. Crutchfield, C. H. Dungan, J. H. Letcher, V. Mark, and J. R. Van Wazer, “Topics in Phosphorus Chemistry,” Vol. 5 Interscience, New York, N. Y . ,1967. (2) M. M. Crutchfield, C. F. Callis, R. R. Irani, and G. C. Roth, Irjorg. Chem., 1,813 (1962). (3) M. M. Crutchfield and R. R. Irani, J. Amer. Chem. SOC.,87, 2815(1965). (4) E. F. Fluck, J. R. Van Wazer, and L. C. D. Groenweghe, ibid., 81,6363 (1959). (5) K. Moedritzer, G. M. Burch, J. R. Van Wazer, and H. K. Hofmeister, Inorg. Chem., 2,1152(1963). (6) J. R. Van Wazer and L. Maier, J . Amer. Chem. Soc., 86, 811 (1964). (7) S. Norval and J. R. Van Wazer, ibid., 88,4415 (1966). ( 8 ) C. A. Reilly. ANAL.CHEM., ?0,839(1958). (9) J. L. Jungnickel and J. W. Forbes, ibid., 35,938 (1963). (10) P. J. Paulsen and W. D. Cooke, ibid., 36, 1713 (1964). (11) C. F. Callis, J. R. Van Wazer, J. N. Schoolery, and W. A. Anderson J . Amt r . Chem. SOL.,79,2719 (1957).
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18 9
4 8
Figure 1. Condensed phosphates
Procedure. Power levels giving the best compromise between satisfactory S/N and minimum saturation together with sweep rates in the range of 2.0 to 4.0 Hz/sec were normally used. Commonly, the faster sweep rates were employed for electronic integration with the chemical shifts being obtained at the slower rates. Chemical shifts are reproducible to *0.5 ppm and are somewhat dependent on impurity levels. The samples were examined as either the neat liquids (phosphines) or as saturated aqueous solutions (phosphates and thiophosphates) in 5.0-mm 0.d. thin wall tubes with spinning, and referenced to an external capillary of 85 HaPO4. Integration was performed electronically on the non-referenced samples with the V-3521A integrator decoupler unit which also provided base-line stabilization. In certain cases very slow sweep “spread out” scans were obtained to allow mechanical integration with a planimeter. The condensed sodium phosphates were those supplied by the ASTM D-12 subcommittee T-4, whereas the phosphine and thiophosphate samples were typical mixtures obtained during laboratory preparation. For the analyses, an average of at least three and most often five integrals was obtained.
z
RESULTS AND DISCUSSION
The analysis of the condensed phosphate system has been previously presented (2, ZI). It is repeated here for a comparison of the accuracy and reproducibility of the 31P technique on ASTM standard samples with the best available chemical methods. Two different integration methods were attempted on two “standard” samples and the results are compared with the “known” analyses obtained by paper
ANALYTICAL CHEMISTRY, VOL. 45, NO. 2, FEBRUARY 1973
Na. PSq
Figure 2. phates
alp NMR spectrum of sodium thiophos-
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Table I. Comparison of Phosphate Analyses Ortho Pyro Tripoly Trimeta ASTM G analysish 1,2 14.8 82.0 2.0 1.0 17.0 80.0 NMR integrator 2.0 1.0 16.0 Planimeter 81.0 2.0 Chemical shifta 0 5.5 4.8(18.9) 25.1 0.8 7.4 ASTM F analysisb 91.1 0.6 ... 6.5 NMR integrator 93.0 0.5 Planimeter ... 6.8 92.5 0.7 ppm with respect to 85
