Near-Infrared Analysis of Mixtures of Primary and Secondary Aromatic


Infrared Quantitative Analysis Data. CS-128. Analysis of Mixtures of N-Ethyl-m-toluidine and N-Ehtyl-N-(2,3-dihydroxypropyl)-m-toluidine. K B. Whetsel...
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Table I. Sulfur Compounds Identified in Agha Jari, Iran, Crude Oil

Compound 2-Propanethiol 2-Methyl-2-propanethiol 2-Thiabutane 3-Methyl-2-thiabutane 2-Butanethiol 3-T hiapentane 2-T hiapentane 2-Methyl-3-thiapentane

Boiling Pooint, C. 52.56 64 22 66.65 84.8 85.0 92.10 95.6 107.38

Tentatively Identified I -Propanethi01

2-Methyl-1-propanethiol 2,2-Dimethyl-l-propanethiol

67.8 88.5 103.7

(Table I) were thus definitely identified. The presence of three compounds identified by emergence time and peak enhancement could not be conclusively confirmed by mass spectrometry of trapped fractions because of insufficient sample. The identification of these compounds must remain tentatire until confirmatory proof of their presence is secured by independent means. Three other compounds, l-butanethiol (98.4’ C.), 2-methyl-2-butanethiol (99.2’ e.),and 3,3-dimethyl-2-thiabutane (99.0” C.),were sought but not found. The emergence time of each

of these three compounds coincides with a major component in the concentrate, and trace amounts mould be most difficult to separate and identify by gasliquid Chromatography. These compounds have been identified (9) in small or trace amounts in Wasson, Tex., crude oil and are believed to be present only in trace amounts, if a t all, in Agha Jari crude oil. CONCLUSIONS

The following eight sulfur compounds were positively identified in Agha Jari crude oil by means of gasliquid chromatography and supplemental mass-spectrometry analysis: 2propanethiol, 2-rnethyl-2-propanethiolJ 2-thiabutane, 2-butanethiol, 3-methyl2-thiabutaneJ 3-thiapentaneJ 2-thiapentane, and 2-methyl-3-thiapentane. In addition, 1-propanethiol, 2-methyl-lpropanethiol, and 2,2-dimethyl-l-propanethiol were tentatively identified in the same crude oil. ACKNOWLEDGMENTS

The authors wish to acknovledge the aid of N. G. Foster and Pearl Tribble in supplying the mass spectral analyses of pertinent fractions used in this investigation.

LITERATURE CITED

(1) Amberg, C. H., Can. J . Chem. 36,

590-2 (1958). (2) Coleman, H. J.. Adams, K. G., Eccleston, B. H., Hopkins, R. L., Mikkelsen, Louis, Rall, H. T., Richardson, Dorothy, Thompson, C. J., Smith, H. M., ASAL.CHEX 2 8 , 1380-4 (1956). (3) Desty, D. H., Harbourn, C. L. 8., Division of Analvtical and Petroleum Chemistry, Sympbsium on .Advances in Gas Chromatography, 132nd Meeting, ACS, Xew York, K. Y., September 1957. 14) Destv, D. H.. Khvman, B. H. F.. ASAL. HEM. 29’. 320-9 (1957). (5) Dimbat, hl., Porter,‘ P. E., Stross, F. H., Zbid., 28, 290-7 (1956). (6) Ryce, S. A., Bryce, W. A,, Ibid., 29, 925-8 (1957). ( 7 ) Sunner, S., Karrman, K. J., Sunden, V., Mzkrochim. Acta 1956, 114P51. (8) Thompson, C. J., Coleman, H. J., Llikkelsen, Louis, Yee, Don, Ward, C. C., Rall, H. T., ANAL. CHEY. 28, 1384-7 (1956). (9) Thompson, C. J., Coleman, H. J., Rall, H. T., Smith, H. M., Ibid., 27, 175-85 (1955). (10) Thompson, C. J., Coleman, H. J., Ward, C. C., Rall, H. T. Southwest Regional Meeting, ACS, Tulsa, Okla., December 1957. RECEIVEDfor review January 11, 1958. Accepted May 23, 1958. Division of Petroleum Chemistry, 133rd Meeting, ACS, San Francisco, Calif., April 1958. Investigation performed as part of American Petroleum Institute Research Project 48.4 on “Production, Isolation, and Purification of Sulfur Compounds and Measurement of Their Properties,” which the Bureau of Mines conducts a t Bartlesville, Okla., and Laramie, Kyo.

Near-Infrared Analysis of Mixtures of Primary and Secondary Aromatic Amines KERMIT WHETSEL, WILLIAM E. ROBERSON, and M A X W. KRELL Tennessee Eastman Co.,Division o f Easfman Kodak

b The analysis of mixtures of primary

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and secondary aromatic amines by near-infrared spectroscopy was investigated. By utilizing the N-H overtone and combination bands near 1.49 and 1.97 microns, respectively, mixtures of aniline and N-ethylaniline containing up to 99% of either constituent can be analyzed with standard A deviations no greater than & 1%. modification of the method permits the determination of aniline with a standover the 0 ard deviation of &O.l% to 10% concentration range. Aliphatic amines and tertiary aromatic amines do not interfere. Similar methods can be used to analyze a variety of other mixtures of primary and secondary aromatic amines.

D

past few years nearinfrared spectroscopy has been recognized as another valuable techCRING THE

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ANALYTICAL CHEMISTRY

Co.,Kingsport,

Tenn.

nique for the analysis of organic compounds. Methods which utilize the first overtone 0-H and K-H stretching bands near 1.4 and 1.5 microns, respectively, have been reported for the determination of water in hydrazines (S) and in nitric acid (9),the analysis of mixtures of A‘-alkyl and A‘-alkyl-K-hydroxyalkyl aromatic amines ( 7 ) ,and the determination of unacetylated hydroxyl groups in cellulose acetate ( 6 ) . Goddu has discussed the advantages of studying the fundamental 0-H stretching bands near 2.8 microns Ivith high-resolution quartz optics and lead sulfide detectors (9). He has also described the determination of terminal and cis unsaturation using overtone and combination bands near 1.6 and 2.2 microns ( I ) . The possible application of near-infrared data to the problems encountered in lipide chemistry has been discussed by Holman and Edmondson (4).

Primary aromatic amines are characterized by two intense absorption bands near 1975 and 1500 mp, Kaye (5) has assigned the band at 1975 mp to a combination of N-H stretching and bending modes and the one a t 1500 mp to the first overtone of the N-H stretching vibration. Secondary aromatic amines exhibit the first overtone band near 1500 mp but not the combination band. The present investigation vias concerned with the application of these absorption differences to the quantitative analysis of mixtures of primary and secondary aromatic amines. APPARATUS AND MATERIALS

Spectrophotometer. A Cary Model 14MS spectrophotometer equipped with a log absorbance slide-Tvire was used. (A per cent transmittance or a n absorbance slide-wire could have been used equally well except for the dif-

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1.2 Figure 1.

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Absorption spectra of aniline and N-ethylaniline

Carbon tetrachloride solutions, 10-cm. cells.

ferential work. Here, the 0.0 to 2.0 absorbance slide-wire normally used would lower the sensitivity slightly, but a 0.0 to 0.1, 0.1 to 0.2 slide-wire, which is now commercially available, would be entirely satisfactory.) The scan speed was 5 mp per second for the complete reference spectra and 2.5 mp per second for the quantitative measurements. Constant slit 1%-idths were maintained for the nondifferential work by adjusting the amplifier gain to give a nominal slit of 0.40 mm. a t 1800 m p with carbon tetrachloride in the reference cell. This setting resulted in nominal slits of 0.29 and 0.48 mm. a t 1493 and 1972 mp, respectively. The slit height control b-as kept in the “In” position. Cylindrical cells (10 cm.) with Corex ends were used. and the reference cell was filled with carbon tetrachloride except where otherwise noted. Any beam unbalance due to imperfect matching of the cells or of two optical paths were eliminated by the Helipot adjustments. The bottom curve in Figure 1 mas obtained with carbon tetrachloride in both cells. Carbon Tetrachloride. Reagent grade carbon tetrachloride was used without further purification. The solvent used in the reference cell was always taken from the same lot used to dissolve the samples. This precaution was necessary to avoid unbalances arising from slight variations in the chloroform content of different lots of solvent. Aniline and N-Ethylaniline. Commercial samples were distilled through a fractionating column and the middle cuts collected. The N-ethylaniline sample was essentially free of N , X diethylaniline according to its infrared spectrum. A commercial sample of N-ethylaniline was used without purification to prepare synthetic mixtures for the precision studies.

I.8 WAVE LENGTH, p 1.6

.

--- N-Ethylaniline.

- Aniline.

Bottom curve, carbon tetrachloride in both cells

PROCEDURES

DEVELOPMENT OF METHOD

Samples should be dissolved and cells emptied and cleaned under a hood to minimize exposure to carbon tetrachloride vapors. Care should also be taken to avoid excessive contact of the solvent with the skin. Keigh 0.500 gram of the sample into a 50-ml. beaker and dissolve in carbon tetrachloride. Transfer the solution quantitatively into a 50-ml. volumetric flask and dilute to the mark with solvent. Fill a 10-em. cell with the solution and measure the spectrum from 2100 to 1400 mp using a lO-cnl. cell filled with carbon tetrachloride for reference. Determine the net absorbance a t 1972 mp by subtracting the absorbance a t 1915 mp from the observed absorbance a t 1972 nip. Determine the net absorbance a t 1493 mp by subtracting the absorbance at 1575 mp from the observed absorbance a t 1493 mp. Substitute the net absorbances into the following equations t o determine the percentages of aniline and hiethylaniline.

Spectra. The spectra of aniline and 3-ethylaniline from 800 t o 2550 m p are shown in Figure 1. Aniline has a strong absorption band a t 1972 mp, whereas N-ethylaniline absorbs Iyeakly a t this wave length. Both amines have an absorption band a t 1493 mp, with aniline absorbing over twice as strongly as A’-ethylaniline. These differences are by far the largest observed, so the remainder of the work was concerned with a detailed study of the bands at 1493 and 1972 mp. Stability. The spectra of a 0.6% solution of aniline and of a 1.0% solution of X-ethylaniline were measured five times over a period of 50 hours to determine the stability of the solutions. Slthough the background of aniline increased considerably with time, the net absorbances were constant to within =+=0.5%of the mean. The net absorbances of .V-ethylaniline were also essentially constant over the 2-day period, with the background absorption considerably more stable than that of aniline. Net absorbances in these tests and throughout the remainder of the work were obtained by subtracting the absorbances a t 1575 and 1915 mp from the observed absorbances a t 1493 and 1972 mp, respectively. The usual base-line technique could have been used equally well a t 1493 mp. At 19’72 mp, however, it is difficult t o find a suitable background point on the long wave-length side of the band, and any chosen would result in a slightly negative net absorbance for pure .I--ethylaniline. The single-point background corrections work satisfactorily and are obtained more conveniently than the usual baseline corrections.

% Aniline = 60.2?IA1972- 3.iOA1493 % ’ iV-Ethylaniline = 207.08im

-

(1)

l?I5.3Aisn ( 2 )

Samples Containing Less Than 10% Aniline. Keigh 2.500 grams of the sample into a 50-ml. beaker and dissolve in carbon tetrachloride. Quantitatively transfer the solution into a 50-ml. volumetric flask and dilute to the mark with solvent. Measure the spectrum of the solution from 2100 to 1900 mp using 10-em. cells and pure solvent as reference. Subtract the absorbance a t 1915 mp from the absorbance a t 1972 mp to obtain the net absorance a t 1972 mp. Substitute the net absorbance into the following equation to determine the concentration of aniline in the sample.

yo aniline

=

12.5Alun - 2.06

(3)

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Figure 3. aniline

SLIT WIDTH, mm.

Figure 2. Effect of slit width on N-H bands of aniline and N-ethylaniline

0.5

Table 1. Beer's Law Data for Aniline and N-Ethylaniline in Carbon Tetrachloride"

0.202 0.012

0 400

Slit Width. Figure 2 s h o w the effect of slit width on the 1972-mp band of aniline and the 1493-mp bands of aniline and A'-ethylaniline. During this investigation slit widths of 0.29 mm. a t 1493 mp and 0.48 mm. at 1972 m p were maintained by means of the slit control adjustment. At this level of slit widths a change of as much as 30% in either direction would introduce less than 1% error. With wider slits, however, a comparable change would cause considerably greater error. Calibration. A series of solutions containing from 0 t o 1.0% aniline was prepared and the spectra were meas-

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0:soo 0.305 0,019 0.508 0.032 0.800 0.406 0.025 0.508 0.031 1,000 0.511 0.030 0.511 0.030 Av. 0.507 0.031 a All absorbances corrected by subtracting background absorbances at 1575 and 1915 mp from peak absorbances at 1493 and 1972 mp, respectively.

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Spectra of mixtures of N-ethylaniline and

570 Solutions in Carbon Tetrachloride, 10-Cm. Cells

Carbon tetrachloride solutions, 1 0-cm. cells A. 1493-mp band of N-ethylaniline, 2% solution 6. 1493-mp band of aniline, 1 % solution C. 1972-mp band of aniline, 1 % solution

Absorbances Measured Calcd. for Absorbances 1.0% Soln. Concn., 1493 1972 1493 1972 % m!J WJ m/J m/J Aniline 1.315 1.740 0.200 0.263 0.348 0.400 0.526 0.698 1.316 1.743 0.600 0.783 1.040 1.304 1.735 0.800 1.043 1.395 1.305 1.745 1.000 1.290 1.735 1.290 1.735 .4v. 1.306 1.739 N-E thy laniline 0.200 0.101 0.006 0.505 0.030

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ANALYTICAL CHEMISTRY

0 1.9

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Figure 4. Differential spectra of mixtures of N ethylaniline and aniline

570 Solutions in Carbon Tetrachloride, 10-Cm. Cells Curve Aniline, % Curve Aniline, 70 1 0.0 4 1.0 2 0.2 5 2.0 3 0.5 6 5.0 ured from 2200 to 1300 mp. The net absorbances a t 1493 and 1972 mp varied linearly with concentration, thus showing that Beer's law is followed over the concentration range of interest. Similar results were obtained with another series of solutions containing from 0 to 1.0% N-ethylaniline. These data are shown in Table I. Kine samples having a total amine concentration of 1.00% but with the

ratio of amines varying from pure aniline to pure AT-ethylanilinem-ere studied. Straight lines were obtained when the net absorbances were plotted against the composition of the mixtures. These results showed that there was no interaction between the two amines, so the absorbance values in Table I were used to derive Equations 1 and 2 for the analysis of mixtures. Precision. Two samples containing

approximately 25 and 70y0 aniline nere analyzed six times by three different operators over a 2-month period. The standard deviations fell between 1 0 . 2 7 and +0.60% (samples 1 and 2, Table II), The low total aniline and X-ethylaniline concentrations shon-n for samples 1 and 2 in Table I1 can be explained by the fact that the N ethylaniline sample used to prepare the mixtures was of commercial grade; it contained 8.1% N,N-diethylaniline according to an infrared determination. The interference of the tertiary amine is negligible a t both analytical wave lengths. Low Concentrations of Aniline. With samples containing less than 10% aniline, the primary amine can be determined with greater precision by using more concentrated solutions and measuring only the intensity of the 1972-mp band. Figure 3 shows the spectra of N-ethylaniline samples containing 0, 2, 5 , and 10% aniline, the total amine concentration being 5.00% in all cases. Ket absorbance a t 1972 mb varied linearly with the concentration of aniline according to Equation 3. I n order to test the precision of this type of determination, two samples of N-ethylaniline containing about 2 and 5% aniline n-ere analyzed for aniline six times over a 2-month period by three different operators. Standard deviations of =!=0.08and 10.10% were obtained (samples 3 and 4, Table 11). Differential Spectra. When a sample contains less than 5% aniline, care must be exercised in obtaining the absorbance a t 1972 mp, because the absorbance changes rapidly with the wave length. This difficulty can be eliminated by using differential spectra obtained with a 5% solution of pure Kethylaniline in the reference beam. Figure 4 shows the differential spectra obtained n ith samples containing from 0 to 5y0 aniline. As little as 0.1% aniline can be detected by this technique. Differential spectra are particularly useful with per cent transmittance or log absorbance recordings, because full advantage can then be taken of the expanded scales in the low absorbance range. The 0.0 to 0.1, 0.1 to 0.2 absorbance slide-wires which are now conimercially available should also be useful for differential measurements. Interferences. The interference of S,N-diethylaniline and of other tertiary amines is negligible a t both analytical wave lengths. Other primary aromatic amines interfere seriously a t both 1493 and 1972 mp, but other secondary aromatic amines interfere only a t the former wave length. Thus, aniline can be determined in a mixture of S-substituted anilines without difficulty. Primary and secondary aliphatic amines do not interfere appreciably with

Table 11.

Precision of Aniline and N-Ethylaniline Determinations Aniline, 7c

Sample

Av,

Range

N-Ethylaniline, Std. dev.

*%v.

Range

70

Std. dev. 0 28

S o . of Detns.a

24 0 27 i0 3 69 9-70 7 0 - - . 8-25 - -- 7 . 68.0 67.4-68.7 0.60 30.2 29.7-30.5 0.36 6 3 2.06 2.00-2.19 0.08 ... 6 ... ... 6 4 5.02 4.85-5.16 0.10 ... a Run by three operators over 2-month$eripd. , * Samples 1 and 2 contained 5.7 and 2.5 A),il -diethylaniline, respectively, by infrared analysis.

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~~

2b

. I .

the determination of aniline or Nethylaniline. The overtone and combination N-H absorptions of aliphatic amines are shiftedato about 1530 and 2000 mp, respectively. As a result, the absorption of these amines a t 1493 and 1972 mp is weak. When the general procedure is followed, the presence of 10% cyclohexylamine, 2-ethylhexylamine, di-2-ethylhexylamineJ or diethylamine in mixtures of aniline and N ethylaniline causes a maximum error of 0.5% in the aromatic amine determinations. The ratio of the two aromatic amine concentrations is not critical. With the modified procedure by which only aniline is determined, the maximum error caused by 10% of the aliphatic amines is 0.2%. Primary aliphatic amines react with carbon tetrachloride a t varying rates, but in the cases studied the reaction products did not interfere. DISCUSSION

-4 detailed study of the analysis of mtoluidine and N-ethyl-m-toluidine mixtures gave results entirely analogous to those reported for the aniline compounds. Mixtures of o-chloroaniline and AT-2-hydroxyethyl-o-chloroaniline and mixtures of aniline and N-2-hydroxyethylaniline have also been analyzed by methods essentially the same as those reported here. %%en the secondary amine contains a hydroxyl group in the alkyl chain, an independent determination of this component can be carried out by using the 0-H band near 1405 mp. A working curve should be prepared for the 0-H band. as hydrogen bonding causes serious deviation from Beer's law ( 7 ) . .4ny S,n'-dihydroxyalkyl derivative that is present will interfere nith the 0-H determination, so the extent of agreement between the secondary amine determinations using the N-H and the 0-H bands provides additional information concerning the concentration of tertiary amine. I n fact, satisfactory analyses of mixtures containing aniline, .Y-2-hj-droxyethylaniline, and S,.1'-dii2-hydroxyethyl)aniline have been made bv using the two K-H bands a t 1972 and 1493 mp and the 0-H band a t 1406 mp. Results obtained rrith more than 50

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primary and secondary aromatic amines indicate that the molar absorptivities of the K-H bands are independent of concentration over the 0 to 1% concentration range in carbon tetrachloride. In most cases this conclusion can probably be extended to cover concentrations up to 2y0, but a t higher concentrations the molar absorptivities definitely decrease with increasing concentration. Thus, one should not attempt to substitute 10% solutions and 1-cm. cells for the 1% solutions and 10-cm. cells recommended in this procedure. The positions and the molar absorptivities of the K-H bands of primary amines vary considerably from one compound to another (8). Similar, but less complete, data have been obtained for secondary amines. Thus, it is generally necessary to calibrate for the particular system being studied. However, the authors believe that many of the absorptivities required for calibration can be taken directly from the literature as more fundamental data on the near-infrared absorption of amines become available. This point is discussed in greater detail in the paper reporting position and intensity data for K-H bands of 40 primary aromatic amines (8). ACKNOWLEDGMENT

The authors wish to thank J . P. Reeves for purifying the samples used in this work. LITERATURE CITED

(1) Goddu, R. F., ANAL.CHEM.29, 1790 (1957). (2) Goddu, R. F., Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Pittsburgh, Pa., March 1958. (3) Gordes, H. F., Tait, C. W.,A s . 4 ~ . CHEM.29, 485 (1957). (4) Holman, R. T., Edmondson, P. R., Zbid., 28, 1533 (1956). (5) Kaye, W.>Spectrochim. Acta 6, 257 (1954). (6) Mitchell, J. A,, Bockman, C. D., Jr., Lee, A. V., ASAL. CHEM.29,499 (1957). ( i )Whetsel, K. B., Roberson, W. E., Krell, ?VI.W., Ibid., 29, 1006 (1957). (8) Ibid., 30, 1598 (1958). (9) White, L., Jr., Barrett, W.J., Ibid., 28, 1538 (1956). RECEIVED for review December 4, 1957. .Accepted April 7, 1958. VOL. 30,

NO. 10,

OCTOBER 1958

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