Objective

To estimate the amount of solute dissolved in a saturated solution of 4-chlorobenzaldehyde in chloroform and methanol, by UV spectroscopy.

Procedure

Solutions of known concentrations of 4-chlorobenzaldehyde was made-up in chloroform and methanol. UV spectra of further diluted solutions of each was obtained to determine the λmax (wavelength of maximum absorbance) of the aldehyde. Absorbance of solutions of different concentrations was also obtained.These solutions were made by diluting the previously prepared solutions. From the absorbance values, molar extinction coefficient (molar absorptivity) ε, was calculated from Beer's law A = εcl, by determining the slope of a plot of absorbance Vs concentration. Using molar absorptivity and absorbance, concentration of the saturated solutions was determined. This was done by obtaining the absorbance of dilute solutions (known dilution) of the saturated solutions of 4-chlorobenzaldehyde in chloroform and methanol.

Results

UV Spectrum
4-Chlorobenzaldehyde - CHCl3 212A-2 (0.01mM in CHCl3, 1cm pathlength)
Overlay CHCl3 solutions of 213A-2, 213A-3, 213A-4, 213A-5 and 213A-6
Overlay CH3OH solutions of 213A-2, 213B-3, 213B-4, 213B-5, 213B-6


Spreadsheet

Discussion

The absorption maxima of 4-chlorbenzaldehyde in methanol was reported to be 255 mu. In ethanol it is reported (p 135)to be and 254.4nm, with ε of 16000 cm-1M-1 and 292nm with ε 13000 cm-1M-1corresponding to K and B bands, however it could not be found in chloroform. A plot of Concentration Vs Absorbance measured for 4-chlorobenzaldehyde in chloroform at 258.5nm wavelength, using 10mm path length gives an extinction coefficient of 14960 cm-1M-1


λmax (nm)
ε (cm-1M-1)
258.1
14885
258.2
14926
258.3
14953
258.4
14954
258.5

14960

258.6
14959
258.7
14958
258.8
14932
258.9
14937
Absorbance obtained experimentally in methanol is strikingly different from observed in this experiment, 220nm. This may be due to the solvent/sample impurities or because of experimental uncertainty. Therefore this experiment will be performed again.
Absorbance of saturated solutions obtained are too low for an accurate measurement of solubility of the sample in the solvents used. Therefore this will also be repeated.

Conclusion

Extinction coefficient of 4-Chlorobenzaldehyde in Chloroform at 258.5nm has been determined to be 14960 cm-1M-1. This is comparable to the reported values in methanol and ethanol.

Log

2008-11-10

13:22 Weighed out 4-chlorobenzaldehyde 140.51mg (for solution 211A in CHCl3); 9.9959mM.
13:40 Weighed out 4-chlorobenzaldehyde 140.54mg (for solution 211B in CH3OH); 9.9980mM
14:00 Made-up 100mL solutions of 4-chlorobenzaldehyde 213A (in Chloroform using 140.51mg) and 213B (in Methanol using 140.54mg).
16:00 Made-up five dilute solutions using 212A, the new solutions are 213A-2, 213A-3, 213A-4, 213A-5 and 213A-6 in chloroform in 10mL volumetric flasks. The concentration of each solution is listed in the spreadsheet Exp213-WS1

2008-11-13

18:00 Obtained UV Wavelength scan for chloroform solution (213A dilute solutions)
19:00 Obtained Absorbance for the chloroform solutions (213A dilute solutions).

2008-11-15

16:00 Made-up five dilute solutions using 213B, the new solutions are 213A-2, 213B-3, 213B-4, 213B-5, 213B-6 in methanol in 10mL volumetric flasks. The concentration of each solution is listed in the spreadsheet Exp213-WS1
17:20 Obtained UV Wavelength scan for methanol solution (213B dilute solutions)
20:00 Obtained absorbance for the methanol solutions (213B dilute solutions).


Judges Questions
1.Anything special about the procedure for doing the UV-Vis, how do I know the instrument is calibrated? Do you use a specific type of cuvette? What is the path length?-CN
Answer: Calibration check for the instrument had not been performed before the experiment, however it was done soon after and found to be accurate for all practical purposes. In the future a calibration report will be uploaded. Calibration or the Wavelength Accuracy check on the Perking Elmer lambda 35 UV-Vis was performed with a Deuterium Lamp Test, (D2 peak at 656.1nm). The instrument passed this test.

The standard procedure for obtaining a UV spectrum on the Perkin Elmer Lambda 35 instrument, includes Autozeroing the instrument in order to allow 100% Transmittance with no samples loaded. The instrument being used is a double beam instrument which is programed to subtract the absorbance of the blank (solvent) from the sample, this makes sure that the recorded absorbance is a result of the solute only. Since polystyrene cuvettes do not provide a window for the absorbance of interest (200-350nm) to be measured, quartz cuvettes (Spectrocell- R3010) are used. The path length of these cuvettes is 10mm