I want to determine oxidation of hydroquinone to p-benzoquinone numerically. For solving this problem i preferred to use spectrophotometry. I ran some calibration experiments in Lab and with analyzing them , I found at 304.6 nm and 293.2 nm hydroquinone and p-benzoquinone can be determined using a liner relationship between Adsorption of light and concentration (please see the figure).
But according to NIST database that shows in the figure as red lines, there isn't a full agreement between my lambda max results and official reports. what's wrong? Do you think I should work based on reports or my results? Do you have a similar problem? I am strongly willing to see your comments.
Regards
M.Keyhani
Dear Mohammed,
I don't see any problem in results You observe. The difference between your data and NIST lambda max is very small. It can be explained as the solvent effect on position of absorption band (solvatochromism). I believe your solvent and solvent used for library spectra recording can be different. Solvatochromic shifts for such compounds as yours can be of several nanometers and even several tens of nanometers. So, of course, you should use for calibration your values of lambda, determined under conditions of your own experiment.
Dear Mohammed,
I don't see any problem in results You observe. The difference between your data and NIST lambda max is very small. It can be explained as the solvent effect on position of absorption band (solvatochromism). I believe your solvent and solvent used for library spectra recording can be different. Solvatochromic shifts for such compounds as yours can be of several nanometers and even several tens of nanometers. So, of course, you should use for calibration your values of lambda, determined under conditions of your own experiment.
Dear Muhammed,
I also agree with Dmitry and Patrick, you should check the solvent for NIST whether is the same or not with your solvent
I concur with my colleagues above ... the effect is most likely based on a difference in solvents. I recommend to work based on your results (and mention the difference in the experiment documentation, just in case a paper referee wants to know somewhere down the line the details).
Two factors influence on the maximum of spectrum: calibration of spectrophotometer and nature of solvent. Second case is more important in the case of big dipole moment of chromophore molecule caused by intramolecular charge transfer and different dipole moments of solvent molecules as well as their donor-acceptor nature. That maybe the case of your molecules. So all comments are write.
My comments may be late, let summarize and complete, if it is still useful.
(i) the spectrophotometer must be calibrated in wavelength and absorbance using appropriate filters; (ii) the bandwidth changes the shape of absorption bands; (iii) solvent effects may be a few nm; (iv) absorption max may be sensitive to temperature (thermochromism); (v) with the band shapes shown, the decimal on wavelength is probably not significant.
Even with an uncalibrated instrument in stable conditions, the Beer Lambert law is respected, and calibration with pure compounds will give exact results, if properly handled. Note that your linear plots (absorbance vs. concentration) must go through the origin (you may add the zero/zero point in your data; it is in fact an experimental point, because the reference cell is zero absorbance by definition, with concentration = zero); a shift indicates a systematic error. It seems to be OK in your case (although I expect a better precision for hydroquinone).
I understand that you wish to use the spectrophotometer for a kind of titration (and /or kinetics, equilibria, ….). The two wavelengths at which you are working are rather close; this may cause a problem because you have to subtract large absorbances to observe small changes. If you can use an alternate wavelength, for example the one of benzoquinone at about 360 nm, the problem will be minimized (but it is an order of magnitude less intense). Check that the isosbestic point(s) is precisely obtained in your mixtures.
Good luck !
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