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Topic: Isobestic Points in UV Spectrum  (Read 8709 times)

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suga1

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Isobestic Points in UV Spectrum
« on: April 02, 2006, 08:32:27 PM »
Hi; Could someone please tell me if I am doing this question right.  The question is "What is the significance of the isobestic points in UV spectrum of phenol in acid, base and two buffers?  What would cause them not to meet?  Would changing the pH of any of hte solutions affect this point? "
I think the signficance of an isobestic point is that it is the point where the absorbance of the phenol equals the absorbance of the phenolate.  Reasons that would cause them not meet are: a change in total concentration of phenol, and also would a change in the ratio of phenol to phenolate also cause them not to meet?  And finally, a change in pH or a change in the position of the equilibrium will not affect their chances of meeting.  Is this true?  Thanks in advance for your help.

ARGOSpp

  • Guest
Re: Isobestic Points in UV Spectrum
« Reply #1 on: April 19, 2006, 06:47:06 AM »
Dear <suga1>,
Your "significance" meets not correctly the definition of an 'Isosbestic/Isobestic Point' in that way, that your point of view is incorrect!
If you answer the question: Which spectrum can you recognise in your host of the pH related spectra?, then you will detect, that (but only in case of two reactands) not their absorbance will meet, but, in this special case, their "Molar Absorbtivity/Absorptivity" will meet.
That means, that The Summation of all your reactands will be the same all the time along the pH-coordinate.
the conclusion of all:  An 'Isosbestic/Isobestic Point' is only an Imaginary Point and NEVER real!!!

If you are more iinterested:  www.p-forster.com/english/themes/Spectroscopy/Spectroscopy_FAQ.htm#FAQ_SP15

Good Luck!!!

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