Determination of Carboxylic Esters by Acid Fusion Reaction Gas Chromatography Richard J. Williams' and Sidney Siggia' Department of Chemistry, University of Massachusetts, Amherst, Massachusetts
A quantitative method has been developed to determine the carboxylic acid content of nonvolatile esters. The determination is carried out In one piece of apparatus by coupling a reaction chamber through a traploop to a gas chromatograph. The method involves hydrolysis of the ester in molten phosphoric acid at 200-300 OC for 15 min. The volatile reaction products are concentrated in the trap-loop prior to gas chromatographic analysis. The precision of the method is f0.1 to 3.0% using a 1- to 10-mg sample. Quantitative results were obtained wlth cholesteryl esters, carbohydrate esters, cellulose esters, poly(viny1 ester), and vinyl acetate copolymers.
Several gas chromatographic methods have been introduced for the determination of the combined carboxylic acid content of poly(viny1 esters) and cellulose esters. T h e methods are based on a preliminary chemical reaction t o release the carboxylic acid from the ester functional group followed by a gas chromatographic analysis of the reaction products. Wandel and Tengler developed a method involving t h e transesterification of cellulose esters with 10% BF3 in absolute methanol (1). T h e analysis was carried out in a sealed tube at 140 "C for 3 h. T h e methyl esters formed in the transesterification were analyzed on a poly(ethy1ene glycol) column. Esposito and Swann reported a procedure involving a 2-h saponification of the sample followed by acidification with hydrochloric acid and gas chromatographic determination (2). Aydin e t al. used acid hydrolysis to determine the total and individual carboxylic acid content of vinyl acetate-vinyl propionate copolymers ( 3 ) . T h e total aliphatic acid content was measured by potentiometric titration after hydrolysis with excess p-toluenesulfonic acid. T h e acetic and propionic acids liberated by hydrolysis were identified and their relative amounts determined by gas chromatography. T h i s paper describes t h e determination of the carboxylic acid content of nonvolatile esters by acid fusion reaction gas chromatography. In the present work, the carboxylic acid is liberated by hydrolysis in molten phosphoric acid at an elevated temperature. The reaction conditions promote a rapid analysis. T h e proposed method is free of solvent limitations which is particularly important in polymer analysis. Another advantage of the method is t h a t the reaction, separation, and quantitative determination are performed in one piece of apparatus. This reduces sample manipulation and losses t o a minimum. T h e entire determination can be completed in 30-35 min on a 1- t o 10-mg sample.
EXPERIMENTAL Reagents. Solid orthophosphoric acid, H,PO,, was found t o be an ideal reagent for acid fusion. The reagent was obtained from K & K Laboratories and required further drying before using. This was accomplished by drying in vacuo for 24-48 h using
Present address, Allied Chemical Gorp., Box 1021R-Bldg. CRL, Morristown, N.J. 07960.
0 1003
phosphorus pentoxide as desiccant. After drying, the reagent was stored in a glove box. A number of other acids were investigated as possible acid fusion reagents. Sodium bisulfate monohydrate, NaHS04.H20, was obtained from J. T. Baker Chemical Co. Phosphorous acid, H3P03, was obtained from Fisher Scientific Co. Benzenephosphonic acid, C6H5P03H2,and p-toluenesulfonic acid monohydrate, C-H;SO3H.H20,were obtained from Eastman Organic Chemicals. All were used without further purification. Cellulose acetate, cellulose acetate propionate, cellulose acetate butyrate, and cellulose acetate hydrogen phthalate were obtained from Eastman Organic Chemicals. Cellulose triacetate was supplied by Polyscience. The carbohydrate esters were obtained from either Aldrich Chemical Co. or Eastman Organic Chemicals. Sucrose benzoate was supplied by Pfalti! & Bauer. The cellulose and carbohydrate esters had a 1-3'70 adsorbed water content and were analyzed wet for their carboxylic acid content by acid fusion. This was necessary since dried samples readsorbed moisture during weighing. Sample weight was corrected by determining the water content of each sample analyzed. This was usually accomplished by oven drying a t 100-105 "C for 2 h. Poly(viny1 acetate) and poly(viny1 propionate) were prepared from 99-100% hydrolyzed poly(viny1 alcohol) obtained from Aldrich Chemical Co. and the corresponding carboxylic acid anhydride also obtained from Aldrich Chemical Co. according to the procedure outlined by Sorenson and Campbell ( 4 ) . A commercial sample of poly(viny1acetate) was also obtained from Polyscience. Poly(ethy1ene-vinyl acetate), 18% vinyl acetate, and poly(viny1pyrrolidonevinyl acetate) were supplied by Polyscience. Cholesteryl acetate and cholesteryl n-butyrate were obtained from Aldrich Chemical Co. Cholesteryl propionate was obtained from Applied Science. Ethynodiol diacetate was furnished by Searle Pharmaceutical Co. The C2-C4 carboxylic acids used in calibration were obtained from the following sources: glacial acetic acid from J. T. Baker, propionic acid from Mallinckrodt Chemical, isobutyric acid from Aldrich Chemical Co., and n-butyric acid from Eastman Organic Chemicals. Benzoic acid, analytical reagent grade, was supplied by Mallinckrodt. Apparatus. Gas chromatographic separations were performed using either a Perkin-Elmer 900 or !390 gas chromatograph equipped with thermal conductivity detectors and linear temperature programming. The chromatograms were recorded on a Varian A-25 and in some cases a Leeds & Northup Speedomax W recorder. A Vidar 6300 digital integrator was used for retention time and peak area measurements. The columns and chromatographic conditions used in this investigation are listed in Table I. The Chromosorb 101 and 105 columns were used to separate the C2-C4 carboxylic acids. The Free Fatty Acid Phase (FFAP) column was employed for separating benzoic acid from water. Since the free carboxylic acids were chrornatographed, every effort was made to reduce or eliminate possible adsorption sites (5, 6). Glass tubing was substituted whenever possible for stainless steel. The glass tubing was silanized with trimethylchlorosilane to remove surface silanol groups (7). Teflon wool (Supelco, Inc.) was used for column end plugs instead of the usual glass wool. The reaction unit consisted of a pyrolysis apparatus obtained from Perkin-Elmer Corp. (Pyrolysis Accessory 154-0825) and modified for analytical reaction gas chromatography by Siggia et al. (8-10). A detailed diagram of the reaction unit used in this investigation including afi major modifications is given by Frankoski and Siggia (9). Two modifications in the reaction apparatus were made during this investigation. The trap-loop ANALYTICAL CHEMISTRY, VOL. 49, NO. 14, DECEMBER 1977
2337
--
Table I, Columns and Conditions Used to Monitor Products of Acid Fusion Column A. Chromosorb 101, 6 0 / 8 0 mesh,a 6 f t X l i s in. 0.d. glass tubing B. Chromosorb 105, 6 0 / 8 0 mesh,a in. 0.d. glass tubing 6 ft x C. FFAP 10%o n 60/80 mesh,b Anakron AbS, 3 ft X I f 8 in. 0.d. stainless steel
Flow rate, mL/min
Isothermal
Programmed
Final
30-40
6-8 min at 90 C
0-6 rnin
30-40
6-8 min a t 90 C
60
3-4 min at 90 " C
1 2 'C/min t o 170-180 C 1 2 "Cimin to 170-180 "C 16-24 C/min to 200°C
a Injector 200 C, manifold 200 IC, hot wire detector 250 "C, current 175 mA, helium carrier gas. manifold 250 C, hot wire detector 250 C, current 1 7 5 mA, helium carrier gas.
was constructed out of glass instead of stainless steel in an effort to decrease adsorption. The overall length of the glass trap-loop was 8-10 inches. The horizontal inlet and outlet sections were about 1 inch long and were constructed from 3.1-mm 0.d. by 1.5-mm i.d. Pyrex glass tubing. The downward and upward legs of the loop were about 3-4 inches long and were constructed from 5-mm 0.d. by 3.5-mm i.d. Pyrex glass tubing. The larger internal diameter of the loop section was necessary to prevent the volatile reaction products from plugging the trap-loop. The bottom section of the loop was loosely packed with about 30-50 mg of Teflon wool. This was required to prevent aerosol formation. Leak-tight connections were obtained with standard '/a-inch silicone rubber O-rings and '/a-inch Swagelok fittings. The glass trap-loop and reaction unit from the oven to the inlet of the trap-loop were silanized with trimethylchlorosilane. The transfer line which connects the reaction unit and trap-loop to the gas chromatograph was previously heated using nichrome wire insulated with asbestos. This method of heating was replaced with a heating tape (Briskeat, Briscoe Co.) which was easier to install and provided a more reproducible temperature control. The temperature was usually maintained a t 180-200 "C with a Variac. Calibration curves were constructed using solutions delivered by microsyringe. The syringe was calibrated by the difference in weight of the syringe filled with water and empty. The volume was calculated from the weight and density of water. This method was found to be convenient and reproducible. The calculated volume of the syringe usually agreed within 1-370 of the syringe reading. Procedure. Samples from 1 to 10 mg were weighed into tared micro-size platinum boats (Fisher Scientific Co.) using a micro-balance (Mettler Instrument Co.). The platinum boats were then transferred to the glove box where the acid fusion reagent was stored. A glove box was necessary to minimize the adsorption of water by the reagent. The phosphoric acid was ground to a fine powder using a mortar and pestle and enough reagent was added to completely cover the sample and fill the reaction boat. The reaction boats were transported t o the reaction unit in a desiccator and then quickly transferred to the storage area of the reaction unit. Samples were reacted a t 180-200 "C for 5-8 min followed by an additional 10-15 min a t 250-275 "C. The volatile reaction products were concentrated in the trap-loop which was immersed in liquid nitrogen. Upon reaction completion, the sample boat was removed from the oven and the liquid nitrogen Dewar was replaced with a trap-heater. The reaction products were then swept directly into the gas chromatograph by the carrier gas where they were separated and analyzed. Calibration was accomplished by injecting a standard carboxylic acid solution into an empty platinum boat positioned beneath the rubber septum (B, Figure 1, Ref. 9). The calibration boat was heated for 2 min at 200 "C. The carboxylic acid and water were volatilized and concentrated in the trap-loop prior to gas chromatographic analysis. This allowed the construction of calibration curves under conditions comparable to those of actual samples. The total flush method of syringe injection was adopted using either a 10-pL or 25-pL Hamilton syringe. Sample weight was usually adjusted to release 800 to 1000 pg of the C2-C4carboxylic acids. When using the 10-pL syringe, a 5-pL injection of a 160 pg/pL-200 pg/pL carboxylic acid solution was made into the calibration boat with a 2-pL distilled water flush. With the 25-pL syringe, a 15-pL injection of a 53 pg/pL-67 pg/pL solution was 2338
ANALYTICAL CHEMISTRY, VOL. 49, NO. 14, DECEMBER 1977
0-6 min
5 min
Injector 250 'C,
Table 11. Reagent Study
Reagent Sodium bisulfate monohydrate p-Toluenesulfonic acid monohydrate Phosphoric acid Phosphorous acid Benzenephosphonic acid
Acetic acid, % recovery poly(viny1 acetate )
Butyric acid, % recovery cholesteryl n-bu tyrate
59.3% i 19% 103.0% 98.0% 97.8%
...
99.9%
...
92.7%
made into the calibration boat with a 5-pL distilled water flush. The 25-pL syringe was preferred since the size of the water peak more closely matched that produced by the reagent. It was also easier to calibrate and make volume readings with the 25-pL syringe. Samples like cellulose acetate propionate which released two carboxylic acids required standard solutions containing both acids. Calibration curves could also be constructed using the anhydrous sodium salts of the C2-C4carboxylic acids. The salts were weighed into tared platinum boats and covered with phosphoric acid. They were then reacted under identical conditions of time and temperature as actual sample boats. In a similar fashion, compounds previously analyzed by acid fusion can be used as standards for calibration. These methods of calibration were more time-consuming compared to calibration by standard solution injection and were rarely used.
RESULTS AND DISCUSSION Reagent Selection. Sodium bisulfate monohydrate was initially investigated as a possible acid fusion reagent. T h e compound has a low melting point (58 'C) and is a fairly strong acid. T h e thermal stability of sodium bisulfate was examined by Paulik e t al. using TGA, DTA, and DTC ( 1 1 ) . They reported a number of thermal reactions and phase transitions over the temperature range of interest. T h e compound first melts and loses water of crystallization in two steps forming the hemihydrate and finally the anhydrous sodium bisulfate. T h e anhydrous salt melts a t 180 "C and decomposes into sodium pyrosulfate by splitting out water of composition between two bisulfate groups. This reaction is critical because, although it provides a source of water for hydrolysis, it converts the reagent into a nonacidic compound which is no longer capable of sustaining the hydrolysis. Sodium bisulfate monohydrate produced only qualitative results. Table I1 shows a low recovery of acetic acid from poly(viny1 acetate) with a large relative standard deviation. p-Toluenesulfonic acid monohydrate was investigated as a substitute for sodium bisulfate. T h e compound is a strong monoprotic acid with a melting point of 104-106 "C. Thermal gravimetric analysis indicated that thermal decomposition commenced at about 200 "C with almost complete volatilization of the reagent. Gas chromatographic analysis showed two products, water and toluene. A peak corresponding t o
-1
c
1 2-
1I
I
- ~ _ _
i@
,hc
Ai,
2cc i
Figure 1, TGA of solid phosphoric Heating rate: 10 'C/min
.
25c
3cc
35c
- -~ a
a0
sulfur dioxide, a decomposition product of the sulfonic acid group, was not observed (12). T h e sulfur dioxide peak may have been masked by the large water peak. Under actual experimental conditions, the reagent releases approximately 75 t o 100 mg of toluene a n d 15 to 20 mg of water which overloads t h e chromatographic column. T h e large toluene peak overlaps t h e acetic a n d propionic acid peaks on both Chromosorb 101 and 105 making it impossible t o analyze samples containing either acid on these columns. Quantitative hydrolysis a n d recovery of butyric acid were, however, achieved with cholesteryl butyrate (Table 11). Further work was abandoned because of t h e thermal instability of t h e reagent. Crystalline phosphoric acid proved to be an ideal acid fusion reagent. I t is a fairly strong acid which melts at 42 "C. T h e thermogram of solid phosphoric acid illustrated in Figure 1 shows an almost constant weight loss from 60 to 425 "C. This was interpreted as a continuous loss of water. Gas chromatographic analysis confirmed t h a t only water was released from phosphoric acid. T h e reagent condenses with itself by the elimination of water when heated, forming pyrophosphoric acid and higher condensed species. This reaction is analogous t o t h e condensation of sodium bisulfate t o form pyrosulfate. T h e condensation serves as a needed source of water for hydrolysis; however, unlike sodium bisulfate, the acidic character of the reagent is preserved. Solid phosphoric acid was able t o quantitatively hydrolyze and liberate acetic and butyric acid from poly(viny1 acetate) and cholesteryl butyrate as shown in Table 11. Phosphoric acid was used in all subsequent quantitative work. T h e only difficulty with the reagent was its hygroscopicity which necessitated t h e use of a glove box. Phosphorous acid and benzenephosphonic acid were briefly examined as possible substitutes for phosphoric acid. Both reagents have acid strength similar t o phosphoric acid a n d can be used without requiring a glove box. Both compounds gave quantitative results as shown in Table 11. Benzenephosphonic acid thermally decomposes like p-toluenesulfonic acid releasing large quantities of benzene and water. Work was discontinued with phosphorous acid because it releases poisonous phosphine gas when heated above 200 "C (13). Phosphorous acid could be considered as a substitute for solid phosphoric acid provided adequate steps were taken t o completely t r a p out t h e phosphine gas. Quantitative Analysis. T h e cholesteryl esters were used in conducting t h e initial quantitative investigation, T h e cholesteryl esters were selected because they were sufficiently nonvolatile and were obtainable in high purity, making a check determination of their acid content unnecessary. A typical chromatogram of the reaction products released during acid fusion of cholesteryl acetate is shown in Figure 2. T h e
-. \
5
16
m wtes
.
Atmosphere: N, at 30 m ~ / m i n
~i
Flgure 2. Gas chromatogram of cholesteryl acetate after acid fusion (Column A, Table I). Peak 1, water: 2, acetic acid
Table 111. Analysis of Cholesteryl Esters by Acid Fusion Gas Chromatography
Compound Cholesteryl Cholesteryl Cholesteryl Ethynodiol
acetate propionate n-butyrate diacetate
Reaction product Acetic acid Propionic acid n-Butyric acid Acetic acid
Re1 Theoretical std recovery, % dev 102.8 (3)' 100.9 ( 5 )
0.8 0.7
99.9 ( 4 )
1.7 4.0
100.5 (4)
Number in parentheses indicates the number of determinations. chromatogram should show two peaks. A large water peak, from 10-20 pL, released by t h e acid fusion reagent a n d a carboxylic acid peak liberated by hydrolysis from t h e ester. T h e hydroxy and in many cases the polyhydroxy compound resulting from t h e hydrolysis of the ester are generally nonvolatile and, therefore, not detected. T h e results in Table I11 show conversion within 3% of t h e theoretically expected value and proved results of sufficient precision and accuracy can be achieved following sample reaction and gas chromatographic separation. I t is possible t o shorten t h e 30-35 min required for a complete determination, 15-20 min for sample reaction and 15 min for chromatographic separation, by overlapping analyses. A sample reaction can be initiated after the elution cf the water peak from the previous sample boat. T h e delay ensures t h a t the contents of the t r a p have been completely volatilized and also allows sufficient time for t h e detector to stabilize after switching from t h e trap heater to the liquid nitrogen Dewar. T h e carbohydrate esters are chemically similar t o the cellulose esters and were used as model compounds to examine the possibility of analyzing these commercially important polymers by acid fusion gas chromatography. Table IV compares the results obtained by acid fusion with t h e theoretical carboxylic acid content. T h e organic acid content of the nonreducing carbohydrate esters was also checked by saponification (14). T h e chromatogram of t h e reaction products from sucrose octaacetate following acid fusion is presented in Figure 3. All of t h e acetylated carbohydrates produced similar chromatograms. In addition to the expected water and acetic acid peaks, Figure 3 contains a number of minor peaks presumably caused by the decomposition of the sucrose moiety. This was confirmed by running a reaction boat containing sucrose and phosphoric acid. Figure 4 shows five decomposition peaks from the sucrose blank. The peaks were identified as carbon dioxide, formic, acetic, and propionic acids. Peak number four was not identified. T h e carbon dioxide, formic a n d propionic acids from the decomposition of the sucrose moiety in sucrose octaacetate are clearly visible ANALYTICAL CHEMISTRY, VOL. 49, NO. 14, DECEMBER 1977
2339
Table IV. Analysis of Carbohydrate Esters b y Acid Fusion Reaction Gas Chromatography
a-D-Cellobiose octaacetate a-D-Glucose pentaacetate p-D-Ribofuranose 1,2,3,5-tetraacetate 1-Thio-D-glucose pentaacetate p-D-Galactose pentaacetate Sucrose octaacetate Sucrose diacetate hexaisobutyrate
Sorbitol hexaacetate Me thyl-a-D-galactopyranoside
2,3,4,6-tetraacetate 0-D-Ribofuranose1-acetate-2,3,5tribenzoate Sucrose benzoate a
Weight percent + standard deviation Acid fusion Check method Theoretical
Reaction product
Compound
Acetic acid Acetic acid Acetic acid Acetic acid Acetic acid Acetic acid Acetic acid Isobutyric acid Apparent acetic acid Acetic acid Acetic acid
81.7 i- 1 . 2 ( 6 ) 65.1 i 0.5 (4)
Benzoic acid
74.9
i
0.2 (4)
72.62
Benzoic acid
78.3
i
1.8 ( 5 )
80.46
73.1 t 78.4 E 76.3 i 71.5 i 76.3 i 71.9 i 14.5 i 59.1 i 54.8 i
0.7 (5)= 1.4 (4) 1.2 ( 4 ) 0.8 (4) 1.9 ( 6 ) 0.9 ( 6 ) 0.2 ( 5 ) 1.1( 5 ) 0.6 ( 5 )
... ...
70.79 76.92 75.47 73.93 76.92 70.79 14.18 62.42
83.17 i 0.80 ( 3 ) 66.9 i 0.1 ( 3 )
82.94 66.29
... 70.43
... 0.36 (3)
i
...
56.8 * 0.9 ( 3 )
Number in parentheses indicates the number of determinations.
_.-i . j -
5
i
8 T
.'2 nutes
'5
Figure 3. Gas chromatogram of sucrose octaacetate after acid fusion (Column A, Table I), attenuation X 64. Peak 1, carbon dioxide: 2, water; 3, formic acid; 4, acetic acid: 5, propionic acid
in Figure 3. T h e small unknown peak and acetic acid peak produced by the decomposition of the sucrose moiety are masked by the large acetic peak liberated by hydrolysis. Results are thus higher by the amount of acetic acid and unknown component produced by decomposition. Correction can be accomplished by running as a blank t h e respective parent carbohydrate. The production of volatile acids as decomposition products is a common occurrence in the analysis of carbohydrate esters (15-17). As an example, fructose had an acetic acid content of 16% under acidic hydrolysis in sulfuric acid (17). Sucrose had a n acetic acid content of 3% by acid fusion, however, the results for sucrose octaacetate were high by just 1.5% since the compound is only 50% by weight sucrose. Results in Table IV were corrected for decomposition by running a blank on the parent carbohydrates. T h e results in Table IV for sucrose benzoate and p-Dribofuranose-1-acetate 2,3,5-tribenzoate indicate t h a t acid fusion can be successfully extended to samples containing aromatic monocarboyxlic acids. Gas chromatographic separation of the reaction products was carried out using the F F A P column. Somewhat more drastic conditions were required t o achieve complete hydrolysis and t o volatilize the 2340
ANALYTICAL CHEMISTRY, VOL. 49, NO. 14, DECEMBER 1977
0
4
8 12 minutes
16
20
Figure 4. Gas chromatogram of sucrose after acid fusion (Column A, Table I), attenuation X 16. Peak 1, carbon dioxide; 2, water; 3,formic acid; 4, unknown; 5, acetic acid; 6, propionic acid
higher boiling reaction product. Samples were reacted a t 200 "C for 5 min followed by an additional 15-20 min a t 300-325 "C. Calibration was achieved by weighing analytical reagent grade benzoic acid into tared platinum boats. Sample and calibration boats were reacted under the same experimental conditions. A typical chromatogram of the reaction products 2,3,5-tribenzoate is shown in from P-D-ribofuranose-1-acetate Figure 5. Although decomposition of the carbohydrate moiety was observed, a correction was unnecessary since t h e parent carbohydrate degrades without production of benzoic acid. T h e acetate content of P-D-ribofuranose-1-acetate 2,3,5-tribenzoate was not determined because of insufficient resolution of the acid hydrolyzed acetic acid from t h e C1-C3 decomposition acids. No attempt was made to determine the acetate content on either t h e Chromosorb 101 or 105 columns. Sucrose benzoate had an elemental carbon content of 68.62 70 and a 4.65% hydrogen content. T h e elemental composition indicated a 7.18 degree of substitution which corresponds to a 80.46% combined benzoic acid content. This is in general agreement with the benzoic acid content determined by acid fusion. The results from the acid fusion analysis of t h e cellulose esters and poly(viny1esters) are summarized in Table V. The
Table V. Analysis of Cellulose Esters and Poly(viny1 esters) by Acid Fusion Gas Chromatography Weight percent + standard deviation Reaction product Acid fusion Check method Polymer Cellulose acetate Cellulose triacetate Cellulose acetate propionate Cellulose acetate butyrate
Acetic acid Acetic acid Acetic acid Propionic acid Apparent acetic acid Acetic acid
54.9 62.2 42.0 18.3 56.8 42.7
* *
0.3 (4)a 0.9 ( 6 ) 0.2 ( 5 ) 0.1 ( 5 ) 0.3 ( 5 )
i
0.4 ( 5 )
i i i
*
55.88 61.96
i
0.21 ( 3 ) 0.32 ( 5 )
56.65
f
0.51 ( 3 )
___ ___
___ ___
21.7 i 0.1 ( 5 ) Butyric acid 56.82 * 0.15 ( 4 ) 57.5 i 0.4 ( 5 ) Apparent acetic acid ___ 29.3 i 0.7 ( 6 ) Cellulose acetate Acetic acid -__ Hydrogen phthalate Phthalic acidb 46.2 i 2.9 ( 6 ) Apparent acetic 62.6 i 2.1 ( 6 ) 62.13 i 0.56 ( 4 ) acid Poly( vinyl acetate)c Acetic acid 69.7 i 1.4 ( 5 ) 68.46 i 0.09 ( 3 ) Poly(viny1 acetate )crd Acetic acid 70.2 t 0.4 ( 4 ) 68.46 i 0.09 ( 3 ) Poly(viny1 acetate)e Acetic acid 69.9 i 1.3 ( 6 ) 69.5 i 0.7 ( 3 ) Acetic acid 0.72 = 0.06 ( 6 ) Poly(viny1 propionate)e ._73.3 i 0.9 ( 6 ) Propionic acid 74.2 i 0.9 ( 6 ) 74.5 * 0.10 ( 3 ) Apparent propionic acid Number in parentheses indicates the number of determinations. Independently determined. Commercial sample. Calibration curve constructed using sodium acetate. e Prepared from 99-100% hydrolyzed poly(viny1 alcohol).
___
2
-
,-. z
I,
Z
Lid .
c
Figure 5. Gas chromatogram of P-D-ribofuranose-1-acetate 2,3,5tribenzoate after acid fusion (Column C, Table I). Peak 1, carbon dioxide: 2, unknown: 3, water; 4, C,-C3 acids: 5 , benzoic acid
Figure 6. Gas chromatogram of cellulose acetate propionate after acid fushion (Column A, Table I). Peak 1, carbon dioxide: 2, water: 3, formic acid: 4, acetic acid; 5, propionic acid
cellulose esters behaved in the same manner as the carbohydrate esters. T h e volatile acids derived from the degradation of the cellulose backbone were corrected with a cellulose blank. T h e accuracy of t h e acid fusion results was checked by saponification (16). It was necessary to express saponification results as percent apparent acetic acid for the mixed esters. T h e acid fusion results were expressed both as the individual carboxylic acid content and as apparent acetic acid content which allows a direct comparison of the results. By acid fusion, it was possible to determine only the acetic acid content of cellulose acetate hydrogen phthalate. Phthalic acid is nonvolatile and not amenable t o gas chromatographic determination under the existing experimental conditions. An independent determination was carried out for the phthalic acid content and combined with t h e acetic acid value from acid fusion t o obtain t h e apparent acetic acid (18, 19). The reaction products released from cellulose acetate propionate are shown in Figure 6. The water, acetic acid, and propionic
acid peaks are well resolved. Two decomposition peaks are also shown in the chromatogram. The acid fusion determination of the poly(viny1 esters) were also checked and compared favorably with the acid content obtained by saponification (14). T h e ability t o use sodium acetate for calibration was demonstrated with poly(viny1 acetate). Poly(viny1 propionate), besides liberating the expected propionic acid, had a n acetic acid content of 0.72%. T h e acetic acid was derived from residual acetate groups in the 99-100 % hydrolyzed poly(viny1 alcohol) used in the preparation. T h e analysis of poly(viny1 propionate) demonstrates the ability of acid fusion to determine mixed esters of widely varying composition with good precision and accuracy. No decomposition products were observed from the poly(viny1 esters). T h e acid fusion analysis of two vinyl acetate copolymers can be found in Table VI. T h e vinyl acetate content of poly(ethy1ene-vinyl acetate) was calculated from the elemental ANALYTICAL CHEMISTRY, VOL. 49, NO. 14, DECEMBER 1977
2341
Table VI. Analysis of Poly(viny1 acetate) Copolymers by Acid Fusion Gas Chromatography
Percent combined vinyl acetate + standard deviation, % weight Vinyl Check pyrrolmethod idone Acid fusion
Polymer Poly(ethy1ene-vinyl acetate), 18% vinyl acetate Poly( ethylene-vinyl acetate), 18% vinyl acetate Poly(n-vinyl pyrrolidone-vinyl acetate) 60140
16.8 f 0.4 (6)a
16.84b
1 6 . 5 i 0.2 (5)c
16.84
41.2 i- 0.7 (4)
58.15d
a Number in parentheses indicates the number of determinations. Calculated from elemental oxygen Calibration curve constructed using sodium analysis. Calculated from elemental nitrogen analysis. acetate.
Table VII. Analysis of Residual Acetate Groups in Poly(viny1 alcohols) by Acid Fusion Gas Chromatography Percent hydrolyzed
standard deviation Manufacturer's Check method value i.
Polymer
Acid fusion
Poly(viny1 alcohol ) Poly(viny1 alcohol) Poly(viny1 alcohol) Poly(viny1 alcohol)
73.5 t 0.3 ( 5 ) a
72.98
0.31 ( 3 )
75
i-
87.6
t
0.6 ( 5 )
88.13 i- 0.23 ( 3 )
88
96.2
*
0.2 ( 4 )
...
96
98.6
i-
0.1 ( 5 )
...
99-100
a Number in parentheses indicates the number of determinations.
oxygen analysis. Sodium acetate was again employed as an alternate method of calibration in the analysis of poly(ethylene-vinyl acetate) and the resulting vinyl acetate value agreed favorably with the value determined using the syringe calibration method. T h e vinyl pyrrolidone content of
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ANALYTICAL CHEMISTRY, VOL. 49, NO. 14, DECEMBER 1977
poly(viny1 pyrrolidone-vinyl acetate) was determined from the elemental nitrogen analysis. T h e vinyl acetate and vinyl pyrrolidone content are in good agreement. Water and acetic acid were t h e only observed reaction products from both copolymers. Poly(viny1alcohol) is prepared commercially from poly(viny1 acetate) by an acid or base catalyzed saponification reaction. T h e polymer usually contains residual acetate groups, the number of which is regulated by the reaction conditions. Four poly(viny1 alcohol) samples were analyzed by acid fusion reaction gas chromatography. Saponification was used as a check method for the 75% and 88% hydrolyzed poly(viny1 alcohol) samples. T h e results in Table VI1 from t h e acid fusion of all four poly(viny1 alcohol) samples are in agreement with results obtained by saponification and the degree of hydrolysis stated by the manufacturer. T h e chromatograms from all four polymers contained only the expected water and acetic acid peak.
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RECEIVED for review February 16, 1977. Accepted June 29, 1977. Work supported by the National Science Foundation, Grant No. GP-37493X and CHE76-07378.
