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ANALYTICAL CHEMISTRY, VOL. 51, NO. 13,NOVEMBER 1979
Table VIII. Comparison of Experimental and Literature Partition Coefficients compound methyl ethyl ketone
K experimentaln
3.90 t 0.01 x 10-3
nitroethane butanol butanol' (sodium sulfate saturation) dioxaned
2.89 i 0.13 x 7.46 i: 0.57 x 13.7 0.1 x 1 0 ' ~ +_
2.78
i
0.54 x
K literatureb
ref.
2.13 x 10-3 3.57 0.03 x 1 0 - 3 N.F.~ 3.6 2 0.4 x 13.3 x 1 0 - 3 d 3 e
(53)
(24) (23) (9)
1.85 x
(23)
Experimental conditions: pH 7 . 1 , 0.00 M salt concentration at 30 "C. Experimental conditions: not reported other than temperature of 25 "C. Experimental condition this study: pH 7 . 1 , 3.35 M salt concentration at 30 "C. Experimental condition this study: 4.23 M salt concentration at 28 'C, pH not reported. e Literature values within 95% confidence limits of experimental data. The remaining values d o not fall within the 95% confidence limits. f N.F. = No value found. organic compound a t the enhanced and drinking water conditions, respectively. Application of the method of head space analysis as described was utilized for GC/MS of a drinking water sample in Philadelphia. The results of mass spectral identification of the compounds found in the drinking water showed the presence of toluene, two C-2 benzene isomers, CHC13, CHC1Br2, CHC12Br, and 1,1,2,2-tetrachloroethane. T h e quantitative effect of salt and temperature was also studied. The optimized head space conditions of 50 "C with a saturated salt solution was compared to 24 "C without salt addition. An increase in peak response was observed of 8- and 22-fold for chloroform and bromodichloromethane, respectively. The analysis was completed in a 125-mL bottle containing 100 mL of tap water. A 50-yL gas sample was injected onto the GC using a 63Nielectron capture detector. Thus the methodology should be capable for analysis of microgram/liter quantities of organics in drinking water.
LITERATURE CITED (1) I. H. Suffetand J. V. Radziul, J. Am. Water Works A m . , 68, 520 (1976); Erratum, 69, 174 (1977). (2) I. H. Suffet and P. R. Cairo, J . Environ. Sci. Health. A13, 117 (1978). (3) T. A. Bellar and J. J. Lichtenberg, J . Am. Water Works Assoc., 66, 739 (1974). (4) I. H. Suffet, G. Dozsa, and S. D. Faust, Water Res., 5, 473 (1971). (5) I. H. Suffet and S.Segall, J . Am. Wafer Works Assoc., 63, 605 (1971). (6) C. Weurman, J . Agr. Food Chem., 17, 370 (1969).
(7) K. Grob and F. Zurcher, J . Chromatogr., 117, 285 (1976) (8) C. D. McAuliie, "Mark Pollution Monitoring (Petrdeum)",Natl. &K. Stand. (U.S.), Spec. Pub/.. 409, Dec. 1974. (9) P. E. Nelson and J. E. Hoff, J . FocdSci., 33, 479 (1968). (10) W. G. Jennings, J . Food Sci., 27, 366 (1972). 1111 A. F. M. Barton. Chem. Rev.. 75., 731 11975). ~, (12j R. A. Kelier, J.'CbrOmatogr. 'Sci., 11. 49 (1973). (13) A. Hartkopf, S. Grunfeld, and R . Delumyea, J . Chromatogr. Sci., 12, 119 (1974). (14) B. L. Karger, L. R. Snyder, and C. Eon, J. Chromatogr., 125, 71 (1976). (15) G. D. Christian and W. C. Purdy, J . flectroanal. Chem., 3, 363 (1962). (16) S. L. Friant. Ph.D. Thesis, Drexel University, Philadelphia, Pa.. 1977. (17) 0. L. Davies, "Design and Analysis of Industrial Experiments", 2nd ed., Hafner. New York. 1971. (18) J. Pr&, "Statisticai Program Package in APL", 5th ed.,State U. College of New York. 1973. (19) R . E. Kepner, H. Maarse, and J. Strating, Anal. Chem., 38, 77 (1964). (20) W. W. Nawar and I. S.Fagerson, Food Techno/., 16(11), 107 (1962). (21) A. Dravnieks and A. O'Donnell, J . Agr. FocdChem., 19, 1049 (1971). (22) E. S.Gould, "Mechanism and Structure in Organic chemistry",M , Rlnehart & Winston, New York, 1959. (23) A. G. Vitenberg, B. V. Toffe, 2. St. Dimitrova, and I. L. Butaeva, J . Chromatogr., 112, 319 (1975). (24) L. Rohrschneider, Anal. Chem., 45, 1241 (1973). (25) "Analysis of Food and Beverages, Headspace Technique", G. Charalambous, Ed., Academic Press, New York, 1978. \~
RECEIVED for review November 15, 1978. Accepted July 30, 1979. Presented before the Division of Analytical Chemistry, 176th National Meeting of the American Chemical Society, Miami Beach, Fla., Sept. 15, 1978. This research was supported by the Philadelphia, Pa., Water Department under the leadership of Water Commissioner Carmen F. Guarino.
Determination of Alkoxy1 Substitution in Cellulose Ethers by Zeisel-Gas Chromatography K. L. Hodges," W.
E. Kester, D. L.
Wiederrich, and J. A. Grover
The Dow Chemical Company, Midland, Michigan 48640
An improved Zeisei gas chromatographic technique has been developed for the determination of molar substitution in cellulose ether derivatives. The method utilizes adipic acid to catalyze the hydriodic acid cleavage of the substituted alkoxy1 groups quantliatlvety to thelr corresponding alkyllodides. An in-sku xylene extraction of the alkyliodides in a sealed vial allows for the determination of methoxyi, ethoxyl, hydroxyethoxy, or hydroxypropoxy substitution in mixed or homogeneous cellulosic ethers.
Cellulose ethers are used extensively as thickeners, binders, lubricants, emulsifiers, and film formers. Their capability to perform this large variety of tasks depends on the number of 0003-2700/79/035 1-2172$01 .OO/O
moles of ether (alkoxyl) substituted per anhydroglucose unit, molar substitution; and on the number of hydroxy groups substituted, degree of substitution. The ability to quantitatively determine the molar substitution is therefore important in adjusting the solubility, thermal gelation point, viscosity, and other physical properties associated with solutions of these cellulose ethers. A wide variety of analytical methods have been developed over the years for the determination of molar substitution, largely owing to the variety of ethers being marketed. T h e classical Zeisel distillation method (I) has been combined with gas chromatography (2-10) to obtain the selectivity needed for the analysis of mixed cellulose ethers. 0 1979 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 51, NO. 13, NOVEMBER 1979
These techniques work well for either homogeneous or mixed 0-methyl or 0-ethyl substituted cellulose. However, the formation of ethylene and propylene (9), in the cleavage of hydroxyethyl and hydroxypropyl ethers, respectively, leads t o low substitution values, which severely limits their utilization. Other methods have been described which provide selectivity through a different approach. These include a chromic acid oxidation and distillation ( 1 1 ) which has also been combined with gas chromatography (12),a spectrophotometric method based on the ninhydrin colorimetric technique (23), and the formation of the alditol acetates of methyl and ethyl cellulose which are suited for gas chromatography (14). The increased selectivity shown by these techniques, however, has caused losses in precision and reproducibility. Proton NMR (15, 16) provides perhaps the most detailed information concerning degree of substitution as well as molar substitution; however, the precision and accuracy is critically dependent on t h e measurement of the intensity of a group of bands which are fairly weak relative t o other bands in the spectrum. As a result, signal-to-noise enhancement through multiple scanning with computer data acquisition makes the technique undesirable as a routine quality control test. T h e described method uses a catalyst, adipic acid, which allows the Zeisel cleavage reaction t o proceed quantitatively for the four types of cellulose ethers described without the formation of ethylene or propylene. An in-situ extraction of t h e resulting alkyliodides with o-xylene allows for the gas chromatographic determination of substitution in either homogeneous or mixed difunctional substituted cellulose ethers in a single 1.5-h analysis without elaborate distillation equipment or complicated apparatus. EXPERIMENTAL Apparatus. A Hewlett-Packard Model 5700 gas chromatograph equipped with a thermal conductivity detector was used. The column was 10 ft X 1/8 in. stainless steel packed with 10% UCW 98, methyl silicone on lOO/l20 mesh Chromosorb WHP, available from Applied Science Laboratories State College, Pa. 16801. The oven temperature was 100 "C; injection port and detector temperatures, 200 "C. The carrier gas was helium a t a flow rate of 20 mL/min. The detector current was 170 mA. A Hewlett-Packard 3380 reporting integrator was used to facilitate data handling and improve method precision. Reactivials, 5 mL, capped with Mininert valves available from Pierce Chemical Co., Rockford, Ill. 61105, were used to contain the Zeisel cleavage reaction and a Reactitherm heating module was used to control the reaction temperature. Three-dram, soft glass vials, 10-mL capacity, capped with smaller Mininert valves were used for preparation and storage of the standards. Reagents. Hydriodic acid, specific gravity 1.7, containing 57% HI (Matheson Coleman and Bell) and adipic acid, mp 151-153 "C (Matheson Coleman and Bell) were used. The o-xylene and toluene were ACS reagent grade. Iodomethane (99% min), iodoethane (97% min), and 2-iodopropane (97% rnin), available from Aldrich Chemical Company, were assayed under the stated chromatographic conditions and appropriate corrections were made in the preparation of the calibration standard. An internal standard stock solution containing 25 mg toluene/mL o-xylene was prepared by weighing 2.50 f 0.01 g into a 100-mL volumetric flask and diluting to volume with o-xylene. Procedure. A 6&70 f 0.1 mg sample of dried cellulose ether was weighed into a 5.0-mL Reactivial. An amount of apidic acid equal to or as much as twice the sample weight was added. Two milliliters of the internal standard stock solution was pipeted to the Reactivial and 2 mL of hydriodic acid added. The vial was immediately tightly capped with the Mininert valve top and weighed before insertion into the hot heating block. Samples were reacted for 1 h at 150 "C with agitation by shaking after 5 and 30 min. Since the glass vials were under pressure during the reaction, the manual agitation was conducted with caution behind a safety shield in a fume hood. Chemical workers goggles and
2173
IY-.rr-i-. $ 1 01 A l k y l la83!deFor STC
FrPidrat.w
Figure 1. Alkyl iodide - alkoxyl equivalency insulated gloves were worn to prevent exposure to hydriodic acid in case any accidental release of reaction contents occurred. After 1 h the vial was removed from the block, cooled for approximately 45 min and reweighed to determine any loss due to leakage.
CALIBRATION Two calibration standards were prepared using the following procedure and the average component response factor determined to calibrate the integrator. Four milliliters of hydriodic acid were pipetted into a 3-dram vial containing 120--140tng of adipic acid. Two 2.0-mL aliquots of the internal standard stock solution were added with the same pipet used for the preparation of the samples and the vial was capped tightly with EL Mininert valve cap using Teflon tape as a sealant for the vial threads. Based on the percent alkoxyl substitution anticipated in the prepared samples, the quantity of each respective alkyliodide needed for calibration was determined according to the graph in Figure 1. Each alkyliodide was introduced to the tared vial through the Mininert valve top with a 100-kL syringe and the vial weighed to the nearest 0.1 mg after each addition. The equivalent weight of alkoxyl added in mg was determined according to the following equations: mg OCH, = g CH,I mg OC2H5= g C2HBI mg OCJH70 = g C3H71
mol a t OCHB x 1000 mol wt CHJ
---___---
mol wt OC2H, X 1000 --___----mu1 w t C2HJ mol wt OC3M70 x 1000 mol wt C3H;I
Two microliters of the upper (xylene) layer of each prepared standard was injected into the gas chromatograph (Figure 2) and the component response factor was determined by programming the integrator. The following equations can be used if a data acquisition device is not available. response factor =
mg alkoxyl in std x toluene peak area alkyliodide peak mea mg toluene in std
A variance of greater than 5% relative between two component factors was considered unacceptable and a third standard was prepared to resolve the difference. The integrator was pro-
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ANALYTICAL CHEMISTRY, VOL. 51, NO. 13, NOVEMBER 1979 X64
Table I . Adipic Acid as Catalyst for Determination of Methoxy and Hydroxy Propyl Substitution Propylene Glycol Monomethyl Ether (Theor. 34.4%OCH,, 65.6%OC,H,)
CH, I
X16
method
% OCH,
Zeisel-GC Zeisel-GC with adipic acid
33.8, 33.6 3 3 . 9 , 33.9
Propylene Glycol (Theor. 76.4%OC,H,) method Zeisel-GC Zeisel-GC with adipic acid
I
1
0
4
% OC,H,
29.2, 28.9 63.9, 63.7
I
I 12
8
I
16
Retention T ~ m eI n Mlnuter
Figure 2. Chromatogram of alkyliodides. Column 1 0 4 UC W98 methyl silicone; helium 20 mLlmin. Temperature 100 "C: 170 mA
grammed to report weight percent alkoxy1by entering the internal standard and sample weights. Two microliters of the upper (xylene) layer of the sample was injected and the weight percent alkoxyl substitution was determined. See Equation 1.
RESULTS AND DISCUSSION The success of the new Zeisel gas chromatographic method depends on the quantitative conversion of the substituted alkoxyl unit to the corresponding iodide by reaction with hydriodic acid. This conversion proceeds through a postulated vicinal 1,2-diiodo intermediate (I).
+
-
% OC,H,
44.5, 44.6 75.0, 74.8
from acid catalyzed reactions showed appreciable lower propylene levels than nonacid catalyzed reactions. If adipic acid was catalyzing the addition of hydriodic acid to propylene, there would also be a significant reduction in the propylene content in the reactor off gas. This was not found to be true since the addition of a measured quantity of propylene to hydriodic acid with and without adipic acid at a constant rate yielded the same quantity of 2-iodopropane. According to Morgan ( l a ) ,the fate of propylene thus formed should be similar as long as the reaction takes place in a sealed tube and propylene is kept in constant contact with hydriodic acid. It has been reported (19-25), however, that the addition of hydriodic acid to ethylene in the gas phase is kinetically faster than the addition to propylene. This then could explain why the propylene generated from the Zeisel reaction of propylene glycol and propylene glycol monomethyl ether does not react quantitatively as does ethylene in ethylene glycol monomethyl ether. The presence of xylene during the course of a reaction is an important factor in obtaining quantitative 2-iodopropane values due to the elimination of the reported disproportionation reaction (24). This reaction occurs when the iodide is not extracted from the hydroiodic acid thus:
Support for this disproportionation step was obtained by heating 2-iodopropane in the presence of hydroiodic acid with and without o-xylene being present. Without xylene, both propane and propylene were present in the reaction headspace and a loss of 2-iodopropane was noted. With o-xylene no propane was observed and essentially 100% of the 2-iodopropane was recovered. Earlier studies by Merz (24) on the hydriodic acid cleavage of polypropylene oxide oligomers indicated the formation of significant quantities of propionaldehyde. I n the current method, there is no evidence for the formation of propionIts existence is presumed to be transitory since it has never aldehyde. Samples of the xylene and hydroiodic acid layers been isolated or prepared synthetically. of reactions performed with and without adipic acid were The intermediate proceeds through two routes to the analyzed by proton NMR. No evidence for components other desired alkyliodide. The first route (A) is the direct conversion to 2-propyl iodide through a biomolecular iodine elimination than 2-iodopropane, which was larger in the adipic catalyzed reaction. This reaction is acid catalyzed (17)and quantitative reactions, was observed. conversion is achieved by using organic acids such as adipic, Quantitative conversion was somewhat dependent on succinic, formic, acetic, citric, or valeric. structure. Model compounds containing ethoximer or proIn the absence of an acid catalyst, the reaction with hypoximer content greater than 2 such as tetraethylene glycol driodic acid proceeds through an alternative route (B) resulting or tetrapropylene glycol required higher temperatures, more in the quantitative conversion of alkoxyl unit to propylene catalyst, and longer reaction times to obtain quantitative and isopropyl iodide. Reactions performed at temperatures recovery. Alkoxy1 groups substituted on aromatic rings also between 130 and 170 O C in the absence of adipic acid yielded required more severe conditions. low, reproducible, hydroxy propoxy values but quantitative Most substitutions evaluated, however, would react methoxyl values (Table I). quantitatively a t temperatures between 130-180 "C. Some Support for this mechanism was obtained when the off gas hydroxypropoxy determinations would be accompanied by the alkyliodide peak area X average component response factor x mg toluene in sample % alkoxyl = x 100 (1) toluene peak area X mg sample ROCHzCHCHj
I
HI
1,
1
R I t CHzCHCH3
I
ANALYTICAL CHEMISTRY, VOL. 51, NO. 13, NOVEMBER 1979
2175
Table 11. Validation and Precision Hydroxypropyl Methylcellulose Sample A method
% OCH,
% re1 20
( # runs)
29.45 30.17
1.4 1.7
(10)
ASTM D-2363-72 Zeisel-GC
(8)
% OC,H,OH
7.71 8.17
Sample B 1.1
(7)
1.2
(8)
ASTM D-2363-72 Zeisel-GC
27.45 27.28
ASTM D-2363-72 Zeisel-GC
21.27 21.32
1.04
ASTM D-2363-72 Zeisel-GC
30.09 30.06
0.72
4.59 4.37
Sample C (8 1 (11) Methyl Cellulose Sample D
1.8
0.80
(5) (7)
12.69 13.52
0.39
10.2
(7)
Ethyl Cellulose Sample E met hod
% OC,H,
% re1 20
46.20 46.35
0.96 0.54
ASTM D-2363-72 Zeisel-GC
Ethyl Cellulose Sample F ASTM D-2363-72 Zeisel-GC
49.01 48.79
1.61 0.42
Table 111. Different Cellulose Ethers b y Zeisel-Gas Chromatography Using Adipic Acid Catalyst
sample
cellulose typea
Henkel Brit. Cellanese Shin Etsu Natl. Starch Stein Hall Stein Hall Hercules Stein Hall
MC MC HPMC no sub. no sub. HP HP CMHP HEHPMC HEMC HEMC MCET
Dow
Dow Hoechst IC1
% OCH,
25.9 26.7 28.0
