A Quantitative Infrared Spectroscopy Experiment Mark D. Krahling and Robert Eliason' Southwest State University, Marshall. MN 56258 Undergraduate courses in analytical chemistry seldom contain experiments that employ infrared spectroscopy as a tool for quantitative analysis. This is due to the difficulties involved in making quantitative infrared absorption measurements. The spectrum of most substances tend to he so complex that there is a high probability that peaks will overlap in a given mixture. This necessitates that the absolute molar ahsorptivities for the individual peaks be either known or measured independently before the concentration of each species could be determined. Beer's Law deviations2 are more common in infrared mectrocoov. because the slit widths usually required to o h t i n a spectrum yield hand widths which are often the same magnitude as the absor~tionoeak. Slieht deviations in wavelength can give rise to iarge variationsin absorbance, and with large slit widths i t is easier for stray radiation to come in and affect the absorbance. The short pathlength of most infrared cells is hard to use because of the difficulty in exactly duplicating the pathlength. One of the ways to avoid some of the aforementioned difficulties is t o construct an empirical standard curve for the substance in question. The concentration of the substance in a sample is then determined from the standard curve. We have developed an experiment which uses this method and which requires no special cell other than the ordinary sodium chloride salt plates normally found in an undergraduate lahoratory. The amount of methanol up to about 12%in 2,2,2-trifluomethanol (TFE) can he quantitatively determined. In the 1100-900 cm-I region TFE has two hands--one at 1075 em-1 and one a t 925 cm-'. Methanol, in this same region, exhibits a single strong band at 1020 cm-'. I t is possible, therefore, to observe qualitatively the presence of methanol in TFE. I t turns out that there is enough of a window between the two TFE peaks that the methanol peak is clearly defined, and a quantitative analysis can he made. From Beer's Law, eqns. (1) and (2) can be written
Figure 1. The %To for each peak was determined by constructing a baseline far each peak and reading the %Tat the point where Me baseline crossed the peak position. Curve shown corres~ondsto 1.98 M MeOH in TFE.
where A is the absorbance, t is the molar absorptivity, I is the cell pathlength, and C is the concentration. The onthleneth can be eliminated by taking the ratio of eqns. (I) and (2). ?he result is eqn. (3). AM~HIATFE = (CM~OHI(CTFECTFE))C~~~OH (3) The coefficient of CM~OH, EM~OH/'TFE~TFE, cannot be a constant since CTFEchanges with changing Cnneox in the solutions. However, if the change in CTFEis relatively small, then eqn. (3) will approximate a linear relationship. It was felt that a maximum 10%change in C T ~ may E fulfill this requirement. Over the concentration range that we used (0-3 M methanol), Versus CTFE changes hy only 12%. A plot of AM~OH/ATFE C ~ O (Fig. H 2) was linear, thus supporting the above assumption. In addition the error in the individual measurements a t the highest CM.OH is about 10% which is the magM MeOH
' Author to whom correspondence should be sent.
Skoog. D. A., and West, D. M., "Principles of Instrumental Analysis." 2nd ed.,Saunders College Publ. Philadelphia, 1980. 886
Journal of Chemical Education
Figure 2. Standard curve fw the determination of the methanol concentration in 2.2.2-hifluoroethanol.For the curve shown the regression slope is 12.5 -L. 0.2 with the intercept set equal to zero. '
nitude of the expected change in the coefficient of eqn. (3). Thus, the error in the individual measurements may render any change in the coefficient to be undetectable. The absorbance of each peak is determined from the percent transmission by eqn. (4), in which To is the transmission of the base line and T the transmission of the peak. A = log(%To/%T)
(4)
In the present case a base line was drawnfor each peakusing as the reference ~h~ the minima either side of the of the peak was read and %Ton the baseline a t the taken to he %To.The %T of the peak was read directly and the absorbance was calculated. A sample calculation is shown in Figure 1. A plot of (AM&H/ATFE)Versus C M ~ is~presented H in Figure 2. A least squares fit of the data produced a slope of 12.5 0.4 and an intercept of -0.09 f 0.9, where the cited errors are standard errors of the mean. The small value of the in. brcept with an error an order of magnitude larger than itself
*
supports that for all intents and purposes the intercept can be taken as zero as required by eqn. (3). If the intercept is taken to be zero, linear regression analysis of the data yields a slope of 12.5 f 0.2. From this standard curve the coucentration of methanol in an unknown solution can be determined. Experimental
A series of methanol solutions in TFE were prepared on a
VIV % basis starting at 2% and ending at 12%.The molarity of the methanol was calculated from its deilsity. Individual solutions were placed between two ordinary NaCl plates and the spectrum was recorded between 1200 cm-I and 800 cm-'. Acknowledgment
This paper was written while RE was on sabbatical leave. The hospitality of the Organic Chemistry Institute, ~ o r w a ~ ' s Technical High School, Trondheim, was much appreciated, and RE wishes to thank the Norwegian Marshall Fund for a fellowship.
Volume 62
Number 10
October 1985
887
