reference electrode. Several sweeps across the voltage range were made before polarograms used to supply data were recorded. This procedure was found by Laitinen and Shoemaker (4) to stabilize the potential of the mercury pool, which is not depolarized by halide ions in liquid ammonia. A Sargent Model XXI Polarograph was employed throughout this work. Initial and final potential measurements were made with a Rubicon portable potentiometer. RESULTS
The polarograms obtained were normal in appearance and no particular experimental difficulties were encountered. The half-wave potentials measured in this study in liquid ammonia at 25" C. and 10-atm. pressure are listed in Table I. Also listed for comparison are the half-wave potentials
obtained by Laitinen and coworkers for the same ions in liquid ammonia at -36' C. and 1-atm. pressure. Except for thallium, the Ellz values a t the two temperatures agree within about 20 to 30 mv. The temperature coefficient of E,,2 of thallium appears to be significantly more negative than those of the other elements. Both the reducible ion and the mercury pool reference electrode have temperature coefficients of potential, A.E/AT. Thus the changes in Eliz observed are actually relative to that of the mercury pool. The half-wave potentials obtained in this study agree more closely with the values of Laitinen and coworkers than with values obtained by Vecchi (9) and by Leonard and Sellers (6) in l;?rH4N03) a t Divers solution (3"s: 0" C. and atmospheric pressure.
LITERATURE CITED
(I) Heus, R. J., Egan, J. J., J . Electrochem.
SOC.107,824 (1960). (2) Jolly, W. L., J . Chem. Educ. 33, 512 (1956): (3) Laitinen, H. A., ISyman, C. J., J . Am. Chem. SOC.70, 2241 (1948). (4) Laitinen, H. A,, Shoemaker, C. E., Ibid., 72, 663 (1950). (5) Ibid., p. 4975. (6) Leonard, G. W., Sellers, D. E., J. Electrochem. SOC.102, 95 (1955). (7) McElroy, A. B., Laitinen, H. A,, J. P$ys. Chem. 57, 564 (1953). (8) Steinberg, M., Nachtrieb, N. H., J . Am. Chem. SOC.72,3558 (1950). (9) Vecchi, E., Atti accad. nazl. Lincci Rend., Clusse sci. jis., mat. e nut. 14, 290-3 (1953).
RECEIVED for review November 14, 1960.
Accepted January 16, 1961. Contribution No. 975 from the Department of Chemistry, Indiana University, Bloomington, Ind. Work supported by the U. S. Atomic Energy Commission under Contract No. AT (11-1)-256.
A n Electron Paramagnetic Resonance Investigation of Vanadium in Petroleum Oils A. J. SARACENO, D. T. FANALE, and N. D. COGGESHALL Gulf Research & Development Co., Piffsburgh, Pa. b Electron paramagnetic resonance (EPR) spectra of petroleum oils containing vanadium show the presence of hyperfine splitting which serves to identify part of the resonance with that of porphyrin complexes of vanadium. Quantitative electron paramagnetic resonance spectroscopy was performed to establish the amount of total vanadium existing in the +4 oxidation state for a large number of oils. Using vanadyl etioporphyrin(1) complex as a standard, nominal EPR vanadium determinations were obtained on a series of distillates, residues, and full crudes having a total vanadium content in the range of 0.1 to 200 p.p.m., and the results were compared to values obtained b y chemical analysis. Good agreement between the EPR determinations and the chemical results was found. The presence of light ends in full crudes alters the line shape of the vanadyl resonance as compared to viscous media, requiring the use of different standards or the removal of the lighter fractions by distillation. Based on the petroleum oils examined, the conclusions from these studies are that with very few exceptions all the vanadium in petroleum oils whether they be distillates, residues, or full crudes exists in a single valence state, namely, the +4 oxidation state; the crystal field environment around
500
ANALYTICAL CHEMISTRY
vanadium in petroleum oils i s essentially the same. Quantitative determination of vanadium in oil distillates in the range of 0.1 p.p.m. and higher is feasible b y EPR spectroscopy.
P
ETROLEUM CRUDES, charge stocks, and heavy distillate oils almost invariably contain trace metals such as vanadium, nickel, copper, and iron. Vanadium and nickel are known to be combined a t least partly in the form of porphyrin complexes (16). Interest in this respect has centered, on one hand, among geologists who are concerned with any facts which might shed light on questions related to petroleum geology, and, on the other, among chemical engineers in the petroleum refining industry who are interested in the relationship of these trace elements to the technology of refinery operations. Several aspects of the nature of vanadium compounds in oil have been revealed by investigations based largely on chemical and spectrophotometric techniques. These studied have included the identification or isolation of vanadium porphyrin in certain crudes (13, 15) and investigations of physical properties (1, 1'7). Chemical evidence for the existence of three classes of metallic complexes in oils has been presented (7') and the discrepancy to the effect that there exists only 10 to 40% enough porphyrin
in oil to satisfy the metals content has been pointed out (7, 11). With regard to asphaltenes, the existence of vanadium as a metallo-organic compound (2) and its probable association with large molecules have been indicated (5). Synthetically prepared vanadyl etioporphyrin (I) complex (4)when dissolved in a high viscosity petroleum oil exhibits a characteristic electron resonance spectrum (18) consisting of a hyperfine pattern with an over-all spread of approximately 1000 gauss. The presence of a single unpaired electron in the 3d orbital of the central quadrivalent vanadium is responsible for the paramagnetism of the metal porphyrin complex. It is not surprising that crude oils, distillates, and residues containing significant amounts of vanadium also exhibit an E P R spectrum similar to that of vanadyl etioporphyrin(1). This resemblance, in fact, indicates that part of the vanadium is present in the +4 oxidation state. This investigation was undertaken with three specific objectives in mind: First, to determine if the vanadium in petroleum oils is present in a single oxidation state using EPR as a tool. Second, in view of the high sensitivity of electron paramagnetic resonance, to evaluate a possible rapid and accurate quantitative method for the determination of vanadium, and third, to gain
knowledge in the application of E P R for analytical purposes. Quantitative analysis by E P R of materials containing trace metals has not been as extensive as the determination of spin concentration in various free radical systems, the most widely exploited quantitative use of E P R (8). Considering the analytical rapidity of the technique, the small amount of sample required, and extremely high sensitivity obtainable, development work on electron spin resonance spectroscopy has not kept pace with the newer instrumental methods of analysis. ELECTRON PARAMAGNETIC RESONANCE (8, 9 )
An electron in a n atom has an intrinsic spin angular momentum and an orbital angular momentum due to circulation about the nucleus. Both of these angular momenta may have only quantized values in an atom and can be assigned specific quantum numbers. Each of these motions of the electron gives rise to a magnetic moment. The total magnetic moment is the vector sum of these separate magnetic moments. Atoms or ions having closed electron shells and paired electrons have zero moments and quantum numbers. Thus, only relatively few of the common elements are paramagnetic, since to be paramagnetic an atom must have one or more unpaired electrons. I n paramagnetic resonance experiments] only the spin component of angular momentum can usually orient freely in the magnetic field since the orbital component is locked by the electric field existing in the complex. The orbital component is thus said to be quenched. There are several methods of detecting and measuring electron spin moments (9). The method employed here depends upon the fact that, when unpaired spins are placed in a direct current magnetic field, a resonant condition can exist when a microwave field of proper frequency is applied. If compared with a suitable standard, the absorption of microwave energy is a measure of the number of unpaired electrons present in a sample. Transition elements occur when electrons enter a n outer s shell before completely filling inner d or f shells; the first atom showing this effect is titanium. The fact that a metal ion possesses an unfilled shell, and hence a permanent magnetic moment, which can have its energy altered by an external field, does not necessarily mean that paramagnetic resonance can be observed in its compounds. Two additional factors govern the observation of paramagnetic resonance. It is possible for all the degeneracy of the energy levels to be removed before application of the external field due to the action
of the very strong electric fields present in the crystal. I n this case, although the applied magnetic field will change the energy of the levels, it may never bring them close enough together for transitions to be induced between them by frequencies in the microwave region. A very general theorem by Kramers (9) states that if there is an odd number of unpaired electrons in the ion no electric field can completely remove the degeneracy. Paramagnetic resonance is always theoretically possible in such cases, the bottom energy level always being a t least twofold degenerate in spin. The other main factor governing the observation of paramagnetic resonance is the question of line width. The two major causes of broad lines in paramagnetic resonance are spread of energy levels by spin-lattice and spin-spin interactions. The latter is associated with the proximity of the neighboring paramagnetic ions and can always be reduced by diluting with an isomorphous salt. The former, on the other hand, is due to the splitting of the orbital levels of the paramagnetic ions themselves, those with relatively close energy levels having short spin-lattice relaxation times and wide absorption lines. It is evident that the crystal field around a metal ion has a profound effect on whether paramagnetic resonance can be observed under usual conditions. Vanadium (+4),with an odd electron in the d-orbital, by virtue of Kramers’ theorem can theoretically be observed in any crystal environment. Whether V+4 can be observed a t room temperature depends essentially on the degree to which the orbital levels are split. An orbital splitting of the order of 10,000 em.-’ is required to maintain a sufficiently long spin-lattice relaxation time a t room temperature and can occur when the symmetry of the field departs strongly from cubic symmetry. Since the reference compound used in this investigation was in the form of > V=O, the presence of oxygen would produce a strongly noncubic field ( l a ) . This type of electrostatic field would account for the fact that the vanadyl resonance is observable a t room temperature with a relatively narrow line width. Apparatus. All E P R measurements were made with Varian Associates X-band spectrometer, Modei V-4500, employing audio-frequency magnetic field modulation and phase sensitive detection. The standard 6-inch diameter pole face magnet (V-400’7) with a 2.00-inch air gap and matching power supply (V-2200A) comprised the magnet system. QUANTITATIVE EPR MEASUREMENTS
The information obtainable experimentally from electron spin resonance
includes line width, saturation behavior, electron spin concentration, and g-factor. The line width is defined as the width (in gauss) between deflection points on the derivative absorption curve. The normal absorption curve is converted approximately to the first derivative in a phase sensitive detection method which is employed in the Varian Associates E P R spectrometer. With ideal experimental conditions, the number of spins in an unknown sample may be obtained by direct comparison with a sample of known concentration. For the case of vanadium in oils, vanadyl etioporphyrin(1) complex, which contains one unpaired spin/molecule as expected for the +4 valence state, was used as a reference. Known amounts of the complex were dissolved in an oil distillate free of trace metals. The quantity which measures the number of spins is the area beneath the absorption curve, or in the case of first derivative presentation, the first moment divided by the modulation amplitude. If a series of similar samples is being compared, relative intensities can be obtained by measuring the height of the normal absorption curve, or, in the case of first derivative recording, the height between points of maximum deflection. It must be understood that obtaining relative intensities in this manner is valid only if the line width and shapes are alike. This condition is fulfilled if the environmental viscosity of the standard and unknown solution are equal. The best choice of solvent for the standard was, therefore, a metal-free heavy oil distillate when E P R derivative intensities were being compared. Some of the important factors which affect intensity or for which corrections must be made are as follows: 1, sample geometry; 2, modulation amplitude; 3, power level, which has to be set so as not to cause saturation; and 4, degradation of the Q of the cavity by the sample. The sample geometry was easily maintained constant by using special clear fused quartz tubing having especially uniform diameter. The modulation amplitude was kept constant during the entire series of experiments. It was adjusted to a value of 200 C.P.S. for maintaining sweep amplitudes a t about 5 gauss in the coils located inside the Varian cavity. An excessive modulation, greater than the line width of resonance line, leads to distorted derivative spectra and artificially broadened lines. A high power output is desirable to obtain maximum sensitivity. However, power saturation which is possible in resonance spectroscopy can result a t certain regions of radio-freVOL. 33, NO. 4, APRIL 1961
501
STANDARDS OF V A N A D Y L E T I O P O R P H Y R I N A T VARIOUS C O N C E N T R A T I O N S
I
Il 5PPM V
3300
3350
2 5 PPM V
IOFFM V
3300
3350
I
1
3300
3350
50 PPM V
3300
3350
Ho G A U S S AT 9 5 6 K M C I S E C
Figure 2. height 01
02
03
04
05
06
07
OB
Measurement of intensity using derivative peak
09
JPOWER IMILLIWATTSIX IO-'
Figure 1. Saturation behavior of vanadyl resonance in viscous oil
quency power. A check against this phenomenon was performed by plotting signal intensity against the square root of the microwave power (Figure 1) for the vanadyl resonance in a high viscosity petroleum oil. The linear relationship shows that it is safe to do quantitative work a t the power levels employed for this system. Changes in the Q of the cavity can result with samples that have different dielectric properties or surface areas. Since all samples were of similar type, no serious changes in the efficiency (Q) of the resonant cavity were expected. EXPERIMENTAL PROCEDURE
Special clear fused quartz tubing was used in making sample containers for insertion in the resonant cavity. The tubes, sealed a t one end and about 11 em. in length, had the following tolerance specifications: 3 mm. i: 12% inner diameter X 4.75 mm. f 0.25 mm. outer diameter. Volume calibrations were performed on a series of tubes to determine if any varied sufficiently to produce experimental errors greater than 10%. Each tube was appropriately labeled and could be positioned to the same point and orientation in the resonant cavity. After heating to approximately 160' F. with thorough agitation, the oil samples were transferred t o the quartz tube by a syringe or eye dropper and allon-ed to cool to room temperature. Exceedingly viscous samples, such as some visbroken tars, were first diluted with a metal-free heavy oil distillate to facilitate handling. Care was taken t o remove air bubbles which became entrapped or dispersed in the oil when placed in the quartz tubes. This was done by reheating the oil in the tube and allowing air bubbles to rise to the surface. The quartz lubes could be easily cleaned for reuse by heating, drawing out the oil by suction nith a capillary, 502 *
ANALYTICAL CHEMISTRY
rinsing several times with benzene, and finally drying in a stream of air. Volume calibration for tubes would have sufficed if all the materials examined had similar densities or if results were being reported in weight per unit volume. Since chemical results were being reported in parts per million by weight, density variations had to be checked. The density of the petroleum oils did not vary by more than 6% (0.93 gram per milliliter being the average). However, several asphaltenes had packing densities of anyn here from 0.5 to 0.9 gram per milliliter and suitable corrections were applied. Standards were prepared by dissolving known amounts of vanadyl etioporphyrin(1) complex in a Taparito heavy oil distillate which yielded no E P R spectrum and contained no trace metals. Portions of each standard were then transferred to quartz tubes and sealed. These were retained for use in checking the spectrometer sensitivity from time t o time. The standards prepared covered the range of 0.1 to 50 p.p.m. of vanadium. The magnetic field mas slowly altered to obtain the value a t which resonance occurred for electronic precession in a microwave field. Since only one line of the hyperfine pattern is necessary for obtaining the relative intensities, just a narrow portion of the vanadium ( f 4 ) resonance needs t o be scanned. Figure 2 shows a series of four concentrations and the derivative peak height of the most intense peak a t room temperature. The magnetic field ranged from about 3300 to 3400 gauss during this scanning time which was about 4 to 5 minutes. At slightly higher fields the free radical peaks occur. A series of samples can be processed in a relatively short time by simply exchanging tubes in the resonant cavity. Rebalancing of the microwave bridge was usually not necessary be-
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/
0
I 8
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za
12
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PLRIS
43
PER
49
5b
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MILLION
Figure 3. Vanadyl etioporphyrin(1) as standard dissolved in heavy oil distillate containing less than 0.1 p.p.rn. V (0-50 p.p.m. V)
cause all samples were of a similar dielectric nature. If slight changes in leakage did occur, rebalancing was done with the slide screw tuner. Before any quantitative work could be undertaken a t all, it was essential to confirm the linearity of the E P R spectrometer with concentration of vanadium. Figure 3 depicts a plot of the derivative E P R peak height of vanadyl etioporphyrin(1) against the concentration of vanadium in the range of 1 to 50 p.p.m. The solvent used in these standards was a heavy oil distillate which was known t o contain no trace metals. The solvent was chosen t o maintain the environmental medium of the standards constant with respect to the oil distillates and bottoms, thus keeping the line widths and shape similar. It is in this manner that intensities can be obtained simply and quickly by comparing derivative peak heights. Linearity in the high sensitivity setting of the spectrometer is shown in Figure 4 where the 0.1- to 1.0p.p.m. concentration range is coverecl. This latter graph is exceedingly iniportant in these studies because i t demonstrates that vanadium can easily be detected down to a t least 0.1 p.p.m. and, therefore, make E P R spectroscopy applicable to practically all distillates. The method of obtaining relative in-
/I E P R S P E C T R I I M OF P E T R S L E L M B 0 7 i O H C O N T I , N , N G 5 2 0 PPM
"
I 00
2'
02
03
04 05 06 V A N A D I U M PARTS PER
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l
l
01
08
09
1 ID
MILLION
Figure 4. Vanadyl etioporphyrin(1) as standard dissolved in heavy oil distillate containing less than 0.1 p.p.m. V (0.1-1 .O p.p.m. V )
tensity consists of measuring the distance between deflection points of the strongest vanadium line and the opposite weaker component as indicated by the dotted arrows in Figure 2. This is the best peak height to use to avoid interference with free radicals and to obtain maximum sensitivity. Using the above plots as calibration curves, a number of oil distillates were scanned for the vanadium resonance and the iiitcnsities were measured in the manner described to obtain concentration of vanadium in the distillates. EXPERIMENTAL RESULTS
Figure 5 shows the complete E P R spectrum of a petroleum residue containing 52.0 p.p.m. of V and that of a solution of vanadyl etioporphyrin(1) complex dissolved in a heavy oil distillate free of trace metals. The spectra are similar except for the overlapping of the free radical peak with the intense, central line caused by the vanadium. I n all cases, the oil samples showed this spectrum of vanadium and free radicals. I n some cases, the free radical peak was nearly absent. Oil Distillates. If the vanadium signal intensity of the oil distillates is simply plotted against total vanadium as obtained by chemical analysis, a good calibration curve in itself is possible. Figure 6 shows the linear plot obtained. Distillates originating from eight different crudes nere purposely chosen in this correlation. A comparison of the slope of the straight line in Figure 6 with that of Figure 3 on the same scale would show that the latter is slightly lower. Using the vanadyl etioporphyrin(1) as a standard could, therefore, lead to slightly higher EPR results by 10%. This similarity in the slopes of the two curves shows that vanadium in distillates is totally present a t +4 and also that it exists
Figure 5. Complete EPR spectra of vanadium resonance in viscous oils
as a type of vanadyl complex with essentially the same crystal field environment as vanadyl etioporphyrin(1). Table I lists a number of "unknown" oil distillates for which vanadium determinations had been made by E P R and checked by chemical analysis. The calibration curves shown in Figures 3 and 4 were used in obtaining these results. Table I1 indicates the repeatability of these results is 1 0 . 1 p.p.m. of V a t the 1-p.p.m. level. Oil Residues. Essentially a similar procedure was applied here as in the case of the oil distillates. Figure 7 depicts a plot of the E P R signal intensity against the total vanadium as obtained chemically. A scatter of points was obtained averaging t o a straight line with a deviation of 13.5%. The most significant feature of this curve is, however, t h a t its slope is close to that of the standard calibration plot of Figure 3. This
Table 11.
Sample
KO. RS-1 RS-2 RS-5 RS-9 RS-13 RS-17 RS-20 RS-25 RS-29 RS-33 RS-37 RS-41 RS-45 RS-49 RS-53 RS-57 RS-59 A
a
Date Sampled
11/17/59 11/18/59 11/24/59 11/25/59 11/26/59 11/27/59 11/28/59 12/7/59 12/8/59 12/9/59 12/10/59 12/11/59 12/21/59 12/22/59 12/23/59 12/24/59 12/25/59 5/10/60 B 5/11/60 Analyses made January 1960. AnalyEes made J u n e 1960.
Table I. Comparison of EPR and Chemical Vanadium Results on Oil Distillates sample Vanadium, P.P.M. NO. Chemical EPR 1-D 0.6 0.7 2-D 11.4 13.5 3-D 0.1 0.1 4-D 1.4 1.7 5-D 0.5 0.5 6-D 0.8 0.7 7-D 2.4 2.4 8-D 0.2 0.1 9-D 21.4 22.8 10-D 0.1 0.2
16-D
17-D
0.07 0.6
0.1 0.5
Heavy Gas Oils
EPR" 0.93 0.83 0.93 0.91 2.6 0.50 0.70 0.30 0.10 0.12 0.30 0.50 1 15 1 38 0 90 0 68 0 71
s h o m that the vanadium in the oil bottoms existed in the same valance state and crystal field environment as in the porphyrin compound used for a reference. Table I11 lists some types
Vanadium, P.P.M. Chemical.
EPRb 1.02
0.58 0.45
