Table 1.
KO. 1 2 3 4 5 6
Titration of Acridine with Perchloric Acid
Acridine, Mg. Taken Found 182 I57 98 49 45 27
2 8 6 5 8 8
181 8 157 4 98 G 49 2 45 5 273
DISCUSSION
0.65 r
The estimation of acridine by potentiometric titration with perchloric acid in glacial acetic acid medium is accurate within 0.5 mg. on quantities of acridine ranging from 28 to 180 mg. Furthermore, the method is more rapid thac sther available methods. Estimatioc of acridine in the presence of other bases of similar strength was found to be not feasible by this method, although nonbasic additives did not interfere. Excess of acetic anhydride had to be avoided in the glacial acetic acid used as solvent, as its presence tended to give a lower value for acridine found.
Error, Mg. -0 4 -0 4
0 0
-0 3
-0 3 -0 5
in e.m.f. during the course of the titration was measured using a Mullard potentiometer unit. Equilibrium was quickly established after the addition of each lot of perchloric acid and the titration could be completed in 10 minutes. RESULTS
Figure 1 shows the e.m.f. (us. S.C.E.) plotted against milliliters of perchloric acid (0.0098N) in a typical case. The exact end point was located by computing the volume a t which the second derivative of the increments of e.m.f. with respect to volume increments, A2E/AVZ, becomes zero (6). The ratio of e.m.f. increments to volume increments was calculated for each 0.1
0
2
-
Z
$0.4
!i
L
0.3 50
10
20
30
d0
ML.OF PERCHLORIC ACID Figure 1. Titration of acridine with perchloric acid
ml. of reagent added and this ratio, AEIAV, was plotted against volume of the reagent to obtain the second derivative. The results obtained in the series of experiments are given in Table I.
LITERATURE CITED
(1) Albert, A., "The Acridines," p. 229, Edward Arnold & Co., London, 1951. (2) Bolliger, A., Analyst 64, 416 (1939). (3) Hall, N. F., Conant, J. B., J . Am. Chem. SOC.49, 3047 (1927). (4) Khmelevskif, V. I., Ovchinnikova, I. I., Org. Chem. Ind. (U.S.S.R.) 7, 626 (1940). (5) KhmelevskiI, V. I., PostovskiI, I. Ya., J . Aaal. Chem. (U.S.S.R.) 17., 463 , (1944j (6) Lingane, J. J., "Electroanalytical Chemistry," p. 70, Interscience, New York, 1953.
:
RECEIVEDfor review July 29, 1960. Accepted November 21, 1960.
Apparatus for High Pressure Polarography with Application to Polarography in Liquid Ammonia at 25' C. WARD B. SCHAAP, ROBERT F. CONLEY,' and F. C. SCHMIDT Deparfmenf of Chemistry, Indiana University, Bloomington, Ind.
b Apparatus is described for use in polarographic studies with a dropping mercury electrode a t elevated pressures. The apparatus was tested in liquid ammonia at 25" C. and 10-atm. pressure. The apparatus can b e used, in general, in the study of polarography in solvents a t temperatures above their normal boiling points and allows polarographic and derived thermodynamic data in different solvents to b e compared a t a standard temperature.
T
HE use of the dropping mercury electrode for polarographic studies is restricted by the physical properties of mercury to temperatures ranging from -39" C., its freezing point, to nearly 356' C., its boiling point. In actual practice, the dropping mercury electrode has been used in liquid am-
498
ANALYTICAL CHEMISTRY
monia a t -36" C. (3-5, 7) and in fused salt media at 160' to 220" C. (8). At higher temperatures solid microelectrodes have generally been used in polarographic studies. iittempts by Steinberg and Nachtrieb to retain the advantages of a dropping electrode by replacing mercury with a higher boiling metal, such as lead, bismuth, or silver, were unsuccessful (8). Recently, however, Heus and Egan reported using a dropping bismuth electrode successfully a t 450" C in a fused LiC1-KC1 eutectic melt ( 1 ) . At temperatures below the freezing point of mercury, which may be encountered in nonaqueous polarography in low boiling solvents, solid microelectrodes are again a possibility. In these latter cases, however, it is also possible to raise the boiling point of the solvent above the freezing point of
mercury by allowing the pressure of the system to increase sufficiently. In this paper, apparatus is described which allows polarographic studies with a dropping mercury electrode to be carried out a t elevated pressures. The apparatus was tested in polarographic studies in liquid ammonia at 25" C. At this temperature, nearly 60" above its normal boiling point, liquid ammonia develops a pressure of approximately 10 atm. The accepted standard temperature for thermodynamics of aqueous systems is 25' C. Jolly (6) has advanced reasons for the choice of 25' C. as the standard temperature for thermodynamics in liquid ammonia. the principal
1
Present address, Georgia Kaolin Co.,
433 Yorth Broad St., Elizabeth, N. J.
reason being the greater ease and accuracy with which comparisons between chemistry in liquid ammonia and water can be made. Tkis argument would apply also to other nonaqueous solvents and to comparisons of polarographic data. Most of the properties of solutes of thermodynamic interest and of importance in polarography are much more strongly dependent upon temperature than upon pressure. It seemed desirable, therefore, to construct apparatus for polarographic studies at elevated pressures which could be used in the study of low boiling solvent systems a t a standard temperature, or more generally, in the study of polarography in solvents a t temperatures above their normal boiling points. APPARATUS
The apparatus consists of a complete polarographic cell setup, including the dropping electrode and mercury reservoir, enclosed in a thick-walled metal container. The pressure a t all points within the container is uniform, so that the operation of the dropping electrode is not disturbed by a change in pressure of the system. A cross-sectional drawing of the apparatus is shown in Figure 1. The larger, lower chamber is a modified Parr calorimetric bomb and holds a 200-ml. tall-form beaker used as the combination polarographic cell and mercury pool reference electrode compartment. The cover plate of the Parr bomb is a 1/4-inch-thick stainless steel disk in which are mounted a small sparkplug (Champion Model V) which serves as a high pressure, insulated electrical connector, the Parr gas release valve (furnished with the bomb), and the 11-inch-long mercury column and reservoir support tube, also of stainless steel. The diameter of the support tube is such that the capillary and plastic (or rubber) tubing connecting it to the reservoir above are loosely fitting, to allow instantaneous pressure equalization between the lower and upper chambers. A glass-enclosed platinum wire provides electrical connection between the sparkplug and the mercury pool anode (reference electrode). A polyethylene gasket between the cover plate and the top of the bomb wall provides a pressure-tight seal when the threaded Parr bomb cover is tightened. On the upper end of the support tube is melded a smaller chamber which houses the mercury reservoir. The mercury is contained in a funnelshaped glass reservoir connected t o the capillary by Tygon tubing of sufficient length to allow the lower end of the capillary to extend to nithin about inch of the bottom of the beaker. A second, smaller stainless steel plate covers the reservoir chamber and is also threaded for a sparkplug electrical connector to make contact, via a platinum wire, with the mercury in the reservoir. The plate rests on a rubber or plastic ring seal and a
pressure-tight seal is formed when the two enveloping threaded brass cups (rhodium-plated) are tightened.
Table I. Half-Wave Potentials of Cations in Liquid Ammonia at Its Normal Boiling Point and a t 25" C.
PROCEDURES
EUZin Liq. NHt, Volts At
A variety of experimental procedures are possible, depending upon the type and accuracy of data desired. The procedure described below was convenient and satisfactory for measuring halfwave pot entials . Solutions were made in a 1-pint Dewar flask. A calibration mark was made on the Dewar to allow measurement of 200 ml. of solution, allowing for displacement of the immersed tall-form beaker. High-purity liquid ammonia was drained from a tank through a delivery tube directly into the flask. Weighed amounts of the solid dried supporting electrolyte and appropriate metal salt were added to the flask slowly to prevent violent boiling. The solution in the Dewar could be stirred with a magnetic stirrer to hasten solution of the salts if desired. The beaker was cooled by immersion in the liquid ammonia solution to prevent frosting and excessive evaporation upon transferring the solution. The resulting solution was allowed to evaporate to the calibration mark, a t which time the beaker to '/3 filled) was removed and transferred to the cold bomb assembly. I t is estimated that the concentrations of salts in solutions prepared
Figure 1. Bomb assembly for polarographic studies at elevated pressures
Ion
T1+ Pb+2
CU+~(+CU') CU+2(-tCU+) c u +(LCUO) Cd +2 Zn +2 '
25" C. and 10 atm.a -0,255 -0.35
At
C. and 1 atm.b (4, 7 ) -36'
-0.31' -0.11
-0.50 -0.75
-0.18
-0.33 -0.34c -0.15 -0.52 -0.77 -1.21
Ell2 values us. mercury pool reference electrode in contact with ammonia solution containing 0.1M KHJ. E112 values us. mercury pool reference electrode in contact with ammonia solution containing 0.lM KXOI or KI. Values reported vs. lead-0.1M lead nitrate electrode ( 7 ) recalculated assuming mercury 001 t o be +0.32 volt us. lead0.1M read nitrate electrode ( 3 ) . Calculated from two observed oneelectron reduction steps. (1
by this procedure are known nith an accuracy of + 2 t o 3%. The bomb apparatus was precooled to below -40" C. in a 1-gallon Dewar flask containing acetone and dry ice. If the bomb were left a t room temperature, excessive boiling of the snimonia solution would occur before the connections could be tightened. Frosting of the boinb intciior n-as minimized by placing a desiccant in the bomb compartment for several hours prior to cooling or by flushing the assembly with dry, precooled nitrogen gas a t a positive pressure during the cooling operation, After transferral of the ammonia beaker to the cold bomb assembly (pressure - release valve open), all threaded joints were tightened, the release valve was closed, and the system was allowed to warm up to 25" C., first in air and then in a water bath over a period of several hours. Connesting the appropriate leads from the polarograph to the two sparkplugs completed the preparations. The assembly was emptied by opening the release valve under a hood and allowing the ammonia t o evaporate. A O.1Jf solution of ammonium iodide served as supporting electrolytcl. Although is an acid in liquid ammonia, Laitinen and Shoemaker (4, 6 ) show that this acidity does not significantly affect the potential of the mercury pool or half-mve potential of thallium. The cell resistance, measured n t 25' C., was about 3000 ohms. Correction of El!? for iR drop was made whenever the correction TT as significant. A concentration of reducible metal ion of about 10-4M gave good polarograms. Both chloride and nitratc salts were used with no differences in El11noted. The mercury pool a t the bottom of the beaker served as both anode and VOL. 33, NO. 4, APRIL 1961
499
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 EPR 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
