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Topic: Isobestic point  (Read 20637 times)

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Isobestic point
« on: July 17, 2005, 08:35:57 PM »
isobestic point in the UV spectrum indicates the presence of an equilibrium between two substances and they have the identical absorbance.  Would changing the pH of any of the substances affect this point?

personally ... i think it would be affected because absorbance is dependent on the extinction coefficent and the concentration.   If we alter the pH, this implies a change in the concentration. Therefore the absorbance will be changed. BUT, I am not sure if changing the concentration/pH will affect the equilibrium.  If changing concentration/pH will not affect the equilibrium, the isobestic point should not be affected also.  

Please give me some comments or ideas ... Thanks in advance!!!

« Last Edit: July 17, 2005, 08:36:21 PM by mcdull1016 »

Offline Eric

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Re:Isobestic point
« Reply #1 on: July 27, 2005, 03:37:20 AM »
well im no spectrophotometerologist, but wouldnt altering the pH only affect the equilibrium if there is an acid/base equilibrium.  I.E.  adding H+ may not result in a reaction shift unless it is a catalyst, product, or reactant, or subproduct.   Im not an expert or anything, but that seems to make sense to me.

On the other side of things, is H+ absorbtivity zero? you may be able to alter pH without disturbing the reaction equilibrium, but you will change absorbance spectrum right? or is that what you want?  I may be right I may be wrong.  so dont quote me.

Walter Mertens

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Re:Isobestic point
« Reply #2 on: September 30, 2005, 09:50:24 AM »
This message is only to tell you that it is not iso-bestic point
but iso-sbestic point, in English isosbestic.  On google you wil find much more items for isosbest :)

Offline ARGOS++

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Re: Isobestic point
« Reply #3 on: March 18, 2008, 04:11:45 PM »

It may be an older Thread, but it is quite often searched,     ─   
and its Definition of an “Isosbestic Point” (for some also “Isobestic”) is very weak, because in the given kind the Definition is not general, it is only for a very special case/situation.

For sure a better presentation/answer in the Forums is:  "Iso(s)bestic Points in UV Spectrum ”.

Apropos:  Of course are most Isosbestic Points affected by pH changes, except you measure only the “Indicator” self as the Chromophor.
(But in this case it is required, that the volume is all time kept constant!  Do you know why?)

Remember:  Most UV/Vis-Chromophors are more or less Indicators! (except yours!)

Good Luck!

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