In the Laboratory
The Use of Disposable IR Cards for Quantitative Analysis Using an Internal Standard
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Ramee Indralingam* and Angelito I. Nepomuceno Department of Chemistry, Stetson University, DeLand, FL 32720; *
[email protected] Infrared spectroscopy is traditionally used in qualitative analysis to identify compounds and aid in structure elucidation. Solid samples require the tedious and sometimes messy preparation of KBr pellets or mineral oil mulls. Solutions cannot be used because there are no solvents that do not absorb in the infrared. About four years ago, disposable polyethylene cards were made commercially available by the 3M Company. These cards have been used satisfactorily to minimize sample preparation time for solids, and to acquire high-quality IR spectra (1). They have also been effectively used in the organic chemistry teaching laboratory for structure determination (2). We have developed an experiment suitable for the instrumental analysis portion of an analytical chemistry course. It employs an internal standard in order to utilize the disposable cards for quantitative analysis. We use potassium thiocyanate (KSCN) as the internal standard because its absorption band at 2065 cm᎑1 does not interfere with the bands of the unknown, benzophenone. Acetone is used as the solvent because it evaporates readily, making no contribution to the spectrum. This experiment effectively illustrates the use of an internal standard and calibration curves and the principle of Beer’s law. Because a graphical method is used to determine the unknown concentration, there is no need to calculate the response factor for the internal standard. Materials and Methods
Chemicals The analyte is benzophenone, the internal standard is potassium thiocyanate (KSCN), and the solvent is acetone. Equipment This experiment requires an FTIR spectrometer and commercial disposable IR cards. Disposable polyethylene IR cards (Type 61) were obtained from 3M (3M Center, Bldg. 220-9E-10, St. Paul, MN 55144). A Nicolet Impact 400 FTIR Spectrometer was used for the acquisition of all IR spectra (Nicolet Analytical Instruments, 5225 Verona Rd., Madison, WI 53711). Twenty-microliter precision pipets were used for depositing the samples onto the film. Experimental Procedure All glassware must be cleaned and dried before use to avoid interference from the IR absorption bands of water.
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Individual students prepare stock solutions of benzophenone and KSCN containing 1.0 g and 0.5 g, respectively, of the solid in 50-mL volumetric flasks with acetone as the solvent. To prepare the standard solutions for generating the calibration curve, volumes of the benzophenone stock solution ranging from 0.5 to 15 mL are then pipetted into 25-mL volumetric flasks. Next, 5 mL of the KSCN internal standard stock solution is pipetted into each flask and the solution is brought to volume with acetone. The students are given a 25-mL volumetric flask with the unknown in it (
