Quantitative Infrared Analysis Hans Veening Bucknell University Lewisburg, Pennsylvania
of Xylene Mixtures Internal standard method
T h e use of quantitative infrared analysis in undergraduat,e chemistry courses has been limited because of the difficulties often encountered with this technique. The advantages and limitations inherent in infrared work were cited recently in a paper by Gastambide, Blanc, and Allamagny ( I ) , which describes a quantitative study of the kinetics of a second order reaction. Undergraduate experiments dealing with quantitative infrared analysis have been described in several textbooks on instrumental analysis (2-5). I n general, these experiments involve spectrophotometric determinations of hydrocarbons in mixtures, and direct applications of such principles as the base-line technique and Beer's Law. In these experiments, fixed thickness cells are used, and standard calibration curves are prepared; however, none of these analyses makes use of the instructive internal standard method, which can easily be incorporated into such work. The infrared analysis of a four component mixture of the xylene isomers and ethylbenzene has been described by Bauman (6) as an example of an ideal mixture which can be treated using the matrix technique. This method requires the measurement of the absorptivities of each con~ponentat four wavelengths and the subsequent formulation of four linear equations which are solved by matrix methods. Although the matrix technique is murh more complicated, it is more reliable than the internal standard technique when there is some overlap of the con~ponent'sabsorption bands. The internal standard method, however, is simpler since it does not depend on absorptivity values, nor does it require the solution of linear equations with several unknowns. I t is capable of giving satisfactory results if the interference of absorption bands is not serious. The characteristics of a satisfactory internal standard for infrared work have been cited (7). This paper describes a student experiment which has been a part of the analytical laboratory work at Bucknell for several years. Meta- and para-xylene are quantitatively determined on a Perkin-Elmer Model 137 Infracord spectrophotometer using ortho-xylene as an internal standard and cyclohexane as the solvent. A11 the necessary data can easily be obtained in one afternoon, and t,he only equipment needed besides the spectrophotonlet,er are two sodium chloride plates and plain white not,ebook paper. It is convenient for students to work in pairs. Each student constructs a calibration curve for one of the two components in the mixture, and ear11 one analyzes his own unknown. The Experimenf
The infrared spectra between 2.5 and l5p of the fol-
lowing are recorded: (1) Nujol, (2) Nujol plus cyclohexane, (3) Nujol plus ortho-xylene, (4) Nujol plus meta-xylene, and (5) Nujol plus para-xylene. These spectra are examined and analytically useful bands for quantitative measurements are selected. These bands are all located between 12 and 1 5 ~(para-, 1 2 . 6 ~ ; meta-, 1 3 . 0 ~or 1 4 . 5 ~ ; ortho-, 1 3 . 5 ~ ) . The solvent cyclohexane does not absorb significantly in this region, nor does Nujol. A set of standard solutions in cyclohexane is then prepared by each student. Each of these solutions contains 20.0% by volume of ortho-xylene, the internal standard, and amounts of the desired constituent varying from 5.0 to 35.0%. Samples for obtaining spectra are then prepared by mixing thoroughly five drops of the solution with five drops of Nujol, and placing a small amount of this mixture between sodium chloride plates. Nujol serves as a satisfactory suspension medium and prevents evaporation of the volatile liquids while the spectrum is being recorded. A spectrum is obtained for each solution between 12 and 15p and is recorded on plain white 8'/2 X 11-in. notebook paper. The paper must he carefully lined up flush with the top edge of the drum. Also, the 0% transmittance line must he marked accurately on each sheet, since subsequent calculations of absorbance depend on its position.
I Wavelength Figure 1.
Illustrotion of the bose-line method.
The absorbance for each isomer in each mixture is then calculated from the data as described by Ewing (4) or as follows (Fig. 1) : A baseline (B) is drawn tangent to both shoulders of the band. A perpendicular Volume 43, Number 6, June 1966
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(P) is drawn through the minimum of the curve. The distances (D) and (R) are measured, and the percent transmittance 100D/R and absorbance are calculated. The ratios of absorbances (desired constituent to internal standard) are calculated for each sample, and these values are plotted against the concentration of the desired constituent. Each student is given an unknown solution containing varying percentages of meta- and para-xylene as well as 20.0% ortho-xylene. Triplicate determinations are run on each unknown, and the absorbances are obtained in the same fashion. The calibration curves are then used to calculate the percentage of each of the xylenes in the unknown.
that the student, while learning quantitative infrared spectrophotometry, also employs such useful analytical methods as the internal standard and base-line techniques. Any instrument which covers the 12-15@ region can he used for this experiment. For example, the experiment has been successfully carried out by students on an instrument with potassium bromide optics, which also covers this range. This experiment elininates, too, the need for the use of bed thickness sodium chloride cells which are considerably more expensive. Acknowledgment
The author wishes to thank Dr. B. R. Willeford, Jr. for valuable suggestions and criticisms. Literature Cited GASTAMBIDE, B., BLANC,J., AND ~ L A M A D N Y.,Y ,J. CEEM. Eouc., 41,613 (1964). REILLEY,C. N., AND SI-R, D. T., "Experiments for Instrumental Methods," McGraw-Hill Book Company, Inc., New York, 1961, p. 191. MELOAN, C. E., AND KISER,R. W., "Problems and Experiments in Instrumental Analysis," Charles E . MerriU Books, Inc., Columbus, Ohio, 1963, p. 30. EWING,G. W., "Instrumental Methods of Chemical Andysis," McGraw-Hill Book Company, Inc., New York,
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10 20 30 Volume peroent met&wlene
40
Figure 2. Calibration plot for meto-xylene. Wavelengthr: ortho.xylene= 13.5 p; meto.rylene= 13.011. (Data of Linda Mdntyre, Buckneil.)
A typical calibration curve constructed by a student is shown in Figure 2. It can he seen that the points are essentially linear and that the line passes through the origin. This indicates that the background absorbance has been eliminated. Results of the analysis of mixtures by several students are given in the accompanying table. These show that a relative accuracy of +3y0 is attainable by this procedure. Analysis of Unknown Mixtures for m- and pXylene -Percent Given
m-xylenFound
-Percent Given
p-xylen~ Found
30.0 22.0 20.0 18.0 16.0 12.0 10.0 fin
I n preparing the unknowns, the concentration of ortho-xylene is held constant at 20.095, while the volume percentages of meta- and paraxylene can be varied between 5.0 and 35.0%. It is, of course, also possible to make up the unknowns without the internal standard, so that the students must add the orthoxylene themselves using standard volumetric techniques. The important advantages of this experiment are 320
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Journal o f Chemical Edumfion
1960., 0 . 409. (5) STROBEL, H. A., "Chemical Instrumentation," AddisonWesley Publishing Company, Inc., Reading, Mass., 1960, p. 614. (6) BAWMAN, R. P., "Absorption Spectroscopy," John Wiley & Sons, Inc., New York, 1962, pp. 403-417.
(7) WIBERLEY, S. E., SPRAGUE, J. W., Anal. Chem., 29,210 (1957).
AND
CAMPBELL, J. E.,
