Anal. Chem. 1989, 6 1 , 1321-1325
LITERATURE CITED (1) Mendema, W.; Zeldenrust, H.; Emels, C. A. Mekromol. Chem. 1979,
180, 1521-1538. (2) Tijssen, R.; Bleumer, J. P. A.; van Kreveld, M. E. J . Chromafogr. 1983, 260, 297-304. (3) Tijssen, R.; Bos, J.; van Kreveld, M. E. Anal. Chem. 1988, 58, 3036-3044. (4) van Kreveld, M. E.; van den Hoed, N. J . Chromatogr. 1973, 8 3 , 111.
1321
(5) Price. C.; Hudd, A. L.; Booth, C.; Wright, B. Po/ymer 1982, 2 3 , 650-653. (6) Tock, P. P. H.; Stegeman, G.; Peereboom, R.; Poppe, H.; Kraak, J. C.; Unger, K. K. Chromatograph& 1987, 2 4 , 617-624.
RECEIVED for review November 17, 1988. Accepted March 10, 1989.
Size Exclusion Chromatography of Poly(ethy1ene terephthalate) Using Hexafluoro-2-propanol as the Mobile Phase Sadao Mori
Department of Industrial Chemistry, Faculty of Engineering, Mie University, Tsu, Mie 514, Japan
A method for obtalnlng a callbratlon curve (CC) In hexafluoro-2-propanol (HFIP) uslng a characterlzed poly(methy1 methacrylate) (PMMA) as a secondary standard was proposed. Polystyrene (PS) standards were used as a prlmary standard and a PS CC In tetrahydrofuran (THF) was converted to a PMMA CC In THF by use of a ConvBcSJon equatbn. An Integral molecular welght dlstributlon curve of cumulative weight percent vs log molecular weight ( M ) for the PMMA secondary standard was constructed by uslng a PMMA chromatogram In THF and a PMMA CC. An Integral dlstrlbutlon curve of cumulatlve welght percent VS retention volume ( V , ) for PMMA In HFIP was obtained from a PMMA chromatogram In HFIP. Finally, a CC of log M of PMMA vs VR In HFIP was obtained by uslng these two Integral curves. PMMA equlvalent number average molecular weights of poly(ethy1ene terephthalate) (PET) samples characterlred by the end-group titration method were determlned and the ratios of two averages were obtalned. The ratlo Is designated as a correctlon factor, I, and an average value was 0.57. Therefore, corrected (true) molecular welght average of PET can be obtained from PMMA equivalent molecular weight by multlplylng f .
INTRODUCTION Solvents most commonly used for size exclusion chromatography (SEC) of synthetic polymers are tetrahydrofuran (THF), chloroform, and toluene, which are good solvents for polystyrene (PS) and other common polymers. The construction of a calibration curve of an SEC column system using these solvents as the mobile phase is easy for handling, because they can dissolve PS standards which are exclusively used for the calibration purpose. Polyacrylonitrile (PAN), for example, does not dissolve in these solvents and dimethylformamide (DMF) had to be used for SEC of PAN (I). However, DMF is a poor solvent for PS and other standard polymers such as poly(ethy1ene oxide) have been used for calibration standards. Poly(ethy1ene terephthalate) (PET) is one of the unmanageable polymers for SEC with respect to solvents and calibration standards. m-Cresol has been used as the mobile phase for P E T at 125 "C (2). However, rn-cresol is viscous and requires a high column temperature, which leads to polymer degradation. Several solvent systems for the SEC of P E T in which degradation was eliminated or kept to a 0003-2700/89/0361-1321$01.50/0
minimum have been reported. A mixture of nitrobenzene and tetrachloroethane has been used for SEC of P E T at room temperature (3) and degradation of P E T was not observed for several months. The mobile phase was 0.5% nitrobenzene in tetrachloroethane. A sample (0.2 g) of P E T was dissolved in 0.8 mL of nitrobenzene by heating at 180 "C and the solution was then diluted with 60 mL of hot tetrachloroethane. A mixture of phenol and tetrachloroethane (3:2, w/w) was also used for the SEC of PET (4). However, difficulties have been encountered in dissolving P E T in these solvents at times. Hexafluoro-2-propanol(HFIP), pentafluorophenol, and their mixtures were used as the mobile phase in an SEC-LALLS system for P E T ( 5 ) . The solvent HFIP has been found to be a superior solvent capable of dissolving P E T at room temperature (6). HFIP has been commercially available and can be obtained in Japan. There are two major drawbacks in the use of HFIP as an SEC solvent. One is the expensive price of HFIP as a SEC solvent. The other is the insolubility of PS standards, which prevents the construction of a PS calibration curve in an SEC HFIP system. To overcome the high cost, semimicro-SEC using a mixture of HFIP and chloroform (1:l)has been proposed (7). However, the flow rate reliability in conjunction with a conventional pumping system for semimicro-HPLC was not sufficient in general for the determination of molecular weights of polymers. For calibration of a SEC system, Provder et al. proposed a general method for obtaining a calibration curve in a SEC trifluoroethanol (TFE) system for polyamide analysis (8). If HFIP alone can be used as a SEC solvent, then the availability of an SEC HFIP system can be expanded to polymers that can be dissolved in HFIP only. In this paper methods to remove these two drawbacks have been proposed. Recycling use of the effluent from the column outlet by redistillation was employed to reduce the cost of the SEC measurement accompanied by the use of HFIP. A method to generate a calibration curve for a column system was similar to Provder's method (8),but the hydrodynamic volume concept and the Mark-Houwink constants were not employed here. PS standards were used as primary calibration standards and poly(methy1 methacrylate) (PMMA) of known molecular weight averages was a secondary standard. A PMMA calibration curve in THF was constructed by using a PS calibration curve and a conversion equation (9). Integral molecular weight distribution curves in both T H F and HFIP for the PMMA secondary standard were obtained from their respective SEC chromatograms and a PMMA calibration 0 1989 American Chemlcal Society
1322 I
ANALYTICAL CHEMISTRY, VOL. 61, NO. 13, JULY 1, 1989
la!PS
M
13IgMMA 'HF
THF
5t 10
-L
L
"F
_ 1
%
Figure 1. Illustrative process for the calibration of columns in HFIP using PS as primary standards and PMMA as a secondary standard: (a) a PS calibration curve in THF, (b) a PMMA calibration curve in THF, (c) an SEC chromatogram of the PMMA secondary standard in THF, (d) an integral molecular weight distribution of the PMMA standard, (e) an SEC chromatogram of PMMA in HFIP, (f) relationship of weight percent vs V , of PMMA in HFIP, (9) a PMMA calibration curve in HFIP.
curve in HFIP was constructed from these two distribution curves. EXPERIMENTAL SECTION Apparatus and Materials. SEC measurements were performed on a Jasco TRIROTAR-V high-performance liquid chromatograph (Japan Spectroscopic Co., Ltd., Hachioji, Tokyo 192,Japan) with a Model SE-51 differential refractometer (Showa Denko Co., Ltd., Minato-ku, Tokyo 105, Japan) and a Model VL-614 loop injector. Two Shodex KF80M high-performanceSEC columns (each 30 cm X 8 mm id.) (Showa Denko Co.) packed with PS gels for polymer separation were used and were thermostated at 35 "C in an air oven. These columns were equilibrated with tetrahydrofuran (THF) and had the number of theoretical plates (A') of 12 000 plates/30 cm by injecting 0.05 mL of a 0.1 % benzene solution at a flow rate of 1 mL/min. After the required data were obtained from the THF mobile phase, the solvent in the columns was changed to HFIP at a flow rate of 0.3 mL/min. After enough HFIP solvent passed through the column to replace THF, the flow rate was increased to 0.5 mL/min and the columns were conditioned overnight by flowing HFIP at the same flow rate. The value of N was 11000 plates/30 cm by injecting 0.05 mL of a 0.1% acetone solution at a flow rate of 0.5 mL/min. PS standards were purchased from Pressure Chemical Co., Pittsburgh, PA, and a PMMA secondary standard was purchased from ArRo Laboratories, Inc., Joilet, IL. The molecular weight averages of these polymers were determined by the manufacturers. PET samples were obtained from Toray Co., Ltd., and their number average molecular weights (M,) were determined by the company by the end-group titration method. Sample concentrations were 0.1% (w/v) in THF for PS standards, 0.2% (w/v) in THF and in HFIP for a PMMA secondary standard, and 0.2% in HFIP for PET. The injection volume of the sample solutions was 0.1 mL and a 0.1-mL loop was used to inject these sample solutions. HFIP was provided courtesy of Central Glass Co., Ltd., Chiyoda-ku, Tokyo 101,Japan. The solvent was reused several times by redistilling the waste solvent with a simple distilliation vessel composed of a round flask, a Liebig type condenser, and a mantelheater. Calibration Procedure. A schematic illustration for calibration procedure is shown in Figure 1. (1)At first, a calibration curve of log molecular weight (M) vs retention volume ( VR) is constructed by using PS standards in the mobile phase of THF (Figure la). (2) The PS calibration curve in THF is converted to a PMMA calibration curve in THF by using the conversion equation (9) (Figure 1b)
log M P M M A= 0.2938
+ 0.918 log Mps
(1)
(3) An SEC chromatogram of the PMMA sample in THF is
I 1L
1
I
I
I
I
1
1
I
15
16
17
18
19
20
21
22
Retention Volume (rnL) Figure 2. Calibration curves of log M vs V , for PS (0) and PMMA (0) in THF.
measured (Figure IC)and an integral molecular weight distribution of cumulative weight percent vs log M for the PMMA secondary standard (Figure Id) is calculated by using the calibration curve (Figure lb) and the SEC chromatogram (Figure IC). (4) The mobile phase is changed to HFIP. The solvent, THF, in the columns is replaced by HFIP at a flow rate of 0.3 mL/min. (5) An SEC chromatogram of the PMMA secondary standard in HFIP is measured (Figure le) and an integral distribution of cumulative weight percent vs V , for the PMMA secondary standard in HFIP is calculated (Figure If). (6) From two integral distributions, one is that of cumulative weight percent vs log M of PMMA in THF and the other is that of cumulative weight percent vs VR of PMMA in HFIP, a calibration curve of log M of PMMA vs VR in HFIP is constructed (Figure lg). The relationship of cumulative weight percent vs log M for PMMA shown in Figure Id is assumed to be invariate for the PMMA secondary standard used and independent of the different mobile phases and their flow rates. Therefore, once the integral molecular weight distribution for the PMMA secondary standard is constructed, the repeat of steps 1-4 is not necessary as far as the same PMMA secondary standard is used for the calibration of the SEC system. Only steps 5 and 6 are required. Calculation of molecular weight averages was performed in the usual manner: the SEC chromatogram was sliced into equal parts, the height of each sliced point was measured, and each height was multiplied or divided by molecular weight at each point. RESULTS AND DISCUSSION Calibration of Columns. Calibration curves obtained by steps 1 and 2 are shown in Figure 2. When a conversion equation for a secondary standard is known, a calibration curve of the secondary standard is easily obtained by using PS standards as primary standards. The conversion equation for PMMA in the literature (9) was
MpMMA = 1.967Mps0'918 (2) By conversion of both sides of eq 2 to a common logarithmic scale, eq 1 was obtained. If a conversion equation for a secondary standard of known molecular weight averages has not been determined, then the equation can be generated according to the method in the literature (9). One of the advantages of the method described here is it is not necessary
ANALYTICAL CHEMISTRY, VOL. 61, NO. 13, JULY 1, 1989
1323
M
Retention Volume (m L )
Figure 3. SEC chromatogram of the PMMA secondary standard in THF: concentration, 0.2%; injection volume, 0.1 mL; detector, RI; attenuation, X2.
a 1
1
1
/
I
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3 I
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Retention Volume (mL) 1
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R e t e n t i o n Volume
I
I
I
,
I
15
14
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12
11
(rnL)
Flgure 4. SEC chromatograms of the PMMA secondary standard in HFIP concentration, 0.2%; injection volume, 0.1 mL; detector, RI; attenuation, X4; (a) injection 10 days after the sample preparation, (b) injected day after the sample preparation.
to use the so-called hydrodynamic volume concept. Equation 1 is not based on the concept. An SEC chromatogram of the PMMA secondary standard in T H F is shown in Figure 3. Molecular weight averages of the sample were calculated by using the PMMA calibration curve in Figure 2. The values were AIw = 5.87 X lo4 and AI1, = 3.39 X lo4,which were nearly equivalent to the manufacturer's data as AIw = 6.08 X lo4 (obtained by light scattering) and AIn = 3.32 x lo4 (obtained by membrane osmometry). An integral molecular weight distribution of the PMMA secondary standard was obtained by using the chromatogram in Figure 3 and the PMMA calibration curve in Figure 2. The next step was the change of the mobile phase from T H F to HFIP. The solvent THF in the column was replaced with HFIP at a flow rate of 0.3 mL/min. For replacing solvent in a column to other type of solvent, it is required to deliver new solvent intoa column as slow as possible for the protection of the decrease in column efficiency. As HFIP is more viscous than THF, a decrease of a flow rate to 0.5 mL/min was required to keep the column inlet pressure as low as possible. The SEC chromatogram of the PMMA secondary standard in HFIP was then measured and an example is shown in Figure 4. The SEC chromatogram of PMMA sometimes showed a small peak at the exclusion limit of the column system (Figure 4b) and this peak was not observed when the PMMA solution prepared more than 10 days before was injected (Figure 4a). Figure 4b was obtained when a sample solution prepared just 1 day before was injected. This small peak is probably the aggregation of PMMA molecules and it disappeared by heating or by ultrasonic treatment as well as leaving the sample solution more than 10 days after preparation at room temperature. The integral molecular weight distributions calculated from the chromatogram having a small peak at the exclusion limit did not give an accurate result due to the peak as well as due to base-line drifts or fluctuations. The integral distribution of the cumulative weight percent vs V , for the PMMA secondary standard in HFIP was calculated by step 5. A PMMA calibration curve in HFIP was constructed by step 6 and is shown in Figure 5. Pairs of two data points, log M P ~ m and V,, were obtained at the same
Flgure 5. PMMA calibration curve of log M of PMMA vs retention volume in HFIP.
cumulative weight percent of two ordinates of two distribution curves (Figure ld,f by sampling at every 5% from 5 wt % to 95 wt % and at every 1%from 0 wt % to 4 wt % and 96 wt % to 100 wt % of the ordinate. Pairs of each two data points were plotted. The slope of the PMMA calibration curve in an SEC THF system (Figure 2) is steeper than that in an SEC HFIP system (Figure 5 ) . In other words, at the same retention volume, a molecule in an SEC HFIP system has lower molecular weight than that in an SEC T H F system. Ideally it should be the same molecular weight, because retention volumes of benzene and acetone in THF and HFIP were almost the same. It can be said that the sum of the interstitial volume V,,and the pore volume Vi was unchanged by changing the solvent in the column from T H F to HFIP. From the comparison of two calibration curves, it is obvious that the value of V,, in an SEC HFIP system became smaller and that of Vi larger. Compared to an SEC T H F system, therefore, it may be assumed that the difference of the slopes of two calibration curves has arisen from one of four reasons. In an SEC HFIP system, (1) polystyrene gel swelled to some extent and the pore size of the gel became small without changing the inner volume of the pore; (2) the elution of PMMA molecules was accelerated by the ion-exclusion effect; (3) aggregation of PMMA molecules in HFIP was considered by the observation of two chromatograms in THF (Figure 3) and in HFIP (Figure 4); (4) the hydrodynamic volume of a PMMA molecule in HFIP was larger than that of the same molecular weight in THF. Among these four assumptions, the last one is the most probable. Exponents and coefficients in a Mark-Houwink equation for PMMA in TFE and in THF were as follows (8): [q] =
KM"
(3)
at M < 31000, a = 0.461, K = 1.81 X in TFE and a = 0.406, K = 2.1 X low3in THF; at M > 31 000, a = 0.791, K = 5.95 X in T F E and a = 0.697, K = 1.04 X lo4 in THF. If the values of a and K for PMMA in HFIP are assumed to be similar to those in TFE, then hydrodynamic volume of a PMMA molecule in HFIP is higher than that of the same molecular weight in T H F and the ratio of hydrodynamic volumes of a PMMA molecule in HFIP and T H F becomes higher at higher molecular weight than lower one. The plots of hydrodynamic volume [VIMvs VR in HFIP (by using the
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ANALYTICAL CHEMISTRY, VOL. 61, NO. 13,JULY 1, 1989
Table I. Molecular Weight Averages of PET Samples and Correction Factor f
sample 1 2 3 4
molecular weight average PMMA other method" correction &fw M n hf. factor f 37600 25800 13300 9320
22500 14900 8640 4450
13000 8300 5000 2400
av Retention Volume
Figure 6. SEC chromatograms of PET samples: M n = 8300;detector attenuation, X4.
(a)M n = 13 000,(b)
values of a and K in TFE) and in T H F were similar, indicating our assumption is positive. The values of K and a for PMMA in HFIP may be obtained by a separate experiment and are discussed elsewhere. However, there was still a small divergence between both plots in the high molecular weight region and the early elution of PMMA in HFIP due to the ion-exclusion effect cannot be ignored, even though the small addition of sodium trifluoroacetate in HFIP did not make any change in the shape of chromatogram for PMMA. Determination of PET. SEC chromatograms of several P E T samples were determined and examples are shown in Figure 6. P E T in HFIP was very stable, though HFIP is slightly acidic, and there was no change in chromatograms or in the values of calculated molecular weight averages after leaving the PET solution for 3 months. A broad peak after retention volume 23 mL was due to the solvent HFIP and it appeared every time even when only HFIP was injected. PMMA equivalent molecular weight averages of PET samples were calculated by using a PMMA calibration curve (Figure 5). The term correction factor f is designated as
f=
M nobtained by t h e end-group titration method P M M A equivalent A?fn (4)
Therefore, PMMA equivalent molecular weight averages are converted to real PET molecular weight averages by multiplying f to PMMA equivalent molecular weight averages. PMMA equivalent molecular weight averages, number average molecular weights obtained by the end-group titration method, and correction factors f of P E T samples are listed in Table I. For the correction factor f concept to be valid, it is necessary to make sure that the P E T is a linear homopolymer with no short or long chain branches. Among several side reactions during the condensation-polymerization of PET, incorporation of diethylene glycol terephthalate units is well-known and the percentages of diethylene glycol in PET were ranging between 0.6% and 3.6% (IO). Copolymerization with diethylene glycol of this amount may not influence the validity of the correction factor method. Two or more types of diacid and/or diol have frequently been incorporated into a given polyester to optimize its properties (11). Besides this incorporation, the author has never seen any reports on short or long chain branches in PET in the literature. Comparative experiment of values for several other PET samples by end group titration with the SEC correction factor method is needed to say that the correction factor method is valid. However, since the correction factor f only varies from 0.54 to 0.58 over a range of molecular weight tested, it can be concluded that the correction factor method proposed here appears quite valid. The higher molecular weight part of an SEC chromatogram affects the value of weight average molecular weight and the
an
Mw
wt
21430 14700 7580 5310
Mn 12820 8490 4920 2530
0.57
Obtained by the end-group titration method. *Averagef equivalent molecular weight. a
(mL)
0.58 0.56 0.58 0.54
corrected
X
PMMA
lower molecular weight part affects number average molecular weight. The absence of the data for the PMMA calibration curve after retention volume 21 mL affects the accuracy and the precision of the value of The calibration curve between 21 mL and 22.9 mL, which was equivalent to retention volume of acetone, was obtained by intrapolation. The accuracy of number average PMMA equivalent molecular weights may decrease by this reason and therefore, one or two more characterized PMMA standards of different molecular weight range should be incorporated to span the entire retention volume range of interest. As benzene molecules adsorbed on the column when HFIP was the mobile phase, acetone was used as the probe for the estimation of column efficiency. Retention volume of benzene when T H F was the mobile phase was 23.6 mL and that of acetone was 23.0 mL, respectively. Acetone can be used as a probe when THF is the mobile phase, but the response of an RI detector for acetone was worse than that for a benzene probe. Flow rate fluctuation also affects the accuracy and the precision of the values of molecular weight. However, the pumping system employed here was very reliable and the precision of flow rate was below 0.04%. For example, flow rate was measured on seven consecutive days by filling the effluent from the column into a measuring flask of 10 mL and reading the time required to fill it with the solvent. The range of the flow rate was 0.5012 and 0.5019 mL/min, average was 0.5015 mL/min, and average deviation was 0.0002 mL/min. The flow rate after 1 month was almost unchanged. Therefore, the effect of flow rate fluctuation on the value of molecular weight averages is concluded to be negligible in our work. After the mobile phase was changed in the SEC columns from T H F to HFIP, the value of N was slightly decreased as described in the Experimental Section. Lifetime of the columns after the solvent change was more than one year. According to the manufacturer's information, the change of solvent in the columns from T H F to HFIP is possible without any damage in the column efficiency, but the reverse change back to T H F should be avoided. HFIP is a very expensive solvent; the price is $560 (Y70000) per 1 kg. Specific gravity of HFIP is 1.59 and the price per 1 L is therefore $890 (Y111300). The volume of HFIP required to measure one chromatogram is at least 26 mL, costing $23 (Y2900). It usually requires more volume because of loss of HFIP during the experiment. Recycling of the solvent reduces the cost. Therefore, the effluent from the columns was collected and distilled for reuse. The initial volume of HFIP was 2 L and after 144 measurements, the volume of HFIP was 1L. Therefore, the consumption of HFIP per one measurement was 6.9 mL and the cost was reduced to $5.80 (Y733). Distillation was repeated 10 times.
an.
ACKNOWLEDGMENT The author is grateful to Akihiro Nishimuar for his technical assistance.
Anal. Chem. 1909, 6 1 , 1325-1327
LITERATURE CITED (I) Mori, %ciao Anal. Chem. 1983, 55, 2414-2416. (2) Overton, J. R.; Rash, J.; Moor, L. D., Jr. Sixth InternationalGPC Seminar Proceedlngs, Miami Beach, FL, 1968,422-432. (3) Paschke, E. E.; Bidlingmeyer, 8. A.; Bergmann, J. G. J. folym. Sci., Porn. Chem. Ed. 1977, 15, 983-989. (4) U g k , c. V.; Alzicovici. A,; Mlhaescu, S. Eur. folym. J . 1985, 21, 677-679. (5) Berkowltz, Steven J. Appl. folym. Sci. 1984,29, 4353-4361. (6) Drott, E. E. Liquid Chromtography of Polymers and Related Materials; Cams, Jack, Ed.; Chromatographic Science Series Vol. 8, Marcel Dekker: New York, 1976;pp 41-50.
1325
(7) Hibi, Kiyokatsu; Wada, Akio; Mori, Sadao Chromatographla 1886, 1 1 ,
--- - . ..
FSS-fiAl
(8) Provder, Theodore; Woodbrey, James C.; Clark, James H. Sep . Sci. 1971, 6 , 101-136. (9) Mori, Sadao Anal. Chem. 1981, 53, 1813-1818. (IO) Janssen, R.; Ruysschaert, H.; Vroorn, R. Makromol. Chem. 1984, 35,
153-158. (11) Russell, G.A.; Henrichs, P. M.; Hewitt, J. M.; Grashof, H. R.; Sandhu, M. A. Macromolecules 1981, 14, 1764-1770.
for review December 7, lg8&Accepted March 13, 1989.
Gas Chromatographic Determination of Water Using 2,2=Dimethoxypropane and a Solid Acid Catalyst Kevin D. Dix, Pamela A. Sakkinen, and James S. Fritz*
Ames Laboratory-US. Department of Energy and Department of Chemistry, Iowa State University, Ames, Iowa 50011
The amount of water in various organic and Inorganic substances Is determined by reaction with 2,2dknethoxypropane (DMP), folbwed by measurement of a product of the reactlon (acetone) by caplllary column gas chromatography. The reaction of water with DMP requires only 5 mln when Nafion Is used as a solid acld catalyst. Various experlmentai parameters are investigated to optlmlre the analytical procedure. The percentage of water In a variety of analytical samples was determined successfully.
for determining water in nitroglycerin-nitrocellulose pastes by GC. In the present method the sample is combined with a solution containing DMP and an internal standard. A small amount of Ndion (7) is added to catalyze the reaction of water with the DMP. Then an aliquot is injected into a gas chromatograph equipped with a capillary column and a flame ionization detector. The amount of water is calculated from the ratio of the peak areas (or peak heights) of the acetone and internal standard. The method is both sensitive and convenient; it has been applied successfully to a wide variety of analytical samples.
INTRODUCTION The determination of small amounts of water in various organic and inorganic compounds is of great practical importance. The Karl Fischer method (1) is perhaps the most widely used procedure for the determination of water. Although this method works well in many cases, the commercial reagents are rather costly, the visual titration end point is difficult to discern, and there are numerous interferences. Although water can be determined directly by gas chromatography using a thermal conductivity detector (TCD) (2), surprisingly few laboratories seem to use this approach. Furthermore, a packed column rather than a capillary GC column must be used because of the large cell volume of the traditional TCD. A few authors have used the acid-catalyzed hydrolysis of 2,2-dimethoxypropane (DMP), the dimethyl ketal of acetone, as a way to determine water.
EXPERIMENTAL SECTION Glassware and Apparatus. The reactions were carried out in 5-mL glass microreaction vessels fitted with Teflon-lined septa (Supelco Glass Co., Bellefonte, PA). Reagents and Chemicals. The 2,2-dimethoxypropane was obtained either from Aldrich Chemical (Milwaukee,WI) or from Eastman Kodak Chemical (Rochester, NY). Nafion 1100 EW resin (60-100 mesh) was purchased from C. G. Processing, Inc., (Rockland, NY) and was dried at 100 "C for 3 h under vacuum before use. Amberlyst-15 resin was obtained from Rohm and Haas (Philadelphia, PA) and dried as above. All other reagents were of reagent grade or better and were "blanked" before use, Distilled water was further purified with the Barnstead Nanopure I1 system before use. Gas Chromatography. A Hewlett-Packard 5790A gas chromatograph equipped with a flame ionization detector (FID) was used in the split mode. The split ratio was 80-100:l and was held constant during a series of experiments. The injection liner was packed with a small amount of 80-100-mesh silanized glass beads to prevent contamination of the column with nonvolatile components. The beads were changed periodically, and the injector was held at 150 "C. Two different carrier gas flow rates were used in this study. Initially, a flow of 2.5 mL/min of zero grade He was used with an oven profile of 5.2 min at 40 "C.Later, it was found that a flow rate of 5.0 mL/min and an oven profile of 2.2 min at 40 "C provided adequate resolution and decreased the analysis time. In each case, the oven temperature was stepped to 220 "C after the initial hold at 40 OC. This rapid increase in temperature served to remove any later-eluting peaks in less than 5 min. The column was a 30 m X 0.53 mm J+W DB-5 Megabore with a film thickness of 1.5 pm. The detector was an FID held at 250 "C. Reactant Solution. A reactant solution was prepared in a dry 100-mL volumetric flask from a 5-mL aliquot of DMP and
+
CH3C(OCH3)2CH3 H 2 0 -%CH3COCH3 + 2CH30H Critchfield and Bishop (3) determined water by reaction with DMP in the presence of methanesulfonic acid and measured the acetone formed by infrared spectroscopy a t 5.75 bm. Hager and Baker ( 4 ) made a cursory investigation of the use of DMP for the indirect GC determination of water. Martin and Knevel(5) proposed a quantitative method for water by reaction with DMP and measurement of the change in height of the GC peaks of DMP and acetone. The method required accurate weighing of both DMP and acetone, as well as the sample itself, and the sensitivity of the method was somewhat limited. Blanco et al. (6) used a somewhat similar method 0003-2700/89/0361-1325$01.50/0
0 1989 American Chemical Society
