Water Content of Tributyl Phosphate by High Resolution Nuclear Magnetic Resonance B. 8. MURRAY' and R. C. AXTMANN Savannah River laboratory,
E. 1.
du Pont de Nemours 8,
b Tributyl phosphate (TBP) was analyzed for water with a high resolution nuclear magnetic resonance (NMR) spectrometer. The method involves the measurement of the chemical shift of the water proton resonance with respect to the methyl proton resonance in tributyl phosphate; hence, it requires no external standard. In contrast to earlier NMR analytical methods, the precision of the present technique does not depend on gain stability and gain linearity in the spectrometer, as a frequency separation rather than an intensity is measured.
T
HE APPLICATIOS+ of nuclear niagnetic resonance (SMR)spectroscopy to problems in chemical structure, equilibrium. kinetics. and molecular and solid state physics have been steadily increasing ( 2 ) The number of reported S l I R methods for quantitative analysis, hon ever, has remained small for reasons recently discussed in detail by Reilly ( 2 1 ) . Most of the XMR analytical methods that have been reported employ lorn resolution or "wide-line" spectrometers which gire the derivative of the absorption signal. The accuracy of each wideline method depends upon the proportionality between the number of the nuclides in the sample and the signal I
Present address, Department of Chemistry, Montana State College, Bozeman, hlont.
Co., Inc.,
Aiken,
S. C.
recorded by the spectrometer. The constancy of that proportionality, in turn, depends on many factors ( I I ) , but chiefly on amplifier gain and linearity and on reproducibility of the sample geometry. I n most of the \\-ide-line methods. frequent substitution of a knonn saniple or a calibrated electrical signal is required to keep the precision of the deterniinations within reasonable bounds (6). High resolution NMR spectroscopy, on the other hand, has been applied to the differential determination of the oxyacids of phosphorus b y Callis et al. (4, but Tvith low accuracy. As in the case of the n ide-line n-ork, the intensity of the signal from an unknown had to be compared n i t h a standard in order to make an absolute determination. In the present work a high resolution spectrometer n as also used, but the frequency of a signal rather than its intensity n as compared with that of a standard. The standard signal was furnishfd by the u n k n o n itself rather than by an added reagent or an evternal standard. THEORY
Tributyl phosphate (TBP) dissolves water to about a 1 to 1 mole ratio; there is optical evidence of hydrogen bonding between the water and the phosphoryl oxygen of tributj 1 phosphate. Alcock et al. reported a shift of 16 em.-' toward l o m r frequency
in the P=O stretching frequency n hen water is added to tributyl phosphate ( I ) . Geddes found the same shift in the isoamyl alcohol-tributyl phosphate system and, in addition, a shift to higher energy for a band a t 990 em. -1 n hich he bond (7'). had assigned to the P-0Shuler (12)found a neir absorption band a t 5180 em.-' in a solution that was equimolar in tributyl phoiphate and water. A number of investigators haxe studied the K l I R chemical shift accompanying hydrogen bonding in systems other than tributyl phospliatenater ( 3 , 8, 9 ) . I n every case the breaking of the hydrogen bonds is accompanied by a shift of the proton resonance to higher fields. For nater, infinitely diluted in a solvent n ith n hich it does not interact-e.g.. benzenethe shift from the position of the pure n ater resonance, divided by the applied field or frequency (IT hichever is used to express the shift), is approximately 4.0 x 10-6 ( 5 ) . The dilution results in nearly all of the hydrogen bonds being broken, since Ogg found an equiralent shift for the water-steam transition (IO). I n the present case, the proton signal from n ater, extrapolated to infinite dilution in tributyl phosphate, is displaced from the pure nater signal to higher fields by 2.0 X 10-6. This smaller shift is interpreted to mean that, in contrast to the benzene-water system ( 5 ) . the nater does react with the solvent. but that the resultant bonds
Methyl RotOnS
n.n I
I
I
-119
-09
Cycles/sec.
lncrmcinn Field +
0
Figure 1. Nuclear magnetic resonance spectrum of tributyl phosphate containing 1.7% water
450
ANALYTICAL CHEMISTRY
-110
Cycledsec. Increasing Field +
0
Figure 2. Nuclear magnetic resonance spectrum of tributyl phosphate saturated with water
are somewhat iveaker than the waterto-water hydrogen bonds. EXPERIMENTAL
The S l I R spectrometer was a Varian Associates Model T'4300-B with a sample spinner, a 12-inch electromagnet, and a Model YK3506 superstabilizer for the magnetic field. -411 spectra were obtained a t 22' to 24" C. with the spectrometer operating a t 40 Me. The magnetic field was varied through resonance with a Varian Model T'K3507 sloiv sweep unit. The spectra vvere recorded on a Varian GI0 recorder that was calibrated at the time of each measurement b y a sideband technique ( 3 ) . The recorder calibration required frequencies from 80 to 120 c.P.s., which were provided by a Hewlett-Packard Model 200CD audio oscillator whose output was monitored by a Hewlett-Packard Model 522B frequency counter. Samples n-ere prepared b y diluting water-saturated tributyl phosphate (c. P., Eastman) with dry tributyl phosphate or by adding water to dry tributyl phosphate. The samples n-ere standardized b y Karl Fischer titration. The sample vials used in the spectrometer were made from 5-mm. borosilicate glasq tubing. The chemical shifts, Av, were defined as the separation in cycles per second of the water resonance from the methyl proton resonance of tributyl phosphate. Av is negative for applied fields less than that for the methyl resonance. On this scale the proton resonance for pure water occurs a t approximately - 160 C.P.Q. DISCUSSION AND RESULTS
Typical tributyl phosphate spectra are displayed in Figures 1 and 2 . I n
both figures the large, sharp peak a t the extreme right of the recording trace is due to the methyl protons. The broad, partially resolved hump to the left of this peak arises from most of the methylenic protons, nhile the quartet at the left of the trace comes from those methylenic protons that are attached to the carbon atoms immediately adjacent to oxygen atoms. With tributyl phosphate containing 1.7 weight water, the water peak occurs approximately midway between the t\vo methylenic proton groups (Figure 1). Figure 2, the spectrum for water) nearly saturated (6.2 weight 70 tributyl phosphate, shon s the water proton signal coincident n ith one line of the quartet. I n this case the signal has both increased in intensity and shifted to a loner field strength. The chemical shift of the n ater proton resonance n i t h respect to the methyl ' measpeak from tributyl phosphate n a, ured for 11 samples whose n-ater concentrations ranged from 0.44 to 6.3 n eight 7c. At least five determinationwere made on each sample. The results. nhen plotted, show a linear relationship betn een the chemical shift and the concentration of nater, n i t h a slope of 4.9 c.p.s. per per cent nater. The precision of individual measurements was 0.65 c.p.s. ( n = 59, p = 95), M hich n as equivalent to 0.13 weight yowater. This method of determining the water content of tributyl phosphate is nondestructive and requires only 2 minutes per determination once the spectrometer is set up. As the technique does not involve intensity measurements, the stability and linearity of the radio-
frequency amplifiers have no effect on the results. The peak height or the area under the proton resonance peak also can be used as a measure of water concentration, b u t the results are less accurate b y about a factor of five. The method is limited, in general, to a two-component system. The presence of nitric acid or any compound that either complexes the tributyl phosphate or reacts with the water would make the analysis invalid because such reactions also shift the position of the nater peak. LITERATURE CITED
S. S., Healyt T. V., Kennedy, J., McKay, H. A. C., Trans. Faraday SOC. 52, 39 (1956). ( 2 ) Ann. Rea. Phys. Chem., annual reviews of nuclear magnetic. resonance spectroscopy published since 1954. (3) Arnold, J. T., Packard, 11. E.. J . Chem. Phys. 19, 1608 (1951). (4) Callis, C. F., Van Kazer, J. R., Shoolery, J. N., ANAL. CHEV.28, 269 (1956). (5) Cohen, A. D., Reid, C., J . Chenz. Phys. 24,790 (1956). 16'1 Elsken. R. H.. Shaw. T. 31.,ASAL. CHEW27,290 (1955). ' (7) Geddes, A. L., J . Phys. Chenb. 58, 1062 (1954). (8) Gutowskv, H. S., Saika, A., J. Chem. Phys. 21,1688 (1953). (9) Huggins, C. &I., Pimentel, G. C., Shoolery, J . S . , J . Phys. Chem. 60, 1311 (1956). (10) Ogg. R. A., Helv. Phys. Acta 30, 89 (1957). (11) Reilly, C. A,, ASAL. CHEY.30, 839 (1958). (12) Shuler, W, E., E. I. du, Pont de Semours & Co., Savannah River Laboratory, .liken, S. C., private communications. RECEIVEDfor review August 13, 1958. Accepted October 23, 1958. Information developed during work under contract AT(O7:2).-1 with the U. S.Atomic Energy Commission. (1) Alcock, K., Grimley,
\
I
Microdetermination of Ammonium and Protein Nitrogen GORDON FELS and ROGER VEATCH Radioisotope Service, Veterans Administration Hospital, Hines, 111.
b A micromethod for the colorimetric determination of protein nitrogen is described for the estimation of 1 y as ammonium nitrogen with quantitative accuracy. The use of 0.5M citrate buffer stabilizes the color against pH effects and minimizes the neutral salt effect.
N
is used as a n analytical reagent for the colorimetric determination not only of amino acids ( 5 ) but also for other amine derivatives (3). Because the reaction takes place IXHYDRIK
equally n ell with ammonia, resulting in the same product (5), it suggests that ninhydrin could be used with advantage in this instance. Boissonnas and Haselbach (1) have utilized the reaction for the determination of protein nitrogen but demonstrated no quantitative recovery. The sensitivity of the colored reaction product t o p H and salt concentration would make such evidence mandatory, especially following protein digestion. Modifications of the original procedure (5) are presented which overcome these obstacles.
EXPERIMENTAL
Reagents.
Sinhydrin
reagent,
1 . O M citrate buffer, 26.26 grams of citric acid (CsHsOi.H20), and 0.4 grams of tin chloride dihydrate. (SnC17. 2H20) are dissoh-ed in 245 ml. of 1 S sodium hydroxide. Thc p H i, adjusted to 5.0 with several drops of 1 0 s sodium hydroxide and the total volunic. is brought to 250 ml. with distilled water. Four grams of reagent grade ninhydrin are dissolved in 100 nil. purified methyl Cellosolve. Standard aiiimonium sulfatc. Exactly 94.3 mg. of dried reagent grade ammonium sulfate are dissolved in 100 VOL. 31, NO. 3, MARCH 1959
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