Citation :
\documentclass[]{article}
\usepackage{ciler}
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\myauthor{}
\mytitle{I.R.~Spectrophotometry}
\mysubject{}
\mykeywords{}
\date{}
\begin{document}
\maketitle
\section{Introduction}
\subsection{Objectives of the experiment}
Commercial xylene (dimethylbenzene) is a mixture of ortho, meta and para isomers, illustrated in figure \ref{fig:xylene} page \pageref{fig:xylene}. These isomers cannot be separated easily by chromatographic methods or distillation and their UV/Visible spectra are hardly distinguishable (thus fluorescence would fail at discriminating them too). They however exhibit sufficient differences in their infrared spectra to allow for independent quantification of each isomer. Hence the use of IR spectroscopy to determine the composition of a commercial xylene mixture in this experiment.
\begin{figure}[htb]
\caption{the 3 xylene isomers}
\centering
\begin{tabular}[c]{c}
\bzdrv{1==CH$_3$;2==CH$_3$}\\
ortho
\end{tabular}
\begin{tabular}[c]{c}
\bzdrv{1==CH$_3$;3==CH$_3$}\\
meta
\end{tabular}
\begin{tabular}[c]{c}
\bzdrv{1==CH$_3$;4==CH$_3$}\\
para
\end{tabular}
\label{fig:xylene}
\end{figure}
\subsection{Theory of infrared spectroscopy}
The range of electromagnetic spectrum refered to as infrared\footnote{later refered as IR} is 0.75$\sim$300 $\mu$m. However, the vast majority of applications occurs between 2.5 and 15 $\mu$m (4000 cm$^{-1}$ to 670 cm$^{-1}$).
IR spectrophotometry studies the molecular \emph{absorption} of radiation in the IR range. The energy level of IR radiations is not sufficient to trigger promotion of electrons to excited states, but is enough to send the molecule to a higher \emph{vibrationnal} state -- refered as such because the energy absorbed is converted to molecular motion in the form a vibrations of two types~: \emph{stretching}, a motion along the bond axis, and \emph{bending} a change in bond angle. These vibrations can be described in several ways but the most common approach is a Hook-type law where atoms acts as masses and the bonds between them as springs.
There are two conditions necessary for the absorption of IR radiations by a molecule~:
\begin{itemize}
\item the frequency of the incident radiation must be equal to that of the particular vibration mode. This can also be seen from an energy point of view as saying that the energy of the incident radiation must be equal to the difference in energy between the two levels the transition is comprised.
\item there must be a change in the \emph{charge distribution} of the molecule -- a change in its \emph{dipole moment} -- for it to interact with the incident radiation.
\end{itemize}
As for other spectroscopic methods, IR consists in exposing a sample to a radiation source and record the transmitted intensity. Therefore, IR spectra are absorption spectra from which information can be obtained. They are composed of bands or peaks -- and not of lines as would be expected -- because the re-emission by the molecule also includes overtones and combinations that overlap with the fundamental vibrations~; each band having a specific intensity. This intensity is depends upon the magnitude of the change in oscillating bond during the transition and is also directly related to the number of bond in the molecule responsibles for that particular absorption.
\subsubsection{Quantitative analysis}
As mentioned above, the intensity of an absorption band for a given bond is proportional to a certain extent to the concentration of that bond in the solution, thus quantitative IR analysis is possible within a certain range and is described by a Beer-Lambert type law~:
\begin{displaymath}
\mathrm{Absorbance} = \log{\frac{I_o}{I}} = \epsilon C l
\end{displaymath}
where $I_o$ is the incident intensity, $I$ the transmitted intensity, $C$ the concentration of the absorbing bond, $l$ the path length of the incident beam through the sample and $\epsilon$ a coefficient.
\subsubsection{Qualitative analysis}
Although it is the most common use of IR spectroscopy, it is not extensively used in this experiment. Since each bond has a specific vibration frequency -- which can obviously vary slightly depending on the surroundings, structural investigation using IR spectra can be relatively efficient. Coupled with NMR and mass spectroscopy data, it is the best approach for complete structural determination even for extremely complex molecules.
\subsection{Instrumentation}
Two kinds of spectrometers are available for IR spectroscopy, \emph{dispersive} and with \emph{Fourier transform} operation. A FT-IR is used in this experiment.
\subsubsection{dispersive spectrometers}
The sample is illuminated with a uniform IR beam containing all wavelengths. After passing through the sample the light is then directed through a monochromator in which a given wavelength is selected to illuminate the detector and allow the transmittance to be determined. By scanning the monochromator across all wavelengths a spectrum is obtained.
These spectrometers exist as both \emph{single-beam} and \emph{double-beam} apparatuses. Double-beam equipment provides slightly more accurate and easier operation but is more expensive. Dispersive spectrometers are generally regarded as slower and less accurate than Fourier transform instruments.
\subsubsection{Fourier transform spectrometers}
Based on the \emph{Michelson interferometer}, and subsequent reconstruction of the spectrum through a \emph{Fourier transform}, those spectrometer allow for both faster and more accurate operation. A more detailed description is available on the following website~: \href{http://membres.lycos.fr/ciler/archives/Lectures/Spectroscopy/Lect3\_IR.pdf}{IR SPECTROSCOPY - instrumentation \& techniques}
\section{Safety}
Two chemicals are used in this experiment, cyclohexane and xylene. Cyclohexane is a common organic solvent, and as such the usual safety precautions should apply (labcoat, safety glasses, gloves). Xylene, being a benzene derivative is most likely similarly -- if not more acutely -- toxic.
\subsection{Cyclohexane}
Cyclohexane is very flammable (flash point at -20$^\circ$C) and may be ignited by contact with a hot surface -- a naked flame is not necessary, it should thus be used in a well vented area (fume-hood recommended) away from hot surfaces and ignition sources (electrical appliances). It must not be flushed down a sink as it is both an environmental hazard and a serious fire risk and should be stored for disposal in a specific container.
Cyclohexane is far less toxic than hexane, due to its cyclic structure, but is still hazardous and immediate medical help should be seeked if swallowed. In case of skin contact, wash off with soap\footnote{this is necessary since cyclohexane is poorly soluble in water} and water (only water in case of eye contact).
\subsection{Xylene}
Xylene is flammable in lesser proportions than cyclohexane, and thus does not need any further precautions. It has however a higher toxicity and is both a respiratory irritant and a narcotic. Consequently, extreme care should be taken to work in a well vented area and limit exposure to xylene vapours.
\section{Results \& discussion}
\subsection{Experimental results}
Procedure is as per manual. Spectra are attached as appendix at the end of the report.
\begin{table}[htb]
\caption{Data sheet}
\centering
\begin{tabular}{l c c c c r}
& \multicolumn{2}{c}{solution A} & \multicolumn{2}{c}{solution B} & commercial \\
isomer & vol (mL/20mL) & abs & vol (mL/20mL) & abs & abs \\
\hline
ortho & 0.1 & 0.153 & 0.25 & 0.263 & 0.245 \\
meta & 0.3 & 0.09 & 1 & 0.255 & 0.142 \\
para & 0.1 & 0.097 & 0.5 & 0.363 & 0.117 \\
\hline
\end{tabular}\\
results courtesy of Gillian Collins and Patricia Coffey.
\label{tab:data}
\end{table}
Considering that~:
\begin{itemize}
\item all 3 solutions (A, B and commercial) have the same initial volume (20 mL) and dilution level (2$\rightarrow$20 mL)
\item all 3 isomers have the same molecular mass and density
\end{itemize}
it is possible to plot the initial volume content instead of concentration in the calibration plot and determine the equivalent volume content of the commercial sample.
\begin{figure}[htb]
\caption{Calibration plots and calculations}
\centering
\includegraphics[width=0.45\textwidth]{plot}
\begin{tabular}[b]{l r}
isomer & equation \\
\hline
ortho & Abs = 0.7333vol + 0.0797 \\
meta & Abs = 0.2357vol + 0.0193 \\
para & Abs = 0.665vol + 0.0305 \\
\hline
\\
isomer & vol (mL/20mL) in com\\
\hline
ortho & 0.22 \\
meta & 0.52 \\
para & 0.13 \\
\hline
\\
\end{tabular}
\label{fig:cal}
\end{figure}
\subsection{Obtaining the \% content}
The commercial sample has been made from 1 mL of commercial mixture. Therefore, the \% content can be calculated from the equivalent volumes obtained above.
\begin{displaymath}
\textrm{\% content}=\frac{\textrm{equivalent volumic content of the isomer (mL)}}{\textrm{total xylene volume (mL)}} \times 100
\end{displaymath}
\subsection{Final results}
In the commercial sample, the first meta peak (690 cm$^{-1}$) is slightly enlarged compared to solutions A and B spectra, and its top is flat instead of the sharp peaks obtained for the other samples. This is most likely due to an interaction with another molecule in the mixture. Since it is not seen in either A or B, it cannot be the solvent or another of the isomers,. Hence the conclusion that there most likely is at least another chemical in the commercial mixture.
This is confirmed by calculating the \% composition and noticing that the total of all 3 isomers does not add up to 100\%, as seen in table \ref{tab:results}.
\begin{table}[htb]
\caption{Content of the commercial mixture}
\centering
\begin{tabular}{l c }
isomer & \% content (vol) \\
\hline
ortho & 22 \\
meta & 52 \\
para & 13 \\
others & 13 \\
\hline
\end{tabular}
\label{tab:results}
\end{table}
It may that the extra compound is ethylbenzene, a structural isomer of xylene, illustrated in figure \ref{fig:eb}. It has similar density and boiling point as xylene, and is obtained in the same fraction of petrol distillate, which explains its non-negligible presence in the commercial mixture.
\begin{figure}[htb]
\caption{ethylbenzene}
\centering
\bzdrv{1==CH$_2$-CH$_3$}
\label{fig:eb}
\end{figure}
\section{Conclusion}
Final results for the experiment are summarised in table \ref{tab:conc}. Values obtained are in the acceptable order of magnitude, which indicates that the experiment has been overall properly carried out. However, it can be noticed that the content value for the ortho isomer falls out of the expected range (and consequently, so does the ethylbenzene one).
\begin{table}[htb]
\caption{Final results}
\centering
\begin{tabular}{l c r}
isomer & \% content (vol) & labeled value \\
\hline
ortho & 22 & 15-20\% \\
meta & 52 & 40-65\% \\
para & 13 & $<$20\% \\
ethylbenzene & 13 & 15-20\% \\
\hline
\end{tabular}
\label{tab:conc}
\end{table}
This could be due to several factors. The accuracy of the calibration must be considered first. It is widely accepted that quantitative IR measurement are not very accurate. The experimental choice of measuring ortho over a narrow concentration range contributes to enhancing the order of magnitude of the error, as a small variation in the absorbance leads to a large variation in the slope of the calibration line. Furthermore, the fact that only two points are used to obtain the calibration means that there is no smoothing of an experimental artifact, as there would be with a 10 points calibration line. Ultimately, it is possible that the pure ortho has been contaminated which would result in a lesser amount present in the calibration than expected, thus leading to higher readings.
As a conclusion it can be said that content for meta and para isomer is acceptable, and that ortho and ethylbenzene should be rechecked in order to confirm their concentrations. This new experiment set could also be used to check if ethylbenzene is the contaminant and whether or not it is pure or just the main non-xylene compound.
Ultimately, it should be pointed that the sample processing through the IR spectrometer was made in poor safety conditions -- with very little venting and quite a noticeable amount of xylene vapour inhaled. In order to enhance safety, it would be advisable for the IR spectrometer to be moved into, or at least very near to a fumehood.
\end{document}
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Ca fait un bye que j'avais pas mis les mains dans \LaTeX 
(mais ce doc n'a rien à voir avec la fac 
)
![[:tinostar]](https://forum-images.hardware.fr/images/perso/tinostar.gif "[:tinostar]")
honte à moi 