Anal. Chem. 1980, 52, 601-602
60 1
Determination of Dissolved Oxygen in Nonaqueous Electrochemical Solvents J. M. Achord and C. L. Hussey" Department of Chemistry, University of Mississippi, University, Mississippi 38677
Considerable interest has developed in recent years concerning the electrochemistry and reactions of superoxide ion. One of the most convenient methods of generating superoxide ion is by the one-electron electrochemical reduction of oxygen dissolved in polar aprotic solvents. Consequently, there is need for an accurate method for determining oxygen dissolved in these solvents on a routine basis. Previous studies involving electrogenerated superoxide ion (1-5) utilized analbtical procedures for oxygen determination based on various modifications of the Winkler method. We have applied these procedures to the determination of oxygen dissolved in a variety of polar aprotic solvents and have found them to be unsatisfactory for use on a routine basis. Considerable dexterity is required in their application and the results obtained often lack precision. A more reliable, less ambiguous analytical procedure based on gas chromatography would be desirable. Gas chromatographic methods have been employed for the determination of gases dissolved in petroleum fractions ( 6 ) and aromatic hydrocarbons (7). These techniques involve direct injection of the fluid to be analyzed onto a precolumn either connected to or physically located in the chromatograph. T h e precolumn captures the fluid and liberates dissolved gases which are swept into a molecular sieve column and separated. In this paper we report a gas chromatographic procedure utilizing a precolumn in tandem with a chromatographic column which is suitable for the routine determination of oxygen dissolved in aprotic solvents commonly used for electrochemistry. This method is especially attractive in that it is inexpensive, employs a standard chromatograph equipped with a thermal conductivity detector, and requires no special auxiliary apparatus. In addition, heretofore unreported data for the solubility of oxygen in several oxygen saturated solvents, obtained using this technique, are presented.
Table I. Solubility of Oxygen in Organic Solvents at 2 5 . 0 "C concent rat i o n of dissolved oxygen, mM solvent acetone (air satd.) acetonitrile .'V,i,,V-dimethylformamide
dimethylsulfoxide 2,2,4-trimethylpentane propylene carbonate pyridine tetramethylene sulfone (30 'C)
this work
literature
?
0.2
2.2 ( 8 )
8.1 k 4.5 I 2.1 ? 14.8 f
0.6 0.3 0.1 0.9
2.3
3 . 6 t 0.2 4.9 i 0.4 0.97 + 0 . 0 2
-
3.1 (5)
2.1 ( 2 ) 14.4(6) -
to analysis. No allowance was made for minor day-to-day changes in atmospheric pressure. Solvent samples, 0.5 mL in size, were withdrawn from the sample bottles using a 2-mL syringe equipped with a hypodermic needle and injected into the inlet of the chromatograph. The syringe was calibrated by weighing the quantity of water that it would deliver.
RESULTS AND DISCUSSION Calibration curves, which consisted of plots of peak area vs. oxygen gas volume were constructed prior t o each series of experiments on each precolumn-chromatographic column. Typical calibration curves, represented by plots of peak area vs. oxygen sample size, are shown in Figure 1. Data for the calibration curves were obtained by injecting samples of pure oxygen gas using a 100-pL gas syringe (Precision Sampling Corp.). Oxygen gas samples were injected between solvent samples t o ensure that the calibration curve remained valid. T h e retention time for oxygen gas under the conditions of analysis was ca. 8.8 min. The procedure was applied to the determination of dissolved oxygen in the oxygen saturated solvents listed in Table I. The values for the concentration of dissolved oxygen are mean values for a minimum of at least five determinations. T h e precision within each set of measurements is shown as the standard deviation. Overall, the accuracy of the method appears to be quite good as can be seen by comparison of the results in Table I, obtained in the present study, with literature data. Our values for oxygen solubility in Nfl-dimethylformamide are somewhat higher than literature values and we suspect that the literature value ( 5 ) may be in error. Additional evidence for this can be found in the extraordinarily high values of the diffusion coefficient for oxygen measured polarographically by these cm2/s, in oxygen saturated N,N-diworkers, 1.4 X methylformamide. We employed rotating disk electrode voltammetry to measure the limiting current for oxygen reduction in oxygen saturated N,N-dimethylformamide as a function of rotation rate a t a glassy carbon electrode a t -1.2 V vs. SCE. Using our solubility data, this limiting current data, and the Levich equation, we calculated a value of 4.7 X cm2/s for the diffusion coefficient of oxygeii. ?'his diffusion coefficient value is much closer t o t h a t measured by other workers ( 2 , 3 )for oxygen dissolved in aprotic solvents. The sensitivity of this method is ccintrolled by the sensitivity of the thermal conductivity detector. Since the response characteristics of the detector change with flow rate, it was necessary to construct a new calibration curve whenever changes were made in the system which affected the flow rate, e.g., after repacking the precolumris. Recali bration also is
EXPERIMENTAL Apparatus. Gas chromatographic measurements were made with a Beckman GC-45 gas chromatograph equipped with a thermal conductivity detection system. Chromatograms were recorded using a Beckman Model 1005 strip chart recorder. The chromatograph was equipped with a precolumn which consisted of a 4-ft length of 0.25-in. copper tubing containing 60-200 mesh silica gel (Fisher Grade 950). The precolumn was connected using Swagelok fittings in tandem with a chromatographic column fabricated from a 5-ft length of 0.25-in. copper tubing containing 5A molecular sieve pellets (Linde) ground t o 2C-30 mesh. The dual column chromatograph was equipped with two identical precolumn-chromatographic column arrangements. The precolumns and chromatographic columns were conditioned prior to use by heating them to 250 "C for 24 h under flowing carrier gas. Helium was used as the carrier gas with a flow rate of 30 mL/min. During analysis, the chromatograph and columns were maintained at room temperature. Materials. Acetone (J. T. Baker, reagent grade), acetonitrile (Aldrich, spectrophotometric grade), N,N-dimethylformamide (Aldrich, spectrophotometric grade), dimethylsulfoxide (J. T. Baker, reagent grade), 2,2,4-trimethylpentane (J. T. Baker, reagent grade), propylene carbonate (Aldrich, 99% 1, pyridine (J. T. Baker, reagent grade), and tetramethylene sulfone (Aldrich, 99% ) were used as received. Procedure. The solvents were placed in small bottles with septum caps and thermostated at 25.0 f 0.2 "C in a constant temperature bath. Two hypodermic needles were inserted into the septum cap, one to admit oxygen gas (Linde, dry grade) and one to permit the escape of gas. The solvents were saturated with oxygen gas at atmospheric pressure for approximately 1 h prior 0003-2700/80/0352-0601$01 0010
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1980 American Chemical Society
Anal. Chem. 1980. 52. 602-604
602
use by discviiriecting them from the chromatographic columns and heating the column oven above the boiling point of the solvent while continuing a flow of carrier gas. Provisions must be made, however, to vent the exhaust from the heated precolumns into a trap or fume hood. T h e gas chromatographic technique for oxygen determination presented in this paper was demonstrated to be a viable alternative to presently used methods for oxygen determination in polar aprotic solvents. T h e method has additional advantage for electrochemical applications in that no allowance for interference of dissolved supporting electrolyte is necessary.
LITERATURE CITED
02(pmoll
Flgure 1. Plot of chromatographic peak area vs. oxygen ban ple size. ( 0 )column 1, 25 'C, (W) column 2, 25 'C, and (0)column 1, 30 O C
advised if there are inordinate changes in the room temperature from one series of measurements t o another. Precolumn repacking was not necessary in the case of the more volatile solvents such as acetone, acetonitrile, and pyridine. T h e precolumns could be regenerated for further
(1) Coetzee, J. F.; Kolthoff, I. M. J . Am. Chem. SOC.1957, 79, 6110-15. (2) Johnson, E. L.;Pool, K. H.: Hamm, R. E. Anal. Chem. 1900, 38, 183-5. (3) Sawyer, D. T.; Roberts, J. L., Jr. J . Electroanal. Chem. 1900, 72, 90-101. (4) Toni, J. E. A. J . Electrochem. SOC. 1969, 116, 212-17. (5) James, H. J.; Broman, R. F. Anal. Chim. Acta 1909, 4 8 , 411-17. (6) Petrocelli, J. A . ; Lichtenfels, D. H. Anal. Chem. 1959, 3 1 , 2017-19. (7) Ford, P. T. Anal. Chem. 1909, 4 1 , 393-5. (8) Damrnersde Klerk, A.; Boot-Meurs, B. Anal. Chim. Acta 1957, 16,
296-7.
RECEIVED for review September 21,1979. Accepted December 10, 1979. This research was supported by Lawrence Livermore Laboratories through subcontract No. 4288309 and through a University of Mississippi Faculty Research Grant.
Determination of 1,2-Diols by Indirect Atomic Absorption with Digested Lead Periodate Barrie Tan," Paul Melius, and Melvin V. Kilgore Department of Chemistry, Auburn University, Auburn, Alabama 36830
Oxidation with the periodate ion causing the specific 1,2-diol scission is the most useful tool in carbohydrate chemistry, as well as in the determination of this functional group. Frequently, the methods of analysis involve the determination of periodate consumed or the products formed, namely, formaldehyde, formic acid, or carbon dioxide ( I , 2). T h e periodate ion absorbs a t 222.5 nm and the decrease in absorbance has been used t o quantify the amount of adjacent dihydric phenols (3,4 ) . Colorimetric procedures, using the dinitrophenylhydrazine for the assay of glyceric acids and related compounds ( 5 , 6) and glycols ( 7 ) have also been reported. Recently, indirect atomic absorption methods were employed for the determination of aliphatic secondary amines, aldehydes, and thiols (8-10). In each of these indirect atomic absorption spectrophotometry (IAAS) methods, a metallic precipitate formed with the analyte or with the product was incorporated in the final procedure before filtration and finally digested for analysis. IAAS has been applied in the determination of 1,2-diols by Oles and Siggia ( 1 1 ) . Periodic acid was used t o oxidize the glycols. T h e iodate formed was then precipitated carefully as t h e silver iodate, filtered, rinsed, and digested before analysis for the silver content. T h e acid and reagent concentrations used, as well as the cooling of the preflocculated AgIO, to -10 t o --15 "C were altogether critical for the preferential precipitation of AgIO, while keeping the periodate in solution. Determination of six samples by this procedure may be accomplished in 90 min. In the present work, 1,2-diols were reacted with an excess of potassium periodate, the remaining periodate was precipitated with excess lead nitrate and the lead periodate formed
was separated from the reaction medium by membrane filand tration. The precipitate was then dissolved in l M "OB analyzed for the lead content by atomic absorption spectrophotometry. Lead nitrate was used to prepare all calibration solutions and only one calibration curve was needed.
EXPERIMENTAL Apparatus. A Perkin-Elmer 103 atomic absorption spectrophotometer was used. The 283.3-nm line from a single element (Pb) hollow cathode lamp, operating a t a modulated half-wave current of 5.5 mA was used. The flame and slit setting were air-acetylene and 0.7 nm, respectively. Details of instrument settings may be obtained in Reference 12. All filtrations were accomplished with Nalgene (Nalge Co.) disposable filtration units (0.2-pm porosity). Each filter was repeatedly used for about 6 samples before discarding. Reagents. Lead nitrate, l,Z-ethanediol, 1,2-propanediol, and 1,2,3-propanetriol(glycerol) were obtained from Fisher Scientific Co., while l-phenyl-1,2-ethanediol(styrene glycol), l-amino-2,3propanediol, and 1,3,5-trihydroxybenzene (phloroglucinol) were obtained from Aldrich Chemical Co. 1,4-Dihydroxybenzene (hydroquinone) and 1,3-dihydroxybenzene (resorcinol) were purchased from MCB Manufacturing Chemists while 1,2-dihydroxybenzene (catechol) and 1,2,3-trihydroxybenzene (pyrogallol) were purchased from J. T. Baker Chemical Co. Finally, ~~-0-3,4-dihydroxyphenylalanine (DOPA) and potassium periodate (or metaperiodate) were obtained from Sigma Chemical Co., and Baker and Adamson Co., respectively. The best available grades of these compounds were used without further purification. All diols were prepared in aqueous solution and kept at room temperature. These compounds should be stable for at least 3 weeks (11). DOPA was stored at 5 OC and 1,2-dihydroxybenzene at room temperature, and both were used within 3 days. An aqueous solution of K104 (500 mL, 2.00 X 10' M) was transferred into an amber container arid stored in the dark when not in use.
