were determined for solutions of bismuth and copper, which had been shown to stream using the other techniques studied. At 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. In 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).
RECEIVEDfor 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 quart 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 the ground sample t o
contain 0.04 t o 0.8 mg. of Ionol. Transfer the sample t o the bottom of a Wiley extractor and add 20.0 ml. of spectrograde cyclohexane. Place the extractor in a boiling water bath 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.
Calibration. Follow the procedure outlined above using standard solutions containing 0.00, 0.02, 0.05, 0.08, 0.1, 0.2, and 0.5 mg. of Ionol per 5 ml. of cyclohexane. RESULTS AND DISCUSSION
Preliminary experiments showed that the rate of oxidation of Ionol in alkaline isopropyl alcohol to form the material with an absorption peak of 365 mp increased with increased alkalinity and temperature. Therefore, the oxidation of Ionol was studied in a refluxing potassium hydroxide-saturated isopropyl alcohol solution. Effect of Oxygen. Dissolved oxygen causes oxidation of the alcohol in a hot alkaline alcohol solution. The oxidation products interfere with the measurement of the Ionol oxidation product. An oxygen-free alkaline alcohol solution produces a negligible quantity of ultraviolet absorbing material when refluxed for 30 minutes. The Ionol oxidation occurs only in the presence of oxygen. If air is bubbled through the oxygen-free alkaline alcohol during the oxidation step, sufficient oxygen is present to complete the desired reaction. Fortunately, the amount of interfering color from the solvent alone is negligible. The peak a t 365 mp is caused only by the Ionol oxidation product (Figure 1). The quantity of Ionol oxidation product found reaches a maximum value in 10 to 15 minutes of refluxing and then remains constant with a gradual increase in the background adsorbance up to 30 minutes of refluxing. This reaction is reproducible and the reaction product is stable for 15 minutes a t room temperature. Effect of Cyclohexane. Since cyclohexane extracts Ionol from polyethylenes readily, the effect of various amounts of cyclohexane on the oxidation of Ionol in alkaline isopropyl alcohol was investigated. Increasing amounts of cyclohexane reduced the absorbance of the Ionol oxidation product, b u t for a given concentration of cyclohexane the measurement is reproducible. The presence of cyclohexane caused some potassium hydroxide to precipitate. The decreased alkalinity probably reduced the extent of the oxidation. Effect of Polymer. Considerable short-chain polymer, especially from the copolymer samples, will be present in the cyclohexane extract. A solution containing short-chain polymer was prepared by extraction of Ionolfree polymer with cyclohexane. An aliquot of this extract, added to Ionol in the alkaline alcohol solution, had no effect on the oxidation. The procedure was checked by extraction of Ionol-free poiymer with
cyclohexane containing a known quantity of Ionol. A portion of the cyclohexane extract was oxidized and the Ionol content measured as described in the procedure. The JX%ults are shown in Table I; Recovery of Ionol by Slurry Extraction. Standard Ionol-containing polymers can be prepared by blending (10). However, a more convenient
Table 11.
0.01’82 0.0160 0.0182 0.0182 ... ... 0.0182 0.0178 Established by nonequilibrium extraction until no further Ionol extracted. 30 60
method for the determination of total Ionol is extraction of the polymer under nonequilibrium conditions. Ground polymer was placed in the cup of a Wiley extractor and hot cyclohexane was refluxed through the sample. After various lengths of time an aliquot of the cyclohexane was analyzed by the described procedure. The absorbance reached a constant value after 2 hours. The same value was obtained after 4 hours extraction. This technique allows “calibration” of the polymer. Two representative copolymers were calibrated (Table 11). Separate portions of these polymers were analyzed by the slurry-extraction
0.8
0.6
i$
’
Ionol, Wt. 70 Presenta Found
Ionol, Kt. 70 Presenta Found 0.0162 0.0142 0.0162 0.0164 0.0162 0.0164
Time,
Minutes 10 15
w
Type Polymer Added Recovered Copolymer 0.00 0.00 Copolymer 0 . 400a 0.408 Copolymer 0.400 0.404 Equivalent to 0.02% in the polymer.
Extraction of lonol from Copolymers with Cyclohexane
Extraction
a
Table 1. Recovery of lonol b y Oxidation-Spectrophotometric Procedure Ionol, Mg.
procedure (extraction of the sample mixed with boiling cyclohexane). The weight per cent Ionol recovered as a function of the extraction time is shown in Table 11. The agreement between the data shows that all of the extractable Ionol will be removed from the copolymer in 30 minutes. Ethylene homopolymers behave similarly. Effect of Santonox. The presence of Santonox [4,4’-thiobis(6-tert-butylm-cresol) 1, another stabilizer sometimes present in polyethylene, interferes with this determination. However, the error in the Ionol determination will not exceed 10% if the amount of Santonox present is less than ten times the amount of Ionol. At equal concentrations Santonox causes no interference. Santonox can be removed from the cyclohexane extract with 1N aqueous sodium hydroxide (10). Identity of Oxidation Product. T h e oxidation of Ionol under a variety of conditions has been studied by numerous workers (1-9, 11-13). However, none of the products found appear to correspond to the oxidation product formed
Table Ill. Precision of lonol Determinations b y Oxidation-Spectrophotometric Procedure
0
04
Sample 1 0.2
2 l
!
l
l
l
l
l
l~
3
00 320
340
360
380
400
WAVELENGTH , MILLIMICRONS
Figure 1. Spectrum of lonol oxidation product in alkaline isopropyl alcohol
4
5
Wt. 7” Ionol Found 0.0192 0,0199 0.0158 0.0159 0.0187 0.0179 0.0216 0,0216 0.0162 0.01691
VOL. 34, NO. 7, JUNE 1962
795
in this study. Further work on the identity of the colored species was not justified. The reproducibility of the oxidation product formation was sufficient for the analysis. The precision of the Ional determinations is shown in Table 111. Since polymers containing appreciable quantities of oxidized Ionol were not investigated, the effect of such products on the method is not known. ACKNOWLEDGMENT
The author gratefully acknowledges the helpful suggestions of Dean J.
Veal in the development of this analytical procedure. LITERATURE CITED
(1) Becconsall, J. K.; Clough, S., Scott, G., Proc. Chem. Soc. 1959,308. (2) Campbell, T. W., Coppinger, G. M., J . Am. Chem. Soc. 74,1469 (1952). (3) Cook, C. D., J . Org. Chem. 18, 261 (1953). (4) Cook, C. D., Nash, E.G., Flanagan, H. R., J . Am. Chem. SOC. 77, 1783 (1955). (5) \ - , Cook. C. D.. Woodworth. R. C.. Zbid,, 75, 6242 (1953). (6) Coppinger,. G. M., Campbell, T. W., Zbzd., 75, 734 (1953).
(7) Gersmann, H. R., Bickel, A. F., J. Chem. SOC.1959,2711. (8) Gersmann, H. R., Bickel, A. F., Proc. Chem. SOC.1957. 231. (9) Kharasch, M. S., Jdshi, B. S., J . Org. Chem. 22, 1439 (1957). (10) Spell, H. L., Eddy, H. L., AKAL. CHEM.32,1811 (1960). (11) Yohe, G. R., Dunbar, J. E., Pedrotti,
R. L., Scheidt, F. M., Lee, Fred G. H., Smith, E. C., J. Org. Chem. 21, 1289 (1956).
(12) Ibid., 24, 1251 (1959). (13) Yohe, G. R., Hill, D. R., Dunbar, J. E., Scheidt, F. M., J . Am. Chem. SOC. 75,2688 (1953).
RECEIVED for review December 15, 1961. Accepted April 4, 1962.
Potentiometric Studies of the Titration of Weak Acids with Tetra butylammonium Hydroxide LELAND W. MARPLE and JAMES S. FRITZ Institute for Atomic Research and Department of Chemistry, Iowa State University, Ames, Iowa ,The sources of amine, carbonate, and silver impurities in tetrabutylammonium hydroxide have been investigated, and techniques for their removal have been evaluated. The stability of the base in water, isopropanol, tert-butyl alcohol, and pyridine was determined. Salt bridge systems of the type
1
I
SCE aqueous phase MCL-HzO-organic organic phase MCL-HsO-organic have been devised for acetone, isopropanol, fert-butyl alcohol, and pyridine. Titrations of weak and very weak acids using the glass indicating electrode were reproducible to within 2 to 5 mv.
T
HERE are a t least two major advantages in using tetrabutylammonium hydroxide as a titrant in nonaqueous solvents. One is that the salts formed, in contrast to the salts of alkali hydroxide or alkoxides, are soluble in low dielectric media. The other advantage is that excellent potentiometric curves can be obtained with an ordinary glass electrode. Tetrabutylammonium hydroxide titrants can be prepared by two methods. Harlow, Noble, and Wyld (4) prepared the hydroxide in isopropanol by anion exchange starting with the iodide salt and obtained a 0.2N titrant with only 0.5% water. The titrant was not stable and decomposed to give a weak base at the rate of 1% in 6 weeks. Harlow aud Bruss (3) later prepared a 1.5.44 titrant by evaporation of iso-
796
ANALYTICAL CHEMISTRY
propanol from a 0.2M solution prepared by ion exchange. Their data do not show whether impurities were present in the titrant, or if the concentrated base solutions decomposed over a period of time. Cundiff and Markunas (1) were the first to prepare anhydrous tetrabutylammonium hydroxide in methanol by the silver oxide process. Starting with the iodide salt, they prepared an equimolar mixture of hydroxide and methoxide in methanol, and then diluted with benzene to give a titrant containing 10% methanol. The base in this solvent was stable for a t least 60 days. Because of the acidity of methanol, it is desirable to reduce the methanol content to 5% or lower. Pritchett (8) found that when this was done, the solution of base decomposed slightly within 1 month, even when stored under nitrogen and protected from light. Previous attempts to prepare anhydrous hydroxide solutions in isopropanol have been both successful and unsuccessful. Harlow and Bruss (3) reacted the iodide salt with silver oxide in isopropanol and prepared a 1.5M titrant. Malmstadt and Vassallo (7) prepared a similar hydroxide, methyltributylammonium hydroxide, in isopropanol without any apparent difficulty. However, Hummelstedt (6) found that when the oxide was added to an isopropanol solution of tetrabutylammonium iodide, a semicolloidal suspension formed that continued to settle for days despite repeated filtrations. I n spite of rigid exclusion of carbon dioxide during the preparation of
tetrabutylammonium hydroxide by the silver oxide process, preparations made in this laboratory frequently contained 1 to 2y0 impurity as a weak base. Cundiff and Markunas (I) noted that their preparations contained severe1 per cent impurities and suggested that the impurity might be tributylamine. However, addition of tributylamine to a strong acid solution did not alter their results for the titration of the acid with pure tetrabutylammonium hydroxide (prepared by ion exchange) in pyridine solvent. Because the impurity was removed by anion exchange, it now seems evident that it was indeed carbonate. However, the fact that tetrabutylammonium hydroxide can undergo Hofmann elimination to give tributylamine, 1-butene, and water should be taken into account when the stability of the base is considered. The instability of the base in 5% methanol-95yo benzene solution (8) and o.5y0water-99.5% isopropanol solution (4) was undoubtedly a result of the Hofmann elimination. The goals of this research were to avoid decomposition of the base when prepared by the silver oxide process, to evaluate methods for removing both carbonate and tributylamine, and to develop an electrode system that would give reproducible potentiometric titration curves in very weakly acidic solvents. Previously, the calomel electrode used was modified by replacing the aqueous saturated potassium chloride solution by methanolic saturated potaasium chloride. While good potentiometric curves for weak acids were
