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Anal. Chem. 1982, 5 4 , 1639-1641
thod of evaluation. As the data given above indicate, the results obtained by the proposed method using the calibration curve and the method of standard additions were in close agreement (0.057% S anti 0.058% S) which showed that a 1-h combustion time was sufficient and that the use of the method of standard additions WELRactually a supeirfluous precaution. The trapping solution yielded no reagent blank signal. It is possible that the shorter than 1-hi burning intervals yielded higher SOz concentrations because of a difference in mobility within the lamlp wick of the sulfur compounds and of the bulk of the oil (13). If this is indeed the case, a 60-min burning interval may not necessarily be a safe choice for all kerosenes. Instead, either the effect of burning time on the results must be studied for each new kerosiene or the samples have to be burned to calmpletion. The finishing method for the determination of sulfur in kerosene studied in this work is sensitive, rapid, and specific for SOz. The specificity of the technique is due to the fact that conceivable interference can only be caused by a volatile organic compound whiclh somehow avoids being burned but does become trapped by the absorbing solution, which also becomes evolved into the gas phase upon injection of the absorbing solution into acid, and which furthermore yields a vapor capable of absoirbing at 210 nm. Test runs of reagent-grade sulfur-free keirosene yielded no signals. The total analysis time for one sample, excluding combustion, is about
10 min. The method requires little specialized instrumentation. The reaction vessel can be constructed in a few hours by any person familiar with basic glassblowing techniques and the atomic absorption spectrophotometer is, of course, a common instrument in most analytical laboratories.
LITERATURE CITED (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13)
Stephens, 8 . G.; Lindstrom, F. Anal. Chem. 1884, 36, 1308. Seefield, E. W.; Robinson, J. W. Anal. Chim. Acta 1880, 22, 61. West, P. W.; Gaeke, G. C. Anal. Chem. 1858, 28, 1816. EPA Reference Method for the Determination of Sulfur Dioxide in the Atmosphere (Pararosaniline Method), Fed. Reglst. 1871, 36 (Nov. 25) 22384. West, P. W.; Ordovesa, F. Anal. Chem. 1882, 3 4 , 1324. Wlnkier, H. E.; Syty,A. Envlron. Scl. Techno/. 1878, 10, 913. Nlcholson, G.; Syty,A. Anal. Chem. 1878, 4 8 , 1481. Syty,A., Anal. Chem. 1878, 57, 911. Muroskl, C. C.; Syty,A. Anal. Chem. 1880, 52, 143. Syty. A.; Simmons, R. Anal. Chlm. Acta 1880, 120, 163. Grieve, S.;Syty,A. Anal. Chem. 1981, 53, 1711. Krueger, J. A. Anal. Chem. 1874, 4 6 , 1338. Slemer, D. D.,private communication, 1982.
Michael L. Ruschak Augusta Syty* Department of Chemistry Indiana University of Pennsylvania Indiana, Pennsylvania 15705
RECEIVED for review February 16,1982. Accepted April 28, 1982.
Dissolution of Perfluorinated Ion Containing Polymers Sir: Much recent research effort has been devoted to the preparation, characterization, and applica.tionsof chemically modified electrodes based on ion-containing polymers (ICPs) (1-10). Of the half dozen or so polymerei studied to date, a 970 equivalent weight (ew) member of DuP’ont’s Ndion family of perfluorosulfonic acid polymers (see I) has proved to be one
I
DuPont does, however, commercially market 1100 and 1200 ew versions of this polymer. While these higher ew versions are also chemically inert and water insoluble and also strongly retain certain counterions ( I I ) , there is no published procedure for dissolving these polymers. Hence, it has not been possible to cast films of these polymers and for this reason applications to CMEs have not been reported. Our research group has initiated a study of the chemical and electrochemicalproperties of a number of families of ICPs. We wanted to include the Nafion polymers in this study. It became necessary, therefore, to develop a procedure for dissolving these polymers; we have succeeded in developing such a procedure. Because of the great current interest in surface modification in general, and in modification via coating with ICPs in particular, we are reporting this Nafion dissolution procedure at this time. A brief electrochemical characterization of films prepared from the dissolved polymers is also included here. A more detailed report of the electrochemical characteristics of CMEs based on the 1100 and 1200 ew Nafions will be presented elsewhere (13).
of the most useful for preparing chemically modified electrodes (CMEs) of this type (4-6,9, 10). This is because the 970 ew Nafion is very chemical1.y inert, is soluble in ethanol so that films may be cast but completely insoluble in water, and it strongly retains certain electroactiveions even in the presence of huge excesses of supporting electrolyte (9, 11). Unfortunately, the 970 ew Ndiori was produced by DuPont in limited quantity; it is not commercially available nor does DuPont have any more to give away (12). Hence, despite the ideal properties of this polymler for preparation of CMEs, future studies and applications seem limited due to the inadequate supply.
EXPERIMENTAL SECTION The Nafion samples were obtained from the DuPont Co. (Wilmington, DE). Methylviologen was obtained from Aldrich (Milwaukee,WI). All other reagents were of analytical reagent grade. All solutions were prepared with triply distilled water. Chemically modified electrodes were prepared as described by Martin et al. (9). Electrochemical studies were performed using an EG&G PARC (Princeton, NJ) Model 175 programmer, Model 173 potentiostat/galvanostat, and Model 179 coulometer. A Houston Instruments (Austin, TX) Model 2000 X-Y recorder was used to record the cyclic voltammograms. All potentials were measured va. a saturated calomel reference electrode. Solutions were degassed with prepurified N2prior to the electrochemical
- CICF:?CFz), CFCFz1,r l
1
0
I
CFz C Fz S 0 3 ti
0003-2700/82/0354-1639$01.25/0
0 1982 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 54, NO. 9, AUGUST I982
V
Flouv 1. Cyclic voltammetric background cunents in 0.2 F sodium triRwfoacetata: potentab measued vs. SCE; sweep rale = 100 mV s.t. , (A) bare glassy carbon electrode. (8)glassy carbon electrode ccated w b a N a h flm containkg 1.73 X IO-' mol an-*Suncnlc add groups. meaauremente. A PARR Instrument Co.(Moliue, IL)Model 4561, 300-mL reactor wan used to dissolve the polymers. RESULTS AND DISCUSSIONS Theomtical Considerations. Yw's study of the swelling behavior of the 1100 and 1200 ew Nafons (14) provided us with mcet of the information we required to devise a procedure for dissolving these polymers. Yeo has shown that the 1100 and 1200 ew Nafions are unique in that they exhibit two solubility parameter, 6, values (14). Therefore, these polymers interact much more strongly with appropriate binary solvent qntems than with any single solvent (14). (Appropriatebinary solvent system means a system composed of a solvent whose 6 value approximates the lower 6 of the polymer and a solvent whose 6 value approximates the higher 6 of the polymer.) For example, 1200 ew Nafion shows an approximately 130 wt % increase in 2-propanol-water as opposed to a, a t most, 70% increase in a pure solvent. For this reason, it seemed likely to us that dissolution would be most probably in an appropriate binary solvent system. Choice of solvent system is, however, not the only consideration in that none of the binary systems inveatigated by Ye0 or by us would dissolve the polymers at rmm temperature. As pointed out by Yea (14), this is undoubtedly due to the presence of regions of crystallinity in the polymer chain material part of the ICP (14,15). Temperatures sufficientto melt these crystalline regions must he applied if dissolution is to be accomplished. Dissolution Procedure. We have found that both 5050 propanol-water and 5050 ethanol-water, when heated to 250 "C in a high pressure reactor, wiU diasolve both the 1100 and the 1200 ew Nafions. A typical procedure for preparation of a 1wt/vol % solution of the polymer is as follows. A 1-g sheet of the as-received polymer was placed in a beaker containing the solvent and this wan placed in an active ultrasonic cleaner for 1h. This procedure, while not necessary to the dissolution process, apparently rids the polymer of some unknown impurity (16) in that, as noted by Lee and Meisel (16), the polymer has a yellow tinge when received and is totally colorless after this treatment. The polymer sheet was then placed in the reactor with 100 mL of the solvent. The reactor was sealed and purged with N2. The stirrer was turned on and the temperature raised to 250 OC. The temperature was held a t 250 OC for 1 h, after which the heater was turned off and the bomb was allowed to ml. After cooling,the 1% solution wan removed from the reactor. We have not attempted to prepare solutions with concentrations higher than 1wt/vol %. Finally, it is of intereat to note that the solutions prepared have a strong ether smell; a plausible explanation is that ether ia produced via acid catalyzed dehydration of the alcohol (17). Characterization of the Polymer after Dissolution. In their brochure on the safety in handling and use of Nafion products (19),the W o n t Co. states that thermal degradation
2. Cyclk voltammograms fw MV2+ contained in ttm polymer Rim of ttm CME desdbed in Flgure 1B: potenUals measwed vs. SCE: sweep rates = 100 mV s-': (A) h 1 X lo3 F MVCI, plus 0.1 F NaF. (E) atter Incorporation of MV2+: in 0.1 F NaF. -IO
Table I. Results of Experimenta To Assess the Ion Exchange Capacity ofthe Reconstituted Films m X %ofsites theoretical, electroexptl, nmol chemicalIy nmol of ew MV" of MV" active 1100 2.18 1.52 IO 1200
2.00
1.4R
74
of Nafion begins a t around 250 OC. There is, therefore, the passihility that degradation of the polymer occurs during the diasolution proceas. We have evaluated some of the response characteristics of CMEs based on films of the 1100 and 1200 ew Nafions to see if evidence for degradation of the polymer is observed. Figure 1A shows background currents (in 0.2 F sodium tritluoroacetate)observed at an uncoated glassy carbon electrode (electrode area = 0.079 cm2). Figure 1B shows analogous background currents for a CME prepared by depositing 3 pL of a 0.5 wt/vol % solution of the 1100 ew polymer, in 5050 ethanol-water, onto an identical glassy carbon electrode. As would be expected (443, 9, IO), no component of the polymer is electroactive over the potential range shown. The electrode in Figure 1B was then placed in a solution l0-S F in methylviolcgen chloride (MVCIJ and 0.1 F in NaF and the potential was scanned over the range where MV2+is reduced to MV+-. The voltammograms shown in Figure 2A were obtained. Clearly, the film is incorporating M W from the solution via ion exchange with the proton of the fixed sulfonic acid group; analogous behavior was o b s e d with the 970 ew polymer (4-6,9,10). Finally, the electrode was rinsed and placed in a solution containii only supporting eledrolyte. The steady-state W voltammogram shown in Figure 2B was obtained. As was the case with the 970 ew Nafon, the majority of the M V + is retained by the polymer even though the fh is immersed in a solution 0.1 M in Na+. Analogous results are obtained with films of the 1200 ew polymer. The results shown in Figures 1and 2 indicate that films cast from the polymer solutions retain the three important attributes which make these polymers ideal for surface modification; the polymers remain water insoluble (indeed, CMEs in the MvL+ form were boiled in water for 1h with no loss of attached W'), retain their ion exchange ability, and still strongly retain hydrophobic ions in the presence of huge excesses of hydrophilic (e.g., Na*) ions (9, 11). It was of interest, however, to see how much of the ion exchange capacity remained in the reconstituted polymer films. In an attempt to assess this, 0.BpL aliquots of accurately prepared 0.6% solutions of the polymers (in ethanol-water) were applied to glassy carbon electrode surfaces. The CMEs thus prepared were soaked overnight in a solution 10-2 F in MVCI?
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Anal. Chem. 1982, 54, 1641-1642
and 10” F in NaF. ThLe CMEs were then soaked for about 30 min in triply distilled1water and placed in lo4 F NaF. The amounts of MV2+cont,ained in the polymer films were then determined by quantitatively reducing the films at -0.8 V (vs. SCE) and counting the charge required to reoxidized the reduced films. The results of this study are shown in Table I. The data indicate that the reconstituted films have somewhat diminished exchange capacities. It is very important to point out, however, that 100% of the theoretical maximum number of sites could not be obseirved electrochemically in the 970 ew polymer either (IO). Hence, it seem likely that the apparently diminished exchange capacities are an artifact of the electrochemical measurement rather than a true loss of -SO3- sites from the films. In any event, the film clearly retains a more than ample supply of exchange sites. As a final attempt t o determine whether thermal degradation occurs during dissolution, we have compared the amount of polymer added to the reactor to the amount of polymer present after the dissolution procedure was completed. For this study, 5-mL aliquots of 0.6% solutions were evaporated to dryness and weighed. The observed weights were within 2% of the expected weights in all cases. Given the volatilities of the solvents used, the propensity of the polymer to absorb atmospheric water and the possibility of adsorption of the polymer to the glassware used, these results indicate that no material is lost (e.g., through thermal degradation and volatilization) (18)as a result of the dissolution process. LITIERATURE CITED (1) Oyama, N.; Anson, F. C. J . Electrochem. SOC. 1980, 727, 247. (2) Oyama, N.; Anson, F. C. Anal. Chem. 1980, 52, 1192.
Shigehara, K.; Oyama, N.; Anson, F. C. Inorg. Chem. 1981, 20, 518. Rubenstein, I.; Bard, A. J. J . Am. Chem. SOC. 1980, 702, 6641. Henning, T. P.; White, H. S.;Bard, A. J. J . Am. Chem. SOC.1081, 703, 3937. Rubenstein, I.; Bard, A. J. J . Am. Chem. SOC. 1981, 703, 5007. Buttry. D. A.; Anson, F. C. J . Electroanal. Chem. 1981, 730, 333. Bruce, J. A.; Wrighton, M. S. J . Am. Chem. SOC. 1982, 704, 74. Martin, C. R.; Rubenstein, I.; Bard, A. J. J . Am. Chem. Soc., In press. White, H. S.;Leddy, J.; Bard, A. J. J . Am. Chem. SOC.,in press. Martin, C. R.; Freiser, H. Anal. Chem. 1980, 53, 902. Fisher, Charles, DuPont Co., Wllmington, DE, personal communication. Mattln, C. R., unpublished results. Yeo, R. S. Po&mer 1980, 27, 432. Qierke, T. D. Presented to the 152nd National Meeting of the Electrochemical Society, Atlanta, QA, Oct 1977. Lee, P. C.; Meisei, D. J . Am. Chem. SOC. 1980, 120, 5477. Morrison, R. T.; Boyd, R. N. “Organic Chemistry”; Ailyn and Bacon: Boston, MA, 1973; Voi. 3, p 554. “Nafion Perfluorosuifonic AcM Products, Safety in Handilng and Use”; Brochure E-24084; DuPont Co., Plastic Products and Resins Depattment: Wllmington, DE.
Charles R. Martin* Teresa A. Rhoades James A. Ferguson Department of Chemistry Texas A&M University College Station, Texas 77843
RECEIVED for review April 2, 1982. Accepted May 17, 1982. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for the support of this research. The authors acknowledge discussions with Allen J. Bard, Keith Wellbourn, and Henry White of the University of Texas a t Austin.
Trifluoromethanesulforiyl Chloride for Identification of Oxygen, Nitrogen, and Sulfur Functional Groups by Fluorine-19 Nuclear Magnetic Resonance Spectrometry Sir: NMR spectrometry has been a powerful tool for determination of organic structure and fiunctionality. A summary of ”, I3C,and I9F NMR methods for organic functional group analysis has been reported (1). ?3i NMR has also been employed recently foi. hydroxy functional group characterization by silylation (2). ?F’ NMR appears to have the greatest potential utility for functional group determination due to its high sensitivity and wide chemical shift range. The fluorine reagents used have been hexafluoroacetone (3-7), trifluoroacetic anhydride (8),and trifluoroacetyl chloride (9) for derivatization of active hydrogen containing compounds (e.g., alcohols, amines, and thiols) and p-fluorophenol (I) for hydrogen bonding with blasic nitrogen compounds. In this paper, we wish to introduce a new fluorine reagent, trifluoromethanesulfonyl chloride, which not only yields sulfonation products with active hydrogen containing compounds but also forms stable complexes with tertiary nitrogen bases. The 19F chemical shifts for the trifluoromethanesulfonyl derivatives of selected model compounds are presented and discussed in relation to structural effects.
as well as nondeuterated solvents were used) in 5-mm NMR tubes. Reactions were carried out in the presence, or absence, of tertiary nitrogen base catalysts at room temperture or 0 “C depending on the reaction rates. The entire reaction can be completed in minutes or hours depending on individual compounds. A Varian T-60 NMR spectrometer was used to obtain 19FNMR spectra at 54.6 MHz. lPF chemical shifta were reported in parts per million in reference to the trifluoromethanesulfonyl chloride signal with a positive sign indicating a downfield shift from trifluoromethanesulfonyl chloride.
RESULTS AND DISCUSSION The 19Fchemical shift data for various trifluoromethanesulfonyl derivatives of model compounds are presented in Table I. With the exception of tertiary nitrogen bases (sample no. 14-17), the I9F chemical shift of all the trifluoromethanesulfonyl derivatives shows little or no solvent effect (the value for chemical shifts is generally lower than 0.01). Tertiary nitrogen bases interact with trifluoromethansulfonyl chloride presumably by a Lewis acid-Lewis base complexation mechanism as shown below:
EXPERIMENTAL SECTION Trifluoromethanesulfonylchloride purchased from Aldrich was
kept in a desiccator under nitrogen. Samples were prepared with an approximate 2:l ratio of trifluoromethanesulfonyl chloride/ substrate in various solvents (e.g., dimethylformamide,dimethyl sulfoxide,tetrahydroban and chloroform, perdeuterated solvents 0003-2700/82/0354-1641$01.25/0
F,CSO,
The 19Fchemical shift of the complex increases as the solvent 0 1982 American Chemical Society
