Monitoring the Course of an Esterification Reaction by FTIR

Monitoring the Course of an Esterification Reaction by FTIR

Objective

Acetic acid and ethanol react to form ethyl acetate in the presence of a mineral acid catalyst:

CH3COOH + CH3CH2OH CH3COOCH2CH3 + H2O

With 1% H2SO4 as the catalyst the reaction will come to equilibrium in about 20 minutes. You will monitor the change in concentration of each of the four components over time using FTIR.

Instrumentation

KVB Analect Diamond-20 FTIR equipped with a DPR-210 ATR Deep Immersion Probe.

Computer Software

Analect FX80 spectrometer control program.

FSP-6701 Multicomponent Analysis software.

Background

Beer’s Law states that the concentration of an absorbing species is related to its absorbance by A = bc. However, this form of Beer’s Law is valid only for a single species at a single wavelength. In a multicomponent system, Beer’s Law is additive and the absorbance at a single frequency is equal to the sum of absorbances for each component.

A Beer’s Law analysis of a multicomponent system generates a series of equations that are best evaluated using matrix algebra. Several matrix forms have been developed for multicomponent analysis. The most widely used is the inverse least-squares method (aka the P-matrix). If you wish to know the mathematics used in building a P-matrix, several resources are available. The main advantage of the P-matrix is that impurities present in the analysis mixture do not have to be quantified; however any impurities in the analysis mixture must also be present in the calibration standards. The quality of the calibration standards is the single most important factor for producing meaningful results.

Procedure

Power up the interferometer and open FX80. At the bottom of the screen you will see a series of command buttons. Within each command button are various function buttons. Within each function button are various parameter input fields. A detailed explanation of all command, function and parameter options can be found in the FX80 User’s Guide.

Select the collect command button; the various function buttons are displayed. Select the AQPARM button. The displayed parameter fields contain information defining the Diamond-20’s physical configuration. This information is stored on an EEPROM (electrically erasable programmable read-only memory) chip in the instruments’ electronics control module. You will not have to change any of these parameters.

Acquiring a Background

Close AQPARM and select Collect  AQBK. Select destination. Type a file name for your background spectrum, choose C:\FTIR\ch_4212 as the destination directory and press OK. Change the number of scans to 64. You may also give your background spectrum a title and description. With the dipper probe open to air, press OK to acquire a background spectrum. After a short pause the scanning will start. When all the scans have been collected, the window will close and the background spectrum will be displayed. The background spectrum is now saved as a *.bkg file in the C:\FTIR\ch_4212 directory. Close the window.

Acquiring Component Spectra

Immerse the dipper probe in pure deionized water. Select Collect  AQSP. Type a file name for the transmission spectrum and choose C:\FTIR\ch_4212 as the destination directory. Change the number of scans to 64. Type in a title and description and press OK. After a short pause the scanning will start. When all the scans have been collected, the window will close and the water spectrum will be displayed. It is saved as a *.asf file in the C:\FTIR\ch_4212 directory. Maximize the window. Select File I/O  PRINT  OK to print the spectrum. Now select AnalyzePEAK calculate. A table will be generated with the wavenumber of each peak maximum (x) and its’ associated transmission value (y). Select print results to print the table. Repeat this procedure for the remaining three components. Using the component data tables, identify a unique, strongly absorbing peak frequency for each component (recall the relationship between transmission and absorbance). The Mcomp program will use these peak frequencies to build the P-matrix and analyze the reaction spectra.

Acquiring Calibration Spectra

Carefully prepare four calibration standards. Each standard must contain all four components. The composition (proportions of components) of each standard must be unique. The four standards should bracket the expected concentration ranges. Prepare 200 ml of each. Acquire and save the spectrum of each standard.

Building the P-Matrix

Close FX80 and exit windows. At the C:\FTIR\USER0 command prompt type mcomp and select P-Matrix. Follow the instructions detailed starting on page 43 of the FSP-6701 Multicomponent Analysis Software User’s Guide. After building the matrix evaluate it with the Evaluate analysis error function. A P-matrix built with carefully prepared calibration standards will have component % errors of less than 2%. If any of your component % errors are greater than 10%, re-build the matrix with more carefully prepared standards and/or different peak frequencies. Be sure to save the analysis file.

Running the Reaction

When you are satisfied with your P-matrix, exit mcomp and open FX80 (type win at the C:\FTIR\USER0 prompt to load windows). Set up the reaction apparatus as diagrammed below. Charge the reaction vessel with 350 ml (70%) ethanol, 150 ml (30%) acetic acid and 5 ml cH2SO4. Bring the mixture to reflux temperature (~75 C) and collect spectra every two minutes for twenty minutes. Be sure to save the spectra and record the time (minutes after reaching reflux) each was acquired.

Analyze the Reaction Spectra

Open Mcomp and follow the instructions on page 60 of the Mcomp manual. Print the results of each reaction spectrum.

Report

In your report include the original prints of your P-matrix error evaluation and reaction analysis data. Produce a plot of component concentration vs. reaction time (all four on the same graph). How do your results compare to the theoretical equilibrium concentrations?