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Direct Determination of Residual Caprolactam in Nylon 6 by Gas Chromatography Francesco Zilio-Grandi, Giovanni M. Sassu, and Paolo Callegaro Laboratorio Chimico di Ricerca Applicata, Stabilimento Petrolchimico Montecatini-Edison, Porto Marghera, Venice, Italy THEDETERMINATION of the content of water extractables, such as caprolactam and oligomers, in polycaprolactam (Nylon 6), is very important; in fact these low molecular weight products have a plasticizing effect, which causes a lowering of the viscosity of the melted polymer and a change of the mechanical properties of product. In addition, their determination is required when the polymer is used for the production of articles employed in the medical and foodstuffs fields. Because of the importance of this determination, the number of analytical methods described in the literature is considerable: a representative selection and critical survey was published by Reinisch et al. ( I ) . From this review it appears that since 1953, gravimetric, refractometric, and later spectrometric techniques were used. These methods always require an extraction and often special procedures. The time needed for each determination is too long (30-36 hr) and, consequently, these methods are unsuitable for both control analyses and polymerization kinetics studies. Among these methods the gravimetric method of Wiloth ( 2 ) is considered the most simple to carry out and the most reliable. Ongemach and Moody (3) were the first t o publish a method which describes the application of gas chromatography to the determination of caprolactam content in aqueous extractions of Nylon 6. This method represents remarkable improvements: much fewer analytical steps and adoption of a n internal standard t o improve the rapidity and precision of the determination. The time required for one determination is reduced t o about 5 hours, which practically corresponds to the extraction time. The gas chromatographic analysis requires not more than 15-20 min. In this study, a gas chromatographic method for the direct determination of caprolactam in the polymer is developed without the preceding extraction which, because of its simplicity, rapidity (ca. 2.5 hr) and precision, is particularly suitable for routine control of the polymer and also for polymerization kinetics studies. This method is based on the dissolution of the polymer sample in 85 formic acid with quinoline added as a n internal standard and followed by direct injection of the obtained solution into the gas chromatograph. EXPERIMENTAL
Instrumentation and Operating Parameters. The instrumentation and the operating parameters are reported in Table I. A 1-pl sample is injected with a 10-pl Hamilton syringe. Quinoline (internal standard) has a retention time (1) G. Reinisch, K. Dietrich, and H. Bara, Faserforsch. u. Textiitechz., 1967,588. (2) F. Wiloth, Mukromoi. Chemie, 15, 106 (1955). (3) G. C. Ongemach and A. C. Moody, ANAL. CHEM.,39, 1005
(1967).
u
r
d
I a
c 0
5 min
Figure 1. Gas chromatogram of two polymers samples containing different caprolactam contents A . 8.51%; B. 0.40%
of 2.3 min and caprolactam, of 4.4 min. Figure 1 shows gas chromatograms obtained from two polymers containing different quantities of caprolactam. Procedure. The concentrations of residual caprolactam content in Nylon 6 is in the range O.l-lO%. A series of calibrated solutions of caprolactam in 85% formic acid in the
Table I. Instrumentation and Operating Parameters Gas chromatograph C. Erba Fractovap mod. GI with flame ionization detector Column 10% (w/w) Carbowax 20 M on 150-175 p Chromosorb W A.W. DMCS, 0.8 m X 4.0 mm i.d. stainless-steel tube Column temperature 200 "C Detector temperature 250 "C Injection port temperature 250 "C Carrier gas flow rate 24 ml/min of nitrogen at 0.5 Kg/cm2 and 200 "C Hydrogen flow rate 20 ml/min at 0.6 Kg/cm2 and 200 "C Air flow rate 330 ml/min at 1.5 Kg/cm2and 200 "C Recorder Leeds & Northrup Speedomax W, 2.5 mV f.s., 1 sec f.s., chart speed 12.7 mm/min
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Table 11. Precision and Accuracy Lactam in formic acid Found, w/v‘ Calcd., % w/v 0.0030 0,0028 0.0047 0.0048 0.0100 0.0104 0.0472 0.0475 0.0920 0.0908 0.1921 0.1906 Average a Average of triplicate analyses.
Standard deviation X lo-‘ 1 .oo
1.73 1.oo 1.22 25.50 29.02 3t9.91
Relative standard deviation, 3.37 20.83 9.61 2.10 1.10 0.52 f6.28
z
Mean error -0.0002 +0.0001 $0.0004 -0.0003 -0.0012 -0.0015 f0.0006
Relative error, Z -6.7 +2.1 +4.0 -0.6 -1.3
-0.8 f2.6
Table 111. Comparison of Caprolactam Determination in a Polymer Sample According to Wiloth’s, Ongemach’s, and Direct Methods Wiloth method Ongemach GC method Direct GC method Found, Deviation Found, Deviation Found, Deviation 8.38 -0.02 8.28 -0.14 8.50 +0.03 8.47 +0.07 8.27 -0.15 8.47 fO.OO 8.38 -0.02 8.48 +O. 06 8.54 +0.07 8.48 $0.08 8.58 $0.16 8.40 -0.07 8.28 -0.12 8.48 +O .06 8.45 -0.02 Average 8.40 8.42 8.47 Standard deviation f0.081 f0.137 f0.059 Rel. stand. deviation 3~0.96% 3t1.62z f O .69 36 hrs 5.5 hrs 2.5 hrs Analysis time, ca.
z
range of 0.002-0,2% (w/w) have been prepared and each analyzed three times : they correspond, respectively, to concentrations of caprolactam in polymer ranging from 0.1 to 10% (w/w); 0.4 (v/v) quinoline had been added as an internal standard to the formic acid. The peak areas were determined by multiplying the peak height times the width a t half-height, with the proper correction made for the sloping base line. A straight line intersecting the ordinate axis a t the origin is obtained by plotting the ratio between the peak areas of caprolactam and quinoline cs. the concentration of caprolactam. The caprolactam content in unknown polymer samples is calculated by the equation:
z
RESULTS AND DISCUSSION
Efficient solvents of polycaprolactam at room temperature are very few. Concentrated inorganic acids being excluded for obvious reasons, only phenols (m-cresol, resorcinol, ochloro-phenol, etc.) and 85 % formic acid are suitable for the gas chromatographic analysis. The latter was preferred for its very low response in a flame ionization detector. The number of liquid phases remaining efficient for a sufficiently long time when used at very high temperature, is very limited. Several solid supports and liquid phases have been tested in order to obtain a good separation without tailing. Apolar liquid phases, like silicone gum rubber SE-30 and Apiezon L, supported at different concentrations from 1.5 to 20 on different supports (DMCS silanized or not Chromosorb W A.W., DMCS silanized Chromosorb G, carbon black) give insufficient separation for quinoline and caprolactam. In addition, the latter and formic acid show a strong tailing which causes a remarkable loss in sensitivity. The best results were achieved with Carbowax 20 M which accomplishes the best separation between formic acid, quinoline, and caprolactam, when supported on any of the above mentioned supports at concentrations of more than 5 Acid washed and silanized with dimethyldichlorosilane Chromosorb W was chosen as solid support. This column represents a good compromise between separation and analysis time. The caprolactam, when its concentration in polymer is less than 1 %, is measured on the tailing of formic acid, but its area is still in the linearity range. Oligomers d o not interfere with the determination, as they are not eluted at the conditions employed. The column begins t o lose its original efficiency after about 400 analyses, even with unaffected retention times of the components. In
z
where: Q = caprolactam content in polymer (% w/w); A , = peak area of caprolactam; Az = peak area of internal standard; P = weight (grams) of polymer sample, corrected for the moisture content; f = correction factor for the peak area of caprolactam; c = internal standard concentration in formic acid (0.438% w/v); and u = volume (10 ml) of acid used in dissolving the polymer. The product fcv is a calibration constant of the method, because the quantity of formic acid used and the quinoline concentration are always the same. If the concentration of caprolactam in polymer is varied, it is not necessary to change the quinoline content in formic acid because of the linear caprolactam response within the entire range considered. In practice, after determining the moisture content, the exact weight (about 0.2 gram) is determined. The sample is then dissolved in formic acid (to which quinoline has been added) in a 10-ml calibrated flask. The polymer is completely dissolved at room temperature in about two hours without stirring (about three hours in the case of high molecular weight polymers). 1848
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fact, the theoretical plates number for the two components is decreased from about 700 to about 350, and because the peak area of quinoline is practically unaffected, the peak area of caprolactam continuously decreases. Consequently, an average difference of 5.5 between the actual and the experimental concentrations is found, which is constant in the entire concentration range. No unexpected peak or anomalous baseline disturbance is observed; and so decomposition phenomena may be excluded which means that caprolactam begins to be partially lost in the column. The mechanism by which this loss occurs is not clear; it may be generally assumed that one of the causes of the irreversible absorption of polar substances, like caprolactam, is hydrogen bonding (#). In addition, a possible interaction between liquid phase, solid support, and analyzed substances should be taken into account. For these reasons it may be similarly supposed that the progressive decrease of caprolactam response is due to a continuous absorption on the solid support, after the solid substrate has been saturated by formic acid. Consequently, after a certain time, calibration should be checked regularly at frequent intervals. The precision of the determination, expressed as average standard deviation, is = t O . O O l ; accuracy, expressed as average relative error, is 1 2 . 6 z ; this value, by excluding the first determination which regards the lowest concentrations, becomes + l .7 (see Table 11). The gravimetric method of Wiloth, the gas chromatographic
method applied to the water extractions, and the direct gas chromatographic method applied to the formic acid solutions of polymers have been compared. The caprolactam content according to the Wiloth method has been calculated as a difference between the water extractables and the oligomers content. The caprolactam content according to the Ongemach method has been determined on an aliquot of the abovementioned water extracts. Quinoline in aqueous-alcoholic solution was used as internal standard. Five aqueous extractions based on 10-gram polymer samples for the first two methods and five determinations on about 0.2-gram polymer solutions for the direct method were carried out. In Table 111, the results obtained from the same polymer sample containing 10% of water extractables are reported. By comparing the results of the three methods, it can be seen that the precision of the direct caprolactam determination is of the same order as that supplied by Wiloth and OngemachMoody methods. The direct one which requires less handling, reduces the possibility of casual errors. The analysis time for a single determination is reduced to about 2.5 hours, because only one weighing and one dissolution step are required. The sensitivity of the method allows the determination of monomer in polymer down to 0.1 The advantages of the present method, compared with the others described in the literature, are: simplicity, high-speed performance, and sensitivity, all of which make it particularly suitable for control analyses. The method can be also applied to polymerization kinetics studies.
(4) V. Kusy, ANAL.CHEM., 37,1748 (1965).
RECEIVED for review April 18, 1969. Accepted July 2, 1969.
z
z
z.
Determination of Pentaerythritol Tetanitrate and Other Nitric Acid Esters with p-Nitroaniline and Azulene V. Hankonyi and V. Karas-Gaiparec Institute of Chemistry and Biochemistry, Faculty of Medicine, Zagreb, Salata 3, YugosIavia
DURING the hydrolysis of nitric acid esters, certain quantities of nitrite ions along with other products are produced. This effect is used for the quantitative determination of esters. In earlier publications ( I , 2), we presented a micromethod, based o n the determination of nitrite ions produced by hydrolysis for the determination of some esters. However, this method is not suitable for the determination of pentaerythritol tetranitrate because of its poor solubility in water and alcohol. Existing methods (3) were tried and found to be unsatisfactory also. This paper presents an application of Garcia's method (4) to the determination of some nitric acid esters (erythritol tetranitrate, pentaerythritol tetranitrate, and glyceryl trinitrate) and particularly of pentaerythritol tetranitrate in pharmaceutical preparations. The principle is as follows: nitrite ions produced by hydrolitic decomposition of esters under the influence of a strong base react with p-nitroaniline to form a diazonium ion, which is then coupled with azulene. The (1) V. Hankonyi and V. Karas-Gasparec, Acta Pharm. Jugoslau,
17, 41 (1967). (2) V. Hankonyi and V. Karas-Gasparec, ibid., in press. (3) T. Bihan, Food and Drug Administration, Zagreb, Yugoslavia, personal communication, 1968. (4) E. F . Garcia, ANAL.CHEM., 39, 1605 (1967).
absorbance of the azo dye produced is measured at 515 nm. The method is rapid and sensitive and permits the determination of nitric acid esters in the concentration range of 5-50 pg in 10 ml of the reaction mixture, EXPERIMENTAL
Apparatus. Absorption intensity measurements were made with a Unicam SP 600 spectrophotometer using 1-cm cells. Reagents. The p-nitroaniline was of reagent grade and the azulene was obtained from Gerhardt Schmidt, Hamburg. The nitric acid esters were obtained from Pliva, Zagreb. The tablets analyzed contained pentaerythritol tetranitrate and had been obtained on the market. ~NITROANILINE. Dissolve 0.4 gram of p-nitroaniline in 100 ml of glacial acetic acid. AZULENE. Dissolve 0.08 gram of azulene in 100 ml of glacial acetic acid. This reagent lasts one week. PERCHLORIC ACID. 7.8 M . PENTAERYTHRITOL TETRANITRATE STOCKSOLUTION.Dissolve 0.04 gram in acetone to a volume of 100 ml. T o prepare the standard solution containing 40 pg of pentaerythritol tetranitrate per milliliter, dilute the stock solution with acetone. ERYTHRITOLTETRANITRATE STOCK SOLUTION.Dissolve 0.015 gram in distilled water at 90 "C and dilute to 50 ml.
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