1
R. were 485" C. and 10 ohms in B of Figure 2, and 490' C. and 30 ohms in B of Figure 3, respectively. The results of Table I and Figures A
HZ
I
2
3
5
4
Time, minutes
Figure 3. Results of a gas 50% mixture (5% HP
+
+ 45% n - C h ) using thermal conductivity
1.5
=
C3HS
cell ( A ) and zinc oxide film (6)
in
1.0
>
..E
2.5
2 and 3 show that the peak widths of the present method are generally large in comparison with those using the thermal conductivity cell. This is because the adsorption and desorption of gases require a relatively long time, depending upon the species of gases, the temperature, and thickness of the zinc oxide film. Selection of proper conditions would improve the results. The above results, however, show that the reproducibility of measurement is sufficient. The experimental and theoretical investigation of the relation between the height or area of peak and the sample quantity is being continued. The sensitivity 1 p.p.m. for the above detection is 50 and about a hundred times that of the thermal conductivity method. The sensitivity will be increased still higher by amplification.
-
LITERATURE CITED
c:
(1) Dimbat, M., Porter, P. E., Stross, F. H., ANAL.CHEM.28,290 (1956).
, 1
2
3
I
4
5
6
Time, minutes
2 and 3 were 41' C. and 40' C., respectively. The quantity of sample used was 1 milliliter in each case. The flow rates of the carrier gas, nitrogen, were 17 ml./min. in A and B of Figure 2 and 26 ml./min. in A
and 20 ml./min. in B of Figure 3. The cell currents of the thermal conductivity cell were 64 ma. in A of Figure 2 and 82 ma. in A of Figure 3. The temperature of zinc oxide film and the value of variable resistance
TETSURO SEIYAMA Am0 KATO KIYOSHI FUJIISHI MASANORI NAGATANI Department of Applied Chemistry Faculty of Engineering Kyushu University Hakozaki-machi Fukuoka-shi, Japan RECEIVEDfor review May 3, Accepted August 10, 1962.
1962.
Determination of Benzoic Acid in Phthalic Anhydride by Gas Liquid Chromatography Sir: R e have developed a method of analysis for benzoic acid in phthalic anhydride. This undesirable by-product has been noted in varying amounts in crude phthalic anhydride (PAA) produced both by fluid- and fixed-bed catalysis. It represents B loss of yield, and in refined PAA, when present in significant amounts, it exerts a "chain stopping action" on the esterification process employed in alkyd resin manufacture. This action results in lower molecular weight polymers and in a sweetish, objectionable odor. The determination of small amounts of benzoic acid in the anhydride by titrimetric methods is complicated by the presence of maleic and phthalic acids. Quantitative chloroform extrac-
tions, and subsequent titration with standard alkali are successful only in amounts greater than 2% (3). Spectrophotometric techniques are also successful but possess 'inherent difficulties
(4)*
Gas liquid chromatography (GLC) does not have these limitations and furnishes a quick, convenient method for carrying out the determination of benzoic acid in crude and refined phthalic anhydride in the presence of other impurities. Because the retention times differ significantly (Figures 1 and 2), this technique permits the simultaneous estimation of other impurities such as toluic acid, l,Cnaphthoquinone, phthalide, and unoxidized naphthalene (or o-xylene) . Hence the method can be used for
checking plant reactor effluent streams for benzoic acid, and for following its concentration during condensing, treating, and refining cycles, as well as for analyzing the finished PAA product. The method is rapid and well suited for a control procedure. Principle of the Method. T h e sample is dissolved in benzene, and the free acids are esterified by diazomethane t o yield the methyl esters and nitrogen. EXPERIMENTAL
Apparatus. Gas chromatograph, Wheelco Model 10, Rockford, Ill. Carrier gas, argon, 7.5 p.s.i. inlet pressure. Detector, @-ionization (Rs 226 source). Column: 8-foot, 6-mm. VOL. 34, NO. 1 1 , OCTOBER 1962
1503
METHYL TOLIIATE 0-XYLENE
L
I
l
I
I
I
l
'
5
6
7
J
I
'
8
I
I
1
1
I
IC
T I V E (m - d e $
Figure 1 . Chromatogram of methyl benzoate plus other impurities in crude phthalic anhydride from o-xylene conversion
Figure 2. Chromatogram of methyl benzoate plus other impurities in crude phthalic anhydride from naphthalene conversion
Solvent: benzene-ether
Solvent: benzene-ether
diameter, acid-washed (Chromosorb W 8OjlOO mesh), 25% silicone grease. Detector voltage; 1000, sensitivity 1 X 1O-SA. for full recorder deflection. Column temperature 185' C., cell temperature 220' C. Hamilton Microliter syringe, 10-11. capacity, K&E compensating polar planimeter. Table I. Benzoic Acid Standards
Average Benzoic area acid under taken, curve, mg./50 ml. No. of sq. in. X solution detns. Std. dev. 5.0 5 49 0.000 3.0 5 27 0.000 1.0 5 11 0.005 0.5 5 8 0.000 0.1 5 5 0,007 Table 11. Benzoic Acid in Several Refined Phthalic Anhydrides
Sample"
Benzoic acid found, yo
0.03 0.11 0.05 0.89 0.57 0.05 a Samples stem from various sources of manufacture, using fixed-, as well as fluidbed catalysis.
Table 111. Benzoic Acid in Various Types of Crude Phthalic Anhydride
Sample0 1
2 3
4 5 6
7
8 9 10
Benzoic acid found, % 0.35 0.44 0.36
0.29 0.31 0.37 0.29 0.27 0.22
0.32 Samples were selected from naphthalene and o-xylene oxidations. a
1504
ANALYTICAL CHEMISTRY
Reagt-it. Diazomethane. An alcohol-free ethereal solution of diazomethane was prepared (1, 2) from N methyl-N-nitroso-p-toluene sulfonamide (Aldrich Chemical Co., Inc.). Standards. Standards were prepared from phthalic anhydride which had been recrystallized from benzene until free of benzoic acid as indicated by the absence of the methyl benzoate peak on the chromatograni. Reagent grade benzoic acid, dissolved in benzene to give a solution of 1 nig. per ml., was added in sufficient increments to 1 gram of phthalic anhydride dissolved in 50 ml. of benzene to give solutions containing from 5 mg. to 0.1 mg. These were esterified and used as standards. PROCEDURE
Weigh 1,000 gram of phthalic anhydride into a 250-ml. beaker. Add 50 ml. of benzene and warm until the sample dissolves, cool to room temperature, and add 15 ml. of an ethereal solution of diazomethane (1, 2), containing about 3 grams of diazomethane in 175 ml. of solution, to the benzene solution. (Diazomethane is a highly volatile toxic material and should not be inhaled or allowed to contact the skin.) I n a well ventilated hood, evaporate the solution to about 30 ml. This step will remove the diazomethane and most of the excess ether. Transfer to a 50.00-ml. volumetric flask and dilute to volume for gas chromatography. A 2.0-11. sample is optimum for sample injection. RESULTS AND DISCUSSION
Figure 1 is a chromatogram of PAA from o-xylene conversion. The methyl benzoate peak is well defined. Methyl toluate, o-xylene, and phthalide are also resolved. Figure 2 is a chromatogram of PAA\ from naphthalene conversion. The methyl benzoate again is well defined, and naphthalene as a-ell as naphthoquinone is resolved. The benzene and remaining ether elute during the first 3 to 4 minutes n-ith some tailing
Methyl benzoate elutes in about 4 t o 5 minutes and is superimposed on the tail of the previous peak. The methyl benzoate peak is isolated for measurement by extending the tail of the peak on which it is superimposed so that the two portions of the tail, which are separated by the peak, are joined in a smooth curve. The resulting enclosed peak area is considered due t o methyl benzoate. The phthalic anhydride elutes in 10 to 11 minutes. Maleic acid probably reacts with diazomethane to form pyrazolines which do not interfere with the methyl benzoate. The phthalic acid esters eIute in about 16 to 18 minutes. Table I is a summary of the data obtained on the standards. Listed in Tables I1 and I11 are representative analytical results on phthalic anhydride from several sources. The areas under the methyl benzoate peaks were measured by a planimeter as indicated in Figure 1. Instrumental variations were compensated for by running frequent benzoic acid standards. ACKNOWLEDGMENT
The authors appreciate the technical assistance of R. B. Jones and Ihor Dmyterko of the Witco Research and Service Laboratories. LITERATURE CITED
(1) Bartsch, R. C., Miller, F. D., Trent, F. M., A N A L . CHEM. 32, 1101 (1960). (2) De Boer, Th. J., Backer, H. J., Rec. Trav. Chim. 73, 229 (1954). (3) Gilman, H., Kirby, J. E., J . Am. Chem. SOC.54, 345 (1932). (4) Murnieks, R., Gonter, C. E., ANAL.
CHEM. 34, 197 (1962).
Chemistry Department Loyola University Chicago, Ill.
CARLE. MOORE
SIEGFRIED MEINSTEIN Research and Service Laboratories Witco Chemical Co., Inc. 6200 West 51st St. Chicago 38, Ill.
