Table 111.
Pr + 3
Molar Absorptivities of Principal Lanthanide Absorption Bands in LiNOa-KNOS Eutectic at
Xd + 3
-
Tb+3
P
E
P
E
P
E
P
E
1.925 1.538 1.445 0.483 0.468 0.445
2.60 3.17 1.83 1.12
3.80 3.10 2.00 19.7 2.48 1.26
1.557 1.500 1.385 1.246 1.088 0,404 0.376
1.87 1.87 1.32 2.10 1.51 3.18 1.79
2.165 2.05 0.465 0.395
I .20
2.55
0.798 0.735 0.597 0.582 0.525 0.511
P
€
P
E
P
E
P
0.910
1.16
0.468 0.452 0.416 0.360
1.06 23.4 2.00 7.20
0.486 0.378
1.22 20.0
1.50
Dyt3
Er + 3
Ho + 3
bands of potential analytical importance are shown in Table 111. B y recourse to both aqueous and molten L i N O r K N 0 3 solutions, the quantitative analysis of even relatively complex mixtures of heavy and light lanthanides should be possible. ACKNOWLEDGMENT
T h e assistance of R. L. McBeth in obtaining the experimental data is gratefully acknowledged. LITERATURE CITED
(1) Banks, C. V., Heusinkveld, hl. R., O’Laughlin, J. W., ANAL.CHEM. 33, 1235 (1961). (2) Banks, C. V., Spooner, J. L., 0 Laughlin, J. W.,Ibid., 30, 458 (1958); ’ (3) Brugel, W., “Einfuhrung in die Ultrarotspektroskopie,” 2nd ed. p. 186, D. Steinkopff Verlag, Darmstadt, 1957.
150” C.
Eu + 3
Sm +a
1.40 0.98
0.B
J !
Tm+3
(4) Butement, F. D. S., Trans. Faraday SOC.44,617 (1948). (5) Cohen, D., Carnall, W. T., J . Phys. Chem. 64, 1933 (1960). (6) Dieke, G. H., Hall, L. A., J . Chem. Phys. 27,465 (1957). (7) Florence, J. M., dllshouse, C. C., Glaze, F. W., Hahner, C. H., J . Research Xatl. Bur. Standards 45,121 (1950). (8) Gobrecht, H., Ann. Phys. Lpz. 31, 755 (1938). (9) Greenberg, J., Hallgren, L. J., Rev. Sci. Znstr. 31, 444 (1960). (10) Gruber, J. B., Conway, J. G., J . Chem. Phys. 32,1178 (1960). (11) Gruber, J. B., ConLLaay, J. G., J . Znorg. & h‘uclear Chem. 14, 303 (1960). (12) Gruen, D. M., Zbzd., 4, 74 (1957). (13) Gruen, D. M., Fried, S., Graf, P , MeBeth, R. L., Proceedings of the Second International Conference on the Peaceful Uses of Atomic Energy, Geneva, 1958, P/940, Vol. 28, p. 112, United Il’ations, New York, 1958. (14) International Series of Monographs on Analytical Chem., R. Belcher and L. Gordon, eds., Vol. 3. “Analytical Chemistry of the Rare Earths,” Vickery,
c
2.275 1.988 1.837
0.93 1.19 0.90
Yb f 3 E
P
E
R. C., Chapter VI, Pergamon Press, Xew York, 1961. (15) Isaac, N. M., Fields, P. R., Gruen, D. M., J . Znorg. & Nuclear Chem. 21, 152 (1961). (16) ‘,‘Nouveau Trait6 de Chimie MinBrale, Paul Pascal, ed., Vol. 7 (Part 11), Masson and Cie, Paris, 1959. (17) Stewart, D. C., Kato, D., ANAL. CHEM.30,164 (1958). (18) Sundheim, B. R., Harrington, G., U. S. Atomic Energy Comm. Rept. NYO-7742, March 9, 1959. (19) Urbain, G., Bourion, F., Compt. rend. 153, 1155 (1911). (20) Young, J. P., White, J. C., ANAL. CHEM.31,1892 (1959). (21) Zbid., 32, 799 (1960). (22) Zbid., p. 1658.
RECEIVED for review December 29, 1961. Accepted February 26, 1962. Based on work performed under the auspices of the U. S. Atomic Energy Commission. Presented in Dart a t the 12th Annual Symposium o i Spectroscopy, Chicago, May 15-18, 1961.
Effects of Streaming on Voltammetric Data JOHN W. OLVER’ and JAMES W. ROSS Department o f Chemistry and laboratory for Nuclear Science, Massachusetts Institute of Technology, Cambridge, Mass.
b
The gross effects of streaming on several voltammetric techniques have been investigated. Methods studied include chronopotentiometry with pool and hanging drop electrodes, controlled current polarography, and linear voltage scan a t a hanging drop electrode. Under appropriate conditions of cell geometry, current density, purity, and composition of solutions, all the experimental techniques showed anomalous results which can b e interpreted as streaming a t the electrodesolution interface. The necessity of using maxima suppressors in these nonpolarographic techniques is demonstrated.
P
maxima caused by convective streaming in the vicinity of the working electrode have been studied extensively (4). Streaming in various voltammetric techniques is important for both analytical and theoretical applications if the techniques depend upon diffusion as the only mass transfer process by which electroactive species reach the electrode. Recently Kolthoff and Okinaka (3) reported t h a t streaming occurs in the vicinity of the mercury drop using the controlled current polarographic technique of Ishibashi and Fujinaga ( 2 ) . However, no maxima were apparent on the current voltage curves. Since OLAROGRAPHIC
this technique is similar to conventional chronopotentiometry, this study was undertaken to examine the gross effects of streaming on data obtained b y several voltammetric methods. The methods investigated include chronopotentiometry at both hanging drop and pool mercury cathodes, and linear voltage scan at a hanging mercury drop, as well as controlled current polarography at the dropping mercury electrode
(D.M.E.).
1 Present address, Department of Chemistry, University of Massachusetts, Amherst, Mass.
VOL. 34, NO. 7, JUNE 1962
791
6t ii A
I I 1 - 0 2 - 0 4 -0.6 - 0 8 -10 E ,volts vs SCE
Figure 1 . solution
Polarograms
of
I -J2
1 -14
bismuth
Curve I. Conventional polovogram of 2.5mM Bi(N03)Z in 0.8M HOAc, 0.8M NaOAc Curve 11. Envelope of potentials a t end of drop life b y present current-scan method Envelope of potentials a t the end of Curve 111. drop life by Ishibashi-Fujinaga current-scan method EXPERIMENTAL
All solutions were preparrd from reagent grade hlallinckrodt chemicals and Du Pont acetic acid using water which had been redistilled from alkaline permanganate. Polarograms were recorded on the Sargent Model XXI Polarograph to check for the presence of maxima by the conventional polarographic method. Current-voltage curves for the rapid potential scan technique a t the mercury hanging drop were recorded on the same polarograph by driving the slide-wire to achieve the desired rate of potential scan. The constant current source for chronopotentiometry a t the hanging drop cathode and for controlled current polarography consisted of a bank of Burgess B batteries totalling 135 volts in series with a large resistance chosen b y a suitable selector switch. The constant current source was synchronized with the dropping mercury electrode by a motor driven switch and solenoidoperated hammer on the D.1LI.E. capillary such that current was applied only during approximately the last half of the drop life. This procedure differs from that used by Ishibashi and Fujinaga ( 2 ) in that a constant current was applied to the D.M.E. continuously in their method. The potential across the electrolysis cell was measured with a Leeds & Northrup p H meter and recorded on a Varian recorder driven by the output from the p H meter. The constant current supply used in experiments using the mercury pool cathode was designed by Reilley, Adams, and Furman ( 5 ) , and chronopotentiograms were again recorded on the Varian recorder. The hanging mercury-drop electrode assembly used has been described by Ross, DeMars, and Shain ( 7 ) . Two drops of total mass 14.0 mg. and area 0 05 sq. cm. from the a u d i a r y dropping electrode were used for each run. The hanging drop was replaced after each electrolysis. The mercury pool cathode of 5.06 sq. em. area was contained in a part Teflon cell similar in nature to that
792
ANALYTICAL CHEMISTRY
described by Reilley, Everett, and Johns (6). A platinum foil anode was used in experiments using the mercury pool cathode. For all other experiments, the saturated calomel electrode was used as the anode. Solutions were deaerated using prepurified tank nitrogen which had been passed through scrubbing solution as an extra precaution against the presence of surface active impurities. In some splutions, to emphasize the effect of streaming, several large pieces of activated charcoal (Norit A) were added t'o remove traces of surface active impurities which might otherwise act as maximurn suppressors. Streaming was observed visually with a horizontally mounted microscope after adding a small amount of fine charcoal to the solution. Approximately 0.0057, phenol red n-as used as a masimum suppressor when it was desired to eliminate streaming entirely. Gelatin proved unsatisfactory as a maximum suppressor because of the difficulty in hanging drops when gelatin was present in suficicnt concentration to eliminate streaming. RESULTS AND DISCUSSION
Controlled Current Polarography. The conventional polarogram of a bismuth solution n i t h typical maxim u m is shown in Figure 1, curve I. Curve I1 is the controlled current polarogram of the same bismuth solution b y t h e present method, scanning from low t o high currents. Curve I11 is the controlled current polarogram obtained by the method of Ishibashi and Fujinaga, scanning from high to low currents. For curves I1 and I11 the smooth curve shown is the envelope of the potentials attained a t the end of the drop life a t each conqtant cuirt n t applied. Curve I1 clearly represent;; streaming similar to that of the conventional polarogram. T o explain these observations one must consider both the potentials can polarograms and the potential-time curves during a single drop life using each of the current-scan polarographic methods. The conventional polarogram for the bismuth solution (cwrve I) has a pronounced maximum approximately twice the height of the diffusion plateau. This maximum disappears abruptly as the potential of the dropping electrode approaches that of the electrocapillary maximum and a level diffusion plateau is then followed cathodically to the reduction of hydrogen ion in this electrolyte. There is a very definite potential range in which streaming can occur and beyond which streaming does not occur. Evamination of the potential-time curve for any single drop when a current is applied over the Thole lifetime of the drop (Ishibashi and Fujinaga) s h o w that the potential is that of reduction of electrolyte (a very cathodic value)
a t the start of each drop. The reduction of bismuth ion cannot supply the current impressed. However, as the drop grows, a point in the drop life is reached, if the applied current is below the conventional polarographic diffusion current, when all of the current passed goes to reduction of bismuth. At that time the potential of the drop breaks (represented by the horizontal in curve 111) toward 3 more anodic value governed by the reduction of bismuth alone. Only after the potential break on n hich the Ishibashi-Fujinaga method depends, does the mercury drop enter a potential range (from curve I) where streaming can occur as shown by conventional polarography and the only effect of streaming then is to cause the electrode t o assume a slightly more anodic potential. If a constant current is applied only over the last half of the drop life (curve 11), the potential when current is applied is on the rising portion of the normal polarographic wave for bismuth. For a cathodic break in potential to occur, the potential of the drop must pass through a range \There streaming can occur. Streaming in this case holds the potential a t the anodic value and a potential break occurs only when the current applied is so great that the streaming process itself cannot provide enough bismuth for reduction a t the eiectrode surface. T h a t current is the current a t the peak in the conventional polarographic maximum for the bismuth solution. While streaming can be observed with the hand microscope using either technique, maxima only appear on
- E , volts
VS
SCE
Figure 2. Rapid potential scan curves at a hanging drop Curve I. 3.714 Cu(NOd2 in 0.245M HOAc, 0.245M N a O A c with phenol red added; potential Icon rate 0.22 volt per minute Curve II. Some solution without phenol red added
12OC
I
I
I
a-
032
016
032 Current
I 0 64
048
I
0 64
~ e n s ~ ,mo/cmz ty
Figure 3. Chronopotentiometric d a t a a t the hanging mercury drop Top, 0 3.3mM Bi(NO& in 0.3M HOAc, 0.3M N a O A c A with charcoal a d d e d - theoreticol Bottom, 0 3mM Cd(N03)z in 0.28M HOAc, 0.28M NaOAc A with charcoal a d d e d 0 with phenol r e d a d d e d and theoretical line superimposed
the curreiit voltage curves obtained by the prescnt method. Rapid Potential Scan a t a Hanging Drop. Some of t h e results for copper are illustrated in Figure 2 . Curve I is t h e current-voltage curve obtained a t a potential scan rate of 0.22 volt per minute with phenol red added. Curve I1 n-ithout phenol red added s h o n s t h e remarkable effect R-hich streaming causes. T h e slope of t h e current rise during streaming is approximately 1, R as n i t h classical polarographic maxima. The huge spike maximum on the rapid potential scan curve drops suddenly as the potential of the electrocapillary maximum of mercury is approached. rlddition of 0.005% phenol red eliminates the streaming n ithout appreciably altering the peak current for copper reduction. Streaming is even more pronounced a t higher voltage scan rates than 0.22 volt per minute. One peculiar feature of curve I1 is t h a t streaming begins a t the peak in the current-voltage curve which would be obtained without streaming. Examination of the conventional polarogram for the same copper solution in acetate media shows that the maximum in the case of copper does not begin on the rising portion of the whve but rather on the diffusion plateau and the range in potential where extensive streaming occurs is preceded by a 20- to 30-mv. range where the currents are very erratic. Bismuth reduction in acetate media provides similar data to those for
copper except t h a t since the conventional bismuth maximum appears on the rising portion of the polarographic wave, there is no indication of what the peak current would be if streaming did not occur. Vastly too high peak currents 15 ith the characteristic 1/R slope similar to that in Figure 2 are obtained. Phenol red again suppresses streaming but causes a 25 to 30% reduction in the peak current expected without streaming, thereby giving analytically erroneous results. Cadmium, \vhich does not give a conventional polarographic maximum in acetate media, gave only .light indication of streaming. I n especially clean solutions n ith charcoal added, a small spike of varying height appeared on the cathodic side of the peak current using voltage scan rates greater than 0.5 volt per minute. The peak current was reproducible and the small spike did not interfere with its determination. I n solutions of copper, cadmium, and bismuth ~3 here streaming did not occur, experimental data were checked using the equation given by Frankenthal and Shain ( I ) and were found to agree n i t h theory within =k3Yc. Chronopotentiometry at the Hanging Drop. Typical results for bism u t h and cadmium are shown in Figure 3. T h e theoretical curves are derived from t h e equation describing chronopotentiometry a t a spherical electrode. For a planar electrode the value of i ~* lshould be constant n i t h changing current applied. Hone\ er, for the cpherical electrode i+* Iaries n i t h current or current density and the approvimation of linear diffusion cannot be used n i t h accuracy a t transition times greater than about 25 seconds. T h e chronopotentiometric data for bismuth solutions n ith no conventional maximum follow theory well but in solutions where large polarographic maxima occur, streaming also occurs in chronopotentiometry a t the hanging drop. Figure 3, top, illustrates deviation from theoretical behavior a t high current densities because of streaming and a t lorn current densities, corresponding to transition times greater than 30 seconds, because of convective stirring oning to density gradients. For a solution of bismuth which had a pronounced polarographic maximum, once streaming had begun in a given chronopotentiometric experiment, no potential transition occurred. The addition of phenol red to a bismuth solution eliminated streaming but caused a decrease in similar to the decrease in peak current observed in rapid potential scan measurements (see above). Data for the chronopotentiometry of cadmium in acetate media are shown in Figure 3, bottom. Khereas conventional polarogranis of a 3mM Cd +* solution in 0.28-If ammonium acetate-acetic acid
53r.: I
c(
1
I
300pa
20
.----
!a2t I IO
I I 20 30 Time I sec
I 40
Figure 4. Chronopotentiometry of copper at the hanging drop Top, Polarogram of 2.9mM C u ( N 0 3 ) ~in 0.245M HOAc, 0.245M N a O A c Bottom, Chronopotentiogram of same solution a t 14 pa. applied current
show no maximum, a t even lower current densities than cause streaming of bismuth, extensive deviation from theoretical behavior occurs. After addition of phenol red, theory is folloi~-edclosely. TYithout a maximum suppressor present, there is no range of transition times where theory is followed. The chronopotentiometric data for copper in acetate media are similar to those for bismuth. Generally in clean solutions, theory is followed only betn.een 30- and 60-second transition timca. Pronounced st'reaming may occur x t current densities which n-ould give transition times lower than 30 seconds. Because polarographic maxima for copper begin on the diffusion plateau, chronopotentiogranis for such solutions exhibit potential timc oscillations. Figure 4, top, is a typical polarogram for copper in acetate media illustrating that streaming starts on the diffusion plateau. Figure 4, bottom, is the chronopotentiogram for the same solution taken at an applied current of 14 pa. These oscillations occur because the potential of the electrode begins to break in the cathodic direction before the potential region >There streaming can begin is reached. Streaming brings excess copper u p to the electrode and causes the potential to shift anodically out of the range where streaming occurs. The process then is repeated several times in succession. Chronopotentiometry at the Mercury Pool Cathode. Chronopotentiograms a t t h e mercury pool cathode VOL. 34, NO. 7, JUNE 1962
793
were determined for solutions of bismuth and copper, which had been shown to stream using the other techniques studied. A t the pool electrode, streaming occurred only for copper a t current densities greater than 0.6 ma. per sq. cm. CONCLUSIONS
Convective streaming can very markedly affect data obtained using any of the techniques studied. I t is, therefore, advisable in analytical applications of these techniques to eliminate with certainty any streaming by the addition of a suitable maximum suppressor. The data confirm that geometry of the electrode plays some role in the phenomenon of streaming. Streaming at a small spherical drop which is
partly shielded by the glass capillary is much more prevalent than streaming a t a pool mercury electrode. A very important factor in streaming is current density. I n the two methods using the hanging mercury drop electrode, streaming was prevalent a t current densitiL2 greater than 0.4 ma. per sq. cm. while no streaming occurred at current densities below 0.25 ma. per sq. cm. I n experiments using the mercury pool, the current density at which streaming began with copper was 0.6 ma. per sq. cm. A certain minimum current density seems necessary to provide sufficient force to start motion of the solution. LITERATURE CITED
(1) Frankenthal, R. P., Shain, I., J . Am.
Chem. SOC.78, 2696 (1956).
(2) Ishibashi, M.,Fujinaga, T., Anal. Chim. Acta 18, 112 (1958). (3) Kolthoff, I. M., Okinaka, Y., J . Am. Chem. SOC.80, 4452 (1968). (4) Nilner, G. W. C., “The Principles and Applications of Polarography,” p. 70, Longmans Green, London, 1957. (5) Reilley, C. N., Sdams, R. N., Furman, N. H., , 4 ~ . 4 CHEM. ~. 24, 1044 (1952). (6) Reilley, C. N., Everett, G. W., Johns, R. H., Ibzd., 27,483 (1955). (7) Ross, J. W., DeMars, R. D., Shain, I., Ibid., 28, 1768 (1956).
RECEIVED for review March 12, 1962. ilccepted April 16, 1962. Taken in part from a thesis submitted by John W. Olver in partial fulfillment of the requirements for the Ph.D. degree, Massachusetts Institute of Technology, June 1961. Work supported in part by the U. S. Atomic Energy Commission under Contract AT(30-1)-905 and by a Summer Fellowship from the National Science Foundation.
Oxidation Procedure for the Spectrophotometric Determination of 2,6- Di-tert-buty I-p-Cresol in PolyoIef; ns CAMILE STAFFORD Research Division, Phillips Petroleum Co., Bartlesville, Okla.
b A method was developed for the determination of lonol (2,6-di-fertbutyl-p-cresol) in polyolefins. It was developed specifically for copolymers, to which a direct ultraviolet method used on homopolymers was not applicable. lonol i s extracted from the polymer sample with cyclohexane and i s oxidized under controlled conditions in alkaline isopropyl alcohol. The base line absorption of the colored oxidation production as measured at 365 mp is a linear function of the lonol concentration. At 0.02070 duplicate determinations did not vary by more than 5%. This procedure eliminates the interference of dissolved polymer and Santonox (4,4’-thiobis(6-tert-butyl-m-cresol)).
I
(2,6-di-tert-butyl-p-cresol) is a thermal antioxidant for polyethylene, The Ionol content of ethylene homopolymers can be determined by extraction of the sample with boiling cyclohexane followed by measurement of the ultraviolet absorbance peak of Ionol at 277 mp (10). This technique fails with ethylene-butene copolymers and other more soluble polyolefins. The dissolved polymer interferes with the measurement at 277 mp. The oxidation of Ionol in alkaline isopropyl alcohol produces a solution with a strong absorption band with a maximum at 365 mp. This absorption ONOL
794
ANALYTICAL CHEMISTRY
band is much stronger than the absorption band of Ionol at 277 mp. Also the polymer interference is less at longer wavelengths. Therefore, work was directed toward reproducibly oxidizing Ionol to form this material. EXPERIMENTAL
Apparatus. Cary Model 11 recording spectrophotometer or equivalent equipped with 5-em. cells. Oxidation reactor consisting of a Wiley extractor modified with a gas inlet tube at the bottom of the extractor. Reagents. Potassium hydroxidesaturated isopropyl alcohol. Purge 800 ml. of spectrograde isopropyl alcohol in a q u a r t bottle for 15 minutes with nitrogen (commercial prepurified grade) a t 50 to 100 ml. per minute. Add approximately 100 grams of potassium hydroxide and continue the nitrogen purge for 15 minutes. Cap the bottle tightly with an aluminum foil-lined screw cap and shake vigorously for 1 hour (a mechanical shaker may be used). Then purge the solution with nitrogen at 50 to 100 ml. per minute for 6 to 8 hours. Store in a tightly capped bottle under a n atmosphere of nitrogen. Purge with nitrogen continuously while using this solution. This solution is only stable for about 3 days. Exposure to air will shorten the useful life of the solution. Analysis. Grind the sample in the Wiley cutting mill t o pass a 10-mesh screen and weigh (*0.005 gram) a portion of t h e ground sample t o
contain 0.04 t o 0.8 mg. of Ionol. Transfer t h e sample t o the bottom of a Wiley extractor a n d add 20.0 ml. of spectrograde cyclohexane. Place the extractor in a boiling water b a t h and reflux for 30 minutes. Pass nitrogen through the oxidation reactor at approximately 50 ml. per minute and add 50 ml. of potassium hydroxide-saturated isopropyl alcohol. Place the reactor in a boiling water bath. When the solution begins to reflux, pipet in 5.00 ml. of the filtered cyclohexane extract. Change the nitrogen to air, filtered to remove oil and dust particles, and reflux for 15 1 minutes with air passing through the system at approximately 50 ml. per minute. Cool the reactor for 1 minute in an ice bath and then transfer the solution to a 100-ml. volumetric flask. Dilute the solution to 100 ml. with spectrograde isopropyl alcohol. To a second 100-ml. flask add the same quantity of cyclohexane extract as used in the reactor and dilute to 100 ml. with spectrograde isopropyl alcohol. Use this solution as the reference for spectrophotometric comparison. Scan the reactor solution us. the reference solution in 5-cm. cells from 400 to 300 mp. Determine the base line absorbance on the spectrum by drawing a line tangent to the absorbance minima near 400 and 325 mp. The difference between this line and the absorbance peak a t 365 mp is the base line absorbance. The base line absorbance is a straight line function of the Ionol concentration up to 2 p.p.m. of Ionol in the final solution.
