solution path length of the drop is not reproducible and is somewhat difficult to measure. For this reason, quantitative spectrophotometric determinations cannot be made until suitable methods for determining the exact thickness of the pendent drop are fully evaluated. The apparatus described has been used for obtaining the spectra of nickel, cobalt, trivalent chromium, and quadrivalent uranium fluorides when dissolved in lithium fluoride-sodium fluoride-
LITERATURE CITED
potassium fluoride (46.5-11.5-42 mole %), melting point 459' C. The results of these and other spectrophotometric studies in molten fluoride salts will be the subject of later reports.
(1) Boston, C. R., Smith, G. P., J . Phys. Chem. 62,409 (1958). (2) Gruen, D. M., McBeth, R. L., Ibid., 63, 393 (1959).
(3) Sundheim, B. R., Greenberg, J., J . Ch.em. Phys. 28, 439 (1958).
ACKNOWLEDGMENT
RECEIVED for review May 21, 1959. Accepted July 27, 1959. Work carried out under contract No. W-740hng-26 a t Oak Ridge National Laboratory, operated by Union Carbide Corp. for the U. S. Atomic Energy Commission.
The authors acknowledge the contribution by L. L. Merritt, Jr., who suggested the possibility of confining molten fluoride salts as pendent drops.
Nuclear Magnetic Resonance Method for Analysis of Polyphosphoric Acids JOE C. GUFFY and GERALD R. MILLER' California Research Corp., Richmond, Calif.
b A rapid and accurate nuclear magnetic resonance method for analyzing polyphosphoric acid in which the answer may be expressed as per cent phosphorus pentoxide, per cent phosphoric acid, etc., is described. The results checked closely with those predicted from the NMR chemical shift data and the detailed paper chromatographic analyses in the literature. The method is at least as accurate as any wet method and is faster.
A
by Popp and McEwen (3)and a book by Van Wazer ( 4 ) make a n excellent introduction to the literature of the physical and chemical nature of the polyphosphoric acids. Paper chromatographic methods of analysis have given by far the most detailed picture of the chemical nature of these systems (I, 2 ) . Nuclear magnetic resonance (NMR) examination of polyphosphoric acids and their salts resulted in finding (6,6)three well separated resonance peaks. These were assigned to isolated orthophosphate, and to end and to middle phosphate groups in linear polyphosphoric acids. The chemical shifts between the three types of phosphorus atoms were large and easily measured: By considering the NMR and paper chromatographic results @), it was concluded that an accurate and rapid NMR quantitative analysis could be developed. REVIEW
APPARATUS
The spectrometer used was a Varian Associates high resolution Model V4300 instrument operating a t 16.2 Mc. Present acldrea~Department of Chemand Chemicai Engineering, Univer-
$try
mty of Illinois, Urbana, Ill.
and about 9400 gauss. I t was equipped with a superstabilizer which gave a sufficiently stable magnetic field throughout all the work. Two sample-tube sizes were used; a large size made from 10-mm. outside diameter Corning 774 Pyrex tuting (nonspinning) and a small size from thin-walled 5 m m . outside diameter Corning 774 Pyrex tubing (spinning). These tubes were specially selected for a constant internal diameter so that the filling factor remained constant. For the standard samples, these tubes were sealed immediately after filling. This procedure gave a calibration series that showed no change over a &month period. Pyrophosphoric acid will crystallize from some samples slowly (106 to llOyo range) but will redissolve readily on warming to give the original composition. Sample tubes for the application of the method were made from 10-mm. outside diameter glass tubing (nonspinning) equipped with a standard taper 12/30 ground-glass joint and cap. REAGENTS
The polyphosphoric acids were prepared in three ways. Two series were made by heating calculated amounts of Baker's analyzed reagent 85% phosphoric acid and Baker's anal zed reagent phosphorus pentoxide. &e series was prepared by diluting and heating a 116% acid supplied by the A. R. Maas Chemical Co. (Richmond, Calif.), a Division of Victor Chemical Works. A few samples above 119% were made by heating a ll6% acid in a platinum crucible over a MBker burner. Different techniques were used for heating and mixing these acid mixturea to obtain equilibrium conditions. The most convenient method was bawd on the use of a 250-ml. suction flask which was partially evacuated and heated on a hot plate equipped for magnetic stirring. A thermometer was inserted through the rubber stopper which sealed
the flask, and it was allow-ed to dip into the acid mixture just far enough to cover the bulb. This gave sufEicient clearance for the magnetic stirrer and made it easy to measure and control the temperature accurately. To prevent etching of the borosilicate glass, it was necessary to keep the temperature below 280" C. Above 300' C. rapid attack of the glass usually occurred and the samples became cloudy. Extended contact times, even a t 250" C., resulted in extensive etching in a few instances. Any sample showing visual evidence of etching was discarded. A few of the samples were analyzed b emission spectroscopy and were founJto contain only a few parts per million of silicon. Some samples were prepared by working in a dry box. However, it was later determined that this was unneceasary. The rate of water absorption from the air was measured on a few samples in open containers on an analytical balance. The very short contact times necessary to transfer the materials did not represent a significant change in composition. In all instances, the samples prepared for calibration purposes were analyzed to check the calculated values. The analyses were made by a magnesium pyrophosphate gravimetric procedure and also by titration with sodium hydroxide after hydrolysis. The results of these analyses normally checked to within 0.3% with the values calculated from the weights of material used. The analyzed value was always slightly lower than the calculated, due probably to the small amount of water picked up during sample preparation. The samples for calibration were tram. ferred to the NMR tubes with a pipet while still hot (150" to 250" C.) and the tubes were sealed immediately. A sleeve insert in the NMR tube prevented the acid from touching the glass wall at the point to be sealed later. The hot transfer is neceasary because of the very high viscosity of many of theae acids a t room temperature. V O L 31, NO. 1 1 , NOVEMBER 1959
* 1895
0,CYCLES
t2I5,CYCLES
t455,CYCLES CALCULATED FROM REFERENCE (2)
4
110.5 *Yo H,P04
n
- A - J L + L 113.8 % H,POI
3
I5
i
10
RI
RI 5
C 98
100
to2
..
ORTHO
END
PROCEDURE
It is very easy to obtain a spectrum of a polyphosphoric acid showing the three peaks. Field requirements, sweep rates, and power settings are not nearly so critical as for high resolution proton work. T o obtain spectra which can be used for quantitative purposes, however, demands careful attention to a number of factors. While highly homogeneous fields were not necessary to see the large chemical shifts, it was easier to detect a pure absorption mode when the field was flat. This was probably due to field gradients through the large tubes which produced line distortion. The field trimmer, a mechanical device designed to make very small adjustments of pole piece alignment, was used to obtain optimufi conditions following the magnet cycling procedures commonly employed for proton work. The cycling procedure takes advantage of a hysteresis effect to obtain a more constant magnetic field over the magnet gap. It involves increasing the field about 10% above the resonance point for a predetermined time and then returning to resonance. The magnitude of the field increase, the time at the higher field, and the rate of return to the resonance point combine to determine the over-all effect of the cycle. The peaks are naturally wide, because of the high viscosity of the acids, and there was no trouble in controlling the scan rates or field drift. The cooling water to the magnet was in a closed circulating system, the temperature re1896
ANALYTICAL CHEMISTRY
I
I
I
100
108
iio
3
112
114
I
% H3P04
MIDDLE
Figure 1. NMR spectra of polyphosphoric acids at 16.2 Mc.
104
Figure 2. Ratio of end phosphorus to ortho phosphorus as function of per cent phosphoric acid
maining constant to about &0.1 O C. at the inlet point. Further, the magnet was enclosed in a box which minimized room temperature variations and prevented drafts from influencing the field. A recent article by Williams (7) discusses some factors which must be controlled to do quantitative NMR work. The RF level and particularly the magnitude of the leakage voltage were of extreme importance. A leakage at least four times the signal size is advisable. Corrections can be calculated based on the approach discussed by Williams (7) if absolute ratios are desired. I n this work it was not necessary to use these corrections. Other instrumental factors which may introduce significant errors include excessive filtering in the receiver, mixed modes due to improper probe balance, nonlinearity due to amplifier overloading a t high probe leakages, line saturation due to too high RF levels from the transmitter, and drifting base lines usually associated with line voltage fluctuations or insufficient warm-up times. It ,is difficult to describe these factors quantitatively. However, proper operating conditions can be determined experimentally with a given instrument. The appropriate conditions will be determined by the particular spectrometer used, the sample size, and the nature of the system being investigated. A reproducibility to 10% in the ratio of the areas under two peaks was easily obtainable; 5% accuracy was not difficult, and with considerable care, results good to 2% were obtained. For this
particular work, the results were consistently better than 5%. This translates into a very accurate measure of acid strength when equilibrium conditions are obtained. Pure absorption modes were judged by close examination of the base of the peak, and adjustments were made to minimum peak width. These peaks were recorded on a Varian recorder a t a sweep rate of about 100 cycles per minute. The sweep rate was not particularly critical and could be increased considerably without bad effect. The area of the peaks was measured with a planimeter. RESULTS AND DISCUSSION
Figure 1 shows spectra obtained a t different concentrations. The separation between the peaks increases with acid strength due to the slight difference in chemical shift associated with groups a t the ends of long or short chains and whether the middle phosphorous atoms are in long or short chains. As shown, the peaks become broader a t the higher acid strengths. I n the range shown, all the solutions can be called liquids even though the viscosities are very high. .4t acid strengths above 120% the peaks overlap and it becomes increasingly difficult to use the method without heating the sample tubes. Evidence of spin-spin splitting can be seen in some of the spectra shown. These spectra were obtained with the nonspinning 10-mm. tubes. Improvement in structure was noted when the
5
e
REFCRENCE ( 2 ) THIS WORK
t
I
m60
-
50
-
40.
IO
-
R2 5 96
98
100
102
104
100
108
110
112
114
I16
I16
120
X H,PO, Figure 4.
0
Per cent phosphorus type as function of acid strength
Figure 3. Ratio of end phosphorus to middle phosphorus as function of per cent phosphoric acid
spinning tubes were run a t elevated temperature, but no analytical advantages were observed. The peak width is determined by the viscosity rather than field homogeneity for all room temperature work. If it is assumed that these systems are a t equilibrium, the ratio of the area of any two peaks can be used to define the acid concentration a t a given temperature. Huhti and Gartaganis (2) give evidence, based on their paper chromatographic work, that a t least a metastable equilibrium condition is obtained when a mixture is heated for a few minutes and allowed to cool to room temperature. A temperature range as high as theirs was not covered, but confirmation was made that systems heated to20Ooto300"C.didnot show significant changes in compositions when they were cooled slowly to room temperature. By cooling some samples very rapidly, it was, however, possible to show that a different metastable equilibrium could be frozen into these acids. These differences were slight and were reversible. If the thermal history of a particular acid was not known or if there was any possibility of pyrophosphoric acid precipitation, the sample was heated to a t least 100" C. for 5 minutes and cooled slowly to room temperature before running. To cover the concentration range of interest, two ratios were used : RI
end phosphorus groups ortho phosphorus groups
Rz
end phosphorus groups middle phosphorus groups
R1was used from 98 to 110% acid, and Re was used from 106 to 120%. Figures 2 and 3 show these values plotted against per cent phosphoric acid. From the data of Huhti and Gartaganis (Z),the fraction of the phosphorus found in each of the three forms recognized by NMR was calculated for each concentration. Figure 4 shows clearly how the three forms change with concentration. R1and Rz values, calculated from these data, checked very closely with the results obtained by NMR, as shown in Figures 2 and 3. Probably, the only significant difference seen was in R, at the higher acid concentrations. These differences can be explained by a small amount of hydrolysis of the polyphosphoric acids during neutralization and dilution prior to the paper chromatographic work. The probability of this hydrolysis was discussed by Huhti and Gartaganis. The sensitivity of the NMR method is high. By considering the rate of change of either R1 or Rs at any particular concentration and using the fact that this R value can be measured to better than 574, the sensitivity of the method is better than +0.27& Both the NMR and magnesium pyrophosphate methods were used to analyze unknown samples after the calibrations were made. The results checked to +0.2% expressed as per cent phosphoric acid. It seems reasonable to say that the NMR method is as accurate as the
best wet methods available and that the sensitivity of the method for showing very slight differences is even better. The NMR method is rapid. -4 reliable spectrum (including probe balancing) can be obtained in 5 to 8 minutes; area measurement normally takes 4 to 6 minutes; and calculations require only a minute or two. Fifteen minutes is a fair average per sample. Electronic integration would, of course, make the method faster. A calibration based on ratios of peak heights was made but was less reproducible than the peak area approach. Approximate answers (+0.5%) can be read visually from the oscilloscopc tracing with a little practice. ACKNOWLEDGMENT
The authors are indebted to S. W. Nicksic for some of the pyrophosphate analyses and to J. N. Erdahl for obtaining some of the spectra used in this work. LITERATURE CITED
(1) cIrowther, J. P., ANAL. C m M . 26, 1383 (1954). (2) Huhti, A.-L., and Gartaganis, P. A., Can. J. Chem. 34, 785 (1956). (3) Popp, F. D., McEwen, W. E., C h a . Reus. 58, No. 2, 321 (1958). (4) Van Wazer, J. ,R., "Phosphorus and
Its ComDounds.
Interscience. New York, 195's. (5) Van Wazer, J. R., Callis, C. F., Shoolery, J. N., J. A m . Chem. SOC.77, 4945 (195 ) (6) Van l k e r , J. R., Callis, C. F., Shoolery, J. N., Anderson, W. A., Zbid., 79,2719 (1957). (7) Williams, R. B., Ann. N . Y . A d . Sci. 70,890 (1958). RECEIVEDfor review April 27, 1959. Accepted August 20, 1959. Division of Petroleum Chemistry, 136th Meeting, ACS, Atlantic City, N. J., September 1959.
VOL 31, NO. 1 1 , NOVEMBER 1959
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