Qua nt ita t ive Dete rmination of M a Ieic An hy d ride, Benzoic Acid, Naphthalene, and 1,4-Naphthoquinone in Phthalic Anhydride by Gas Liquid Chromatography HELEN TRACHMAN aiid FREDERICK ZUCKER Eastern Resin Division, Reichhold Chemicals, Inc., Elizabeth,
b The quantitative analysis of the various related substances commonly found in phthalic anhydride, obtained from the naphthalene conversion process, has previously required separate and time consuming Imalytical procedures for each component. Separation of all the related components present has been. achieved by isothermal gas liquid chromatography using a Silicone SF9i5 column and a hydrogen flame detecior. The phthalic anhydride is dissolved in o-dichlorobenzene and a gas chromatogram is prepared. Prior to cinalysis, calibration curves are prepared and used to relate detector response to concentration. The quantitative analyses for these impurities can thus be completed in a matter of minutes.
N. 1.
RETENTDN TIME
Figure 1.
Chromatogram
of
phthalic anhydride and related components
Naphthalene Process Maleic anhydride B. o-Dichlorobenzene C. Benzoic acid D. Naphthalene E. Phthalic anhydride F. 1,4-Naphthoquinone
A.
T
DEVELOP MEN^ of a rapid analytical method for the determination of related substances present in phthalic anhydridc (PAA) manufactured from the naphthalene oxidation process, has long been desired by the anhydride producers. .inalyses of crude and process samples of PAA for maleic anhydride (RIAA), benzoic acid, naphthalene, and 1,4n:tphthoquinone is necessary to obtain and maintain optimum plant operation. Maximum product yield can only be achieved through a study of analytical data. Many applications by the anhydride consumers require close control of the minor impurities. Fcr example, the presence of benzoic acid leads to objectionable odors when refined PAA is used for making plasticizers. Benzoic acid, if present in significant amounts, exerts a “chain stoppin; action” during the esterfication procew of alkyd and polyester manufacture. This results in a lower molecular weight polymer with different properties. The presence of l14-naphthoquinone gives objectionable color to many produ-ts as well as changing the velocity of certain chemical processes. Thus, close quality control of the refined product is necessary to meet these special needs Until the present, the quantitative analysis of these trace impurities has HE
been conducted by conventional chemical, polarographic, and ultraviolet spectrophotometric procedures. All of these determinations are time consuming, and applicable only to the refined product. Maleic anhydride is most commonly determined as maleic acid by polarography. A number of publications have appeared which describe this determination (2, 3, 7 , 8). Sample preparation, careful pH adjustment, and equipment maintenance make this method time consuming and difficult to maintain on a routine basis. Ultraviolet spectrophotometry has been widely used for the determination of benzoic acid ( 5 ) . Isolation by solvent extraction or chromatography is necessary prior to spectral measurement, making for a lengthy determination. I n addition, interference from oxidation products in analysis of crude anhydride makes quantitation impossible. The determination of naphthalene and 1,4naphthoquinone by ultraviolet spectrophotometry has been described (1, 6). Because of photochemical
changes in the naphthoquinones which occur during the extraction period, it is difficult to obtain good results. Gas liquid chromatography (GLC) furnishes a fast and convenient approach to the simultaneous determination of all the related substances found in PAA. Note is made of a previous GLC determination for benzoic acid in PAA (4). Moore and Meinstein chromatographed
Table
I.
Retention Data
Retention time, minutes (measured from Relative Sample inject) retention5 Acetone (if used) 1.1 0.24 Maleic anhydride 2.4 0.52 o-Dichlorobenzene 4.6 1.00 Benzoic acid 6.2 1.35 Naphthalene 8.4 1.83 Phthalic anhydride 1 1 . 4 2.48 1,4-Naphthoquinone 16.3 3.54 a Relative to solvent peak o-dichlorobenzene = 1.00.
VOL. 36, NO. 2, FEBRUARY 1 9 6 4
269
II. Calibration Data Weight present in Average 3-pl. inject, area count, mg. x 103 0.015 4624 0.0009 260 0.015 8550 0.0009 500 0,015 6386 0.0009 310 0.006 6080 0.0015 1465
Table
Component Benzoic acid Benzoic acid Naphthalene Kaphthalene Maleic anhydride Maleic anhydride 1,CKaphthoquinone 1,PNaphthoquinone
Table 111.
Recovery Data on Synthetic Samples
Standard Maleic anhydride Benzoic acid Naphthalene
ReReAdded, covered, covered, mg. mg. 70 0.250 2.500 2.500
10.000
5.000 10,000
1,4-Naphthoquinone
0.231 2.256 2.495 9.630 4.800 9,710
92.2 90.3 99.8 96.3 96.0 97.1
0,025 0.023 92.5 0.0025 0.0022 88.0
the methyl esters of benzoic acid and related substances; however, severe tailing in the separation obtained is undesirable for quantitative work. Methyl maleate was not separated in the procedure and quantitation was not attempted on naphthalene and 1,4naphthoquinone. A new GLC method has been developed for the separation and complete quantitation of benzoic acid, maleic anhydride, naphthalene, and l,-l-naphthoquinone. After detector calibration has been completed, routine analysis for the four impurities can be completed in about 30 minutes.
No. of detns. 6 6 6 6 6 6 6 6
Std. dev. 1 0 ,000 f0,008 fO.000
10,006 1 0 .000 * O . 005
*o. 000 izo. 001
Reference Materials. Phthalic anhydride, technical (Reichhold), recrystallized twice from carbon tetrachloride. Maleic anhydride, technical (Reichhold), recrystallized twice from carbon tetrachloride. Separation. -4 synthetic mixture of phthalic anhydride, maleic anhydride, naphthalene, and 1,4-naphthoquinone was prepared by intimate grinding together of the reference materials. Complete solution was obtained by dissolving the mixture in odichlorobenzene. Experimentation with various column packings and operational variables of column temperature and carrier gas flow rate, resulted in finding the optimum operating conditions for a good separation. Figure 1 shows a typical chromatogram of the synthetic mixture. Symmetrical peaks, a good baseline return, and full resolution of all the components indicated that quantitation was feasible. Retention data are presented in Table I. Relative retentions for the components are calculated in relationship to
EXPERIMENTAL
Apparatus and Materials. Chromatographic Knit. The instrument used t o obtain the chromatograms was a Model 1609 gas chromatograph (F & 31 Scientific Co.) equipped with a Brown Electronik recorder (Xinneapolis-Honeywell Regulator Co.) and Disc chart integrator (Disc Instruments, Inc.). Operating conditions were as follows: detector cell temperature, 200' C.; injection port temperature, 250' C.; column temperature, isothermal, 220' C.; helium flow rate, 60 cc. per minute (30 p s i . at inlet). Column Preparation. The column was prepared from a 6-foot length of 1/4-inch stainless steel tubing packed with30% SiliconeSF96on 6040 80-mesh, acid-washed Chromosorb W. The column was conditioned a t operating temperature with helium flowing until a stable baseline was attained. 270
ANALYTICAL CHEMISTRY
the solvent peak. This is particularly useful for confirming identification, if any extraneous peaks complicate the chromatogram. Preparation of Standard Calibration Curves. The direct calibration method was used as the means t o obtain quantitation of the individual impurity components present in PAA. The area count data were determined using ACS reagent grade naphthalene and 1,4-naphthoquinone, and recrystallized PAL4and 11IAA. Exactly 250.0 mg. of the standard is weighed on an analytical balance and quantitatively transferred to a 50-ml. volumetric flask. Approximately 25 ml. of o-dichlorobenzene is added to effect solution. Dilute to the mark with solvent and stopper. Measured aliquots are then transferred by pipet to 50-ml. volumetric flasks to yield solutions containing 125.0, 62.50, 31.25, and 15.63 mg., respectively. A series of 3-p1. injections are made into the chromatograph using a microliter syringe and the operational conditions of the chromatograph as previously listed. Peak area counts are obtained from the Disc Integrator tracing. The count in all cases is normalized to maximum sensitivity by multiplying by the attenuation used. See Table I1 for a partial listing of the data obtained, The average area count for the upper and lower limits of each component under study, together with the standard deviations are given. Good linearity of response was obtained. The calibration curves are prepared for each component by repeating the above procedure, for the concentration ranges desired. The magnitude of the area counts obtained are large, so that p.p m. levels are easily achieved by the use of more sensitive attenuation positions on the chromatograph, Procedure. -4 well-ground sample of phthalic anhydride, 2.500 grams, is transferred to a 50-ml. volumetric flask and dissolved in o-dichlorobenzene. If the solution is hazy use about 5 ml. of acetone to replace part of the solvent. Dilute to the mark. -4 10-pl. sample is injected into the chromatograph. Attenuate as necessary to keep all peaks at least 15% and not more than 85% of full scale on the recorder. The peak area count is normalized for the attenuation used. The weight in milligrams of each component present is obtained directly from the calibration curve. The per cent present is calculated by dividing the weight of the component by the sample weight present in inject, 0.5 mg., X 100. DISCUSSION AND RESULTS
Figure 2. Chromatogram of phthalic anhydride and related impurities o-Xylene process
A. B. C. D.
E.
F.
Maleic anhydride o-Xylene o-Dichlorobenzene Benzoic acid Phthalic anhydride Phthalide
To test the procedure, synthetic samples of phthalic anhydride containing known amounts of maleic anhydride, benzoic acid, naphthalene, and 1,4naphthoquinone were prepared and analyzed. The impurity components were added as solutions to obtain the low levels desired. Table I11
lists the weights of the various components added, the w2ights recovered, and the per cent recokery. The choice of impurity levels taken for study simulates actual impurity levels of samples. The calibration curves were prepared using a low sensitivity range of 1000. The range of the Standards covers 0.18-3,070 for all exc2pt 1,4naphthoquinone. I n 1,4naphthoquinone, the standards cover 0.3-1.2% when using the proposed sample procedure. With the sensitive hydrogen flame detector, we can readily extend the range of coverage to lower levchls by factors of 10, 100, and 1000 by means of attenuation. In our exlier work, the separations were achieved using a gas chromatograph with a hermal detector. Quantitation of low-level concentrations, necessitated a switch to the hydrogen flame detector. This sensitivity mas particularly needed for 1,4naphthoquinone where levels in refined PAA is in p.p.m. Typical analyses of crude and refined phthalic anhydride are shown in Table IV. There is a possibilitl that the maleic impurity is present as maleic acid rather than anhydride. Mix tures of maleic acid and maleic anhydride were chromatographed from a rlolvent solution. The resulting chromatcegram gives only one peak, whose relative retention corresponds to the anhjrdride. KOpeak
Table IV.
Typical Analysis of PAA Samples
Impurity
Run 1
Maleic anhydride Benzoic acid Naphthalene 1.4-Naphthoquinone
Crude
Run 2
Run 1
0.08%
0.37%
0.127, 0.317,
0.01%
0.287,
0.1270
0.52%
for water is obtained because the hydrogen flame detector does not respond to water. Phthalic acid, another possible impurity in P..1A, also converts to the anhydride form during chromatography. If phthalic acid is present, the solution of o-dichlorobenzene will be hazy and requires the addition of acetone to clarify. Phthalic acid is not determined in this procedure. The accuracy in the determination of the other impurities is not affected because direct calibration was used for quantitation. A considerable amount of phthalic anhydride is currently being manufactured by the o-xylene conversion process. Our concern to date has been largely directed toward our own manufactured product. However, a brief investigation indicates the column and conditions used in this study are applicable to the separation. See Figure 2 for a chromatogram prepared on PAA and the expected impurities from the o-xylene process. Good separation for
0.61%
Refined
0.16%,
... 1 . 5 p.p.m.
Run 2
0.0370 0.297, ... 1.9 p.p.m.
phthalide, o-xylene, PAA, MAA, benzoic acid, and solvent are obtained. Future work is planned to complete the quantitation of these impurities. ACKNOWLEDGMENT
The authors thank Don Murray of the Reichhold staff for his helpful suggestions on the separations. LITERATURE CITED
(1) Coggeshall, N., Olessner, A., ANAL.
CHEM.21.5.50 11949’1. , \ -
~
( 2 ) Elvingi‘P. J., Martin, A., Rosenthal, I., Ibid., 25, 1082 (1955).
(3) Elving, P. J., Teitelbaum, C., J. Am. Chem. Soc. 71,3916 (1949). ’ ’ (4) Moore, C., Meinstein, S., ASAL. CHEM.34,1503 (1982). rnieks, R., Bonter, C. E., Ibid., ’
RECEIVED for review August 19, 1963. Accepted Xovember 7, 1963.
Determination of 2,6,-Di -terf- butyl-p-c resol a nd 2(2’-Hydroxy, 5’-methylphenyl) benzotriazole in Polystyrene by Gas Liquid Chromatography C. B. ROBERTS and J. 1). SWANK East Analytical laboratory, The Dow Chemical Co., Midland, Mich.
b Two polystyrene additives, lonol (2,6-di-tert-butyl-p-cresol) and Tinuvin
P [2(2’-hydroxy, 5’-methylphenyl)benzotriazole] are determined by gas liquid chromatography. Standards for each are made by dissolving the pure compound in carbon disulfide or methylene chloride. After the chromatograms are obtained, peak heights are measured and standard curves prepared. The polymer samples are prepared by dissolving them in the same solvent, obtaining the desired peak, and computing the amount of the component by mearis of the standard curves. No separations are required and the result is (1 rapid method which produces results of good accuracy and precision.
I
necessary to determine the amount of additives present in finished polystyrene. This has usually been accomplished by spectrophotometric methods such as infrared and ultraviolet adsorption (6, 6). Two compounds commonly used in the manufacture of polystyrene are Ionol (2,6-di-tertbutyl-p-cresol)(Shell Oil Co.) and Tinuvin P [2(2’-hydroxy, 5~-methylphenyl)benzotriazole] (Geigy Ind. Chem.) Although both of these compounds have fairly high boiling points, they have an appreciable vapor pressure. Short columns had been employed to analyze similar compounds (1, 3 , 4 ) and thus the prospect of analyzing them by gas liquid chromatography (GLC) seemed quite reasonable. T IS OFTEN
By selecting the proper conditions and measuring the peak height, a straight line calibration curve was obtained for each compound. When applied to the analysis of the polymer, the technique is quite simple. The polystyrene is dissolved in a solvent such as carbon disulfide or methylene chloride, the sample is injected into the instruF e n t , and the peak heights are compared t o those obtained by running standards under the same conditions. By using flame ionization detection only a small sample is required, thus reducing the amount of polymer retained in the injection block. No separation of the polymer from the solvent is required, and the analysis can be carried out quickly t o produce accurate and precise results. VOL. 36, NO. 2, FEBRUARY 1964
271
