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A1 and/or HKO3 would necessarily stabilize the Mo absorption in systems where Ptln, Fe, and possibly other ions were varying in concentration. Effect of NH4+Addition. The significant feature of the above experiments was t h a t in no case was an absorption reading obtained equal t o that given by the same molybdenum concentration in an aqueous ammonium molybdate solution. Further experiments, in which the object was to achieve maximum Mo absorption from a given sample solution, showed that if the solution was just neutralized with ammonia the full Rlo absorption was obtained; the same result could be achieved more conveniently, and with no risk of hydroxide precipitation, by the addition of ammonium chloride. The experimental results given in Table I11 show that the addition of ,hH4Cl effectively destroys the interference by Mn and Fe on Mo absorption
in solutions analogous to those produced by dissolution of a ferrous alloy sample. The addition is less effective in the presence of added A1 and NO,-, and it is also apparent that the latter combination does not overcome interference by Mn and Fe in these conditions. Method of Additions. Analytical results obtained by the method of additions were often seriously in error, despite the fact that a straight line calibration graph was obtained. The errors may arise from the change in ratio of Mo:Mn, Mo:Fe, and Mo :acid as successive Mo additions are made. The effect of the NH4Claddition is to produce a shift in the calibration line to give a different, usually higher, concentration intercept. Results illustrating this are presented in Table IV. Analysis of Standard Alloys. The results of trial analyses on a number of British Chemical Standard alloys (Bureau of Analysed Samples, Ltd.,
Middlesborough, England) are presented in Table V. The proposed method has proved to be simple and accurate, with a precision of the order 3 to 5%. The method of additions (Procedure B) appears slightly more accurate in these tests, while direct comparison with prepared standards (Procedure A) is slightly more rapid, There is no clear advantage in either technique and both appear satisfactory for routine metals analysis. LITERATURE CITED
(1) David, D. J., Analyst 86, 730 (1961). (2) David, D. J., Nature 187, 1109 (1960). (3) Firman, R. J., Spectrochim. Acta 21, 341 (1965).
R.A. MOSTYN A. F. CUNNINGHAM Chemical Inspectorate Royal Arsenal London, S.E. 18 England
Determination of Extractables Content of Nylon 6 by Differential Refractometry and Determination of Caprolactam Monomer Content in Nylon 6 Extractables by Infrared Spectrophotometry SIR: Methods previously reviewed in the literature for the estimation of cyclic oligomers or caprolactam or both in nylon 6 extractables have generally been based on the weight differences and often involve the application of special techniques. Anton ( 1 ) evaporated the aqueous extract of nylon 6, dissolved the residue in tetrafluoropropanol, and determined the oligomers content in the residue by measuring the absorbance of the N H deformation band a t 1550 cm.-' using the base line method. Our results, however, show that caprolactam interferes with this determination. Caprolactam can be partly separated from oligomers by carbon tetrachloride extraction and determined by the sharp absorbance band it exhibits in solution a t 3430 cm.-' The total caprolactam present in the extractables solution may then be calculated from the caprolactam CClJH20 distribution ratio. Oligomers content is found from the total nylon 6 aqueous extractables by difference. By differential refractometry the total aqueous extractables are determined to a precision of =!=lo%, and, by infrared spectrophotometry, the caprolactam content itself is determined to a precision of *2%. EXPERIMENTAL
Reagents and Instruments. The cyclic oligomers standard was obtained by concentrating the aqueous extract of nylon 6 to dryness, washing the residue several times with distilled water, and drying the residue a t 60"
to 70' C. under vacuum for 4 hours. The caprolactam standard, supplied in crystalline form by National Aniline Division, Allied Chemical Corp., contained less than 75 p.p.m. water. Carbon tetrachloride, supplied as Baker and Adamson reagent grade by General Chemical Division, Allied Chemical Corp., was used without further treatment. Tetrafluoropropanol, obtained from Organic Chemicals Department, E. I. Du Pont de Nemours and Go., was redistilled (b.p. 109' C.), and dried over molecular sieve to 500 p.p.m. water. Differential refractive indices were measured using a Waters Associates Model 1000 digital differential refractometer equipped with an external highsensitivity null meter and a sealed reference liquid cell of distilled water. Measurements were made a t 38" C., which is the temperature of the instrument's optical system. Infrared spectra were recorded using either a PerkinElmer Model 221 double-beam infrared spectrophotometer with sodium chloride optics, or a Beckman Model IR 8 double-beam grating infrared spectrophotometer. Differential spectra were recorded using matched 5 c m . solution cells with near infrared silica windows, and quantitative measurements were made a t the following instrument settings: Perkin-Elmer Model 221 : attenuator speed, 1100; slit program, 927; gain, 3; speed, 32 (60 cm.-l/ minute) ; suppression, 4; scale, 1 X ; and source, 0.36 amps; Beckman Model IR 8: gain, 4.5; and speed, slow (136 cm.-l/minute). Procedure. A calibration curve was made for the differential refractometer by using a series of solutions
ranging from 0.025 to 1.00 gram of dry caprolactam per 250 ml. of aqueous solution (equivalent to 0.5 to 20% extractables of a 5-gram sample), Five grams of polymer or finish-free yarn of unknown extractables content were extracted with distilled water in a Soxhlet extractor for 5 hours. The solution was filtered and diluted to volume (250 ml.). The differential refractive index of the solution was measured, and the extractables content calculated from the calibration curve. Normally, a 100-ml. aliquot of the aqueous extract solution remaining was concentrated without boiling to 15 to 20 ml. and diluted upon cooling to 25 ml. For samples of extractables content of 4 to 20%, however, a 25-ml. aliquot of the extract solution was used without concentration. This was placed in a 125-ml. jacketed separatory funnel maintained a t 30.0 i 0.1' C., and 7.0 f 0.1 grams of potassium chloride (for "salting out") were added. After 75 ml. of carbon tetrachloride were pipetted into the funnel, the mixture was shaken thoroughly and allowed to separate. The carbon tetrachloride layer was withdrawn and diluted to 100 ml. A calibration curve was made by using a series of solutions ranging from 0.2 to 10 mg. of caprolactam per 100 ml. of carbon tetrachloride solution. Each solution was filtered through glass wool into the solution cell, and the spectrum from 3800 to 3200 cm.-* was recorded using the solvent as reference. Using a base line drawn between 3500 and 3400 cm.-l, the absorbance a t 3430 cm.-1 was measured. Solutions of unknown concentrations were analyzed in the same way. The caprolactam content VOL. 38, NO. 1, JANUARY 1966
123
in the original polymer sample was calculated from the equation
+
250C (1 4 M = lO0OWV where 250 =
=
caprolactam content (%), Of
(ml'), = capro1actam 'Oncentration (mg./100 ml. of CCL), d = caprolactam H20/CC14distribution ratio, w = weight of original sample (grams), and V = volume of aliquot (ml.).
Table 1.
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Solution No.@ 1 2 3 4 5 6
RESULTS AND DISCUSSION
Caprolactam monomer and oligomers can be removed from nylon 6 polymer and yarn by either water boiling under reflux or by a Soxhlet extraction in which condensed water is recycled over the sample. Equal quantities of caprolactam and oligomers were extracted from polymer using either a %hour reflux extraction or a 5-hour Soxhlet extraction. Equal quantities of caprolactam were extracted using either a 5-
Extraction of Aqueous Solution by Carbon Tetrachloride
ml. H20
Caprolactam in CC4, mg./100 ml. (by IR)
Caprolactam removed,
%
Caprolactam left in HzOt mg./25 ml.
Distribution ratioc
25.0 50.0 100.0 25.0 50.0 100.0
4.7 9.4 18.3 4.4 8.6 15.5
18.8 18.8 18.3 17.6 17.2 15.5
20.3 40.6 81.7 20.6 41.4 84.5
4.3 4.3 4.5 4.7 4.8 5.5
Caprolactam, mg./25
a Solutions 1, 2, 3 each extracted.with four 20-ml. portions of CC4; solutions 4, 5, 6 each extracted with one 75-ml. portion of CCla. b By difference. c Distribution ratio = (mg. caprolactam left in HzO)/(mg. caprolactam in CC14).
Table II. Absorbance of Standard Oligomer Solutions in 2,2',3,3'-Tetrafluoropropanol- 1
CaproOligomer lactam concn., concn., mg./ral. mg./ml. 0.00
5.CO 2.50 2.50
0.00 0.78 1 2,s 2.50 0.00 0.75 1.25 2.50
2 50
2.50 1.25 1.25 1.25 1.25 a
u
=
Beer's law.
Abs. a t 1550 cm. -1
ua ( X 108) ( X lo4)
415 210 185 165 160 100 85 65 65
166 168 148 132 128 160 136 104 88
hour or a 24-hour Soxhlet extraction however, the amount of oligomera extracted in the latter case was greater. Hydrolytic degradation and an equilibrium partition of caprolactam between solution and polymer are both possible in the reflux extraction. Therefore, we chose the 5-hour Soxhlet extraction as a suitable compromise for routine purposes. Schenker, Casto, and Mullen (9) showed that the refractive index of aqueous solutions of caprolactam varied linearly with concentration. The dif-
absorptivity, calculated from
80
-
o 60
-
Table 111. Absorbance of Standard Solutions of Caprolactam in Carbon Tetrachloride
Concn., Oligmg./ omers 100 ml. present Yes 2.0 Yes 2.0 No 3.0 Yes 5.0 5 Yen - __ - . 06.0 6.0 7.5 7.5 9.0 9.0 10.0 10.0 10.0
124
No
Yes NO No NO
Yes Yes No
Yes
Abs. a t 3430 cm.-1
7:
8;O
,
E
u 0 Y DZ
a
( X 103)
( x lo4)
95 94 133 237 218 272 265 346 348 402 398 440 45 1 450
95.0 94.0 88.7 94.8 87.2 90.7 88.3 92.3 92.8 89.3 88.4 88.0 90.2 90.0
ANALYTICAL CHEMISTRY
WAVELENGTH [MICRONS) 6;s 7;O
h06;
u r
E z
z
s + 40
-50 mg OLIGOMERS
:: 2c'
17b0
---- 50 mg. OLIGOMERS AND5 mg.MON.
1600 10:0 1400 FREQUENCY (CM.1)
A 0
'
Figure 1. Spectrum of caprolactam and oligomers in 2,2',3,3'-tetraf)uoropropanol-1
ferential refractive indices of aqueous solutions of caprolactam and of oligomers were directly proportional to solute concentrations. Because most routine samples have extractables of about 2%, the error due to calibrating with caprolactam solutions would only be 4% relative in this range. Water can remove materials other than caprolactam and oligomers-e.g., aminocaproic acid-from nylon 6. However, the refractive index increment of aminocaproic acid in wateF is similar to that of oligomers, so no great error is entailed here. In routine use the differential refractometer readings are reproducible to i1%, and overall results to f10%. This inchdes the error due to the calibration with caprolactam when oligomers are present in the extractions, After the determination of extractables, a n aliquot of the aqueous solution is concentrated and extracted with carbon tetrachloride so that the monomer content may be estimated by infrared spectrophotometry. To provide a sufficient monomer sample (1 to 10 mg./ 100 ml. of cC14 solution) for the infrared measurement, we found that a 150-ml. aliquot for samples of less than 1% total extractables and a 100-ml. aliquot for samples of 1 to 4% total extractables must be concentrated to 25 ml. before extraction with carbon tetrachloride. For samples having extractables of 4 to 20%, a 25-ml. aliquot can be used without concentration. Caprolactam is more soluble in water than in carbon tetrachloride, but this solubility can be decreased by adding 7.0 f 0.1 grams of potassium chloride to the 25-ml. aliquot or concentrate. Oligomers precipitate with this addition, but this precipitate does not hinder or affect the technique. Extraction with four 20-ml. portions of carbon tetrachloride remove 18.6% of the caprolactam from the 25-ml. aqueous solution at 30" C., and extraction with one 75-ml. portion removes 16.9%. The latter technique was preferred as it requires less analytical time and reduces errors. The results are given in Table I. Results obtained a t ambient temperature and at 25" C. were not reproducible, and experiments done at 35" C. were unsatisfactory because of the increased water content in the carbon tetrachloride solution. Results in our laboratory show that if caprolactam is present in the residue which is dissolved, according to Anton ( I ) , in 2,2', 3,3'-tetrafluoropropanol-1, large apparent fluctuations can occur in the measurement of the absorbance peak at 1550 cm.-l (Figure l), and hence in the calculation of absorptivity. This scatter is shown in Table 11. However, caprolactam, in a dilute carbon tetrachloride solution, exhibits a strong, sharp absorption band at 3430 cm.-1 caused by a free NH stretching vibration (Figure 2). This portion of
,
2;
WAVELENGTH [MICRONS) 2; 2;
3; ..
3+
Table IV.
Infrared Determinationof Caprolactam Content of Known CaprolactamOligomer Mixtures
Abs. at 3430 cm.-l
a
0.252 0.390 0.211 0.329 0.176 0.095 0.030" 0.276 0.449 0.414 Value too low to
Caprolactam concn., mg./100 ml. CClr Found Calcd.
Caprolactam in mixture, yo Found Calcd.
Rel. error, yo
5.57 8.62 4.66 7.27 3.82 2.10
5.62 8.48 4.73 7.33 3.77 2.00
57.4 77.2 61.7 63.8 35.5 70.0
57.9 76.0 62.6 64.4 35.0 66.7
-0.9 +1.6 -1.4 -0.9 +1.4 $4.9
6.10 9.92 9.14
6.00 10.00 9.00
67.2 85.2 71.4
66.1 85.8 70.3
+i:s
...
...
...
...
-0.7 +1.6
be determined accurately.
FREQUENCY (CM.1)
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Figure 2. Spectrum of caprolactam in carbon tetrachloride
the spectrum is ideally suited for quantitative measurements because carbon tetrachloride is transparent in this region up to a path length of 100 mm. When such a path length is used, low concentrations can be employed so that no deviation from Beer's law would be expected (Table 111). Further, no interference will occur from oligomers (which are completely insoluble in carbon tetrachloride) , nor from any other compounds with the exception of large amounts of polyglycol material such as are used in finish oils. However, these are normally removed before water extraction. The OH band of alcohols or glycols present in the
sample does not interfere with the determination. Based on analysis of known caprolactam-oligomer mixtures (Table IV) the precision of the determination of caprolactam content in the residue is & 1.8% relative to total extractables content. One consideration in this investigation was whether carbon tetrachloride could be used to extract only caprolactam from polymer (oligomers being insoluble in carbon tetrachloride). This solution could then be scanned in the infrared without further treatment. However, using 5-hour Soxhlet extractions, carbon tetrachloride did not remove any caprolactam from a control polymer, and removed only about half of that present in polymers of high monomer content-e.g., 8.2% by HzO, 4.7% by CCl,.
ACKNOWLEDGMENT
We gratefully acknowledge the assistance of the personnel of the Fibers Division Technical Department who contributed to the success of this work. LITERATURE CITED
(1) Anton, A., J. A p p l . Poly. Sei. 7, 1629 (1963). (2) Schenker, H. H., Casto, C. C., Mullen, P. W., ANAL.CHEM.29, 825 (1957).
GEORQE C. ONGEMACH VICTORA. DORMAN-SMITH WERNER E. BEIER~ Technical Center Fibers Division Allied Chemical Corp. Hopewell, Va. SIXTEENTHPittsburgh Conference of Analytical Chemistry and Ap lied Spectroscopy, Pittsburgh, Pa., d r c h 1965.
Present address, Fiber Industries, Shelby, N. C.
Refractometric Estimation of Total Fat in Chocolate-Type Products SIR: The estimation of fat in chocolate by a refractometric technique is not novel. The procedures developed largely by Leithe (6) and coworkers have been modified by Kleinert (3, 4) who reviewed the available methods for fat estimation in cocoa products. Reasons for discrepancies between values obtained refractometrically and gravimetrically by official methods have been discussed by Scharrer and Lame1 (Y), who assert that false refractometric values are usually obtained through the use of additives in sample preparation. The difference between these refractometric procedures lies in the solvent used and the methods of interpretation of measurements. The main requirements of a suitable solvent are good solvent action, low volatility, refractive index as different as possible from that of the fat being extracted, and low toxicity. From the
viewpoint of solvent action and refractive index, monochloro- and monobromonaphthalenes have found favor. However, from the viewpoint of low toxicity and suitable solvent property, tricresylphosphate, dibutyl phthalate, and diethylphthalate are commendable and have been employed by Stanley (8) for chocolate analysis. Interpretation of refractometric measurements has required either the empirical calibration for a suitable graph or table of per cent fat in the product us. refractive index of the extract from a solvent-sample mixture or insertion of refractometric measurements in a theoretically derived equation (3). Slight variations of this equation have been adopted by different workers, but the essential requirement, that in a binary solution the volumes employed should be additive, still holds. Experimental verification of
such equations has been obtained by their proponents. The application of refractometric measurements to an empirically prepared table necessitates constancy of refractive index in the fats examined, while the insertion of data in theoretically derived equations necessitates the determination of both refractive index and density of the fat or fat mixtures concerned. The presumption, that refractive indices and densities of fats of different species or from different sources are sufficiently uniform to accept as constant for the purpose of refractometric work, is invalid. This view is supported by the published variations in refractive index obtained on oils and fats likely to be encountered in chocolatetype products. The values in Table I are quoted ( I , 2) for illustration. A refractometric method obviating the requirements of prior knowledge of VOL. 38, NO. 1, JANUARY 1966
125
