How to Determine Acid/Base Ratio with Overlapping Absorption Wavelengths?

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Discussion Overview

The discussion revolves around determining the acid/base ratio of an indicator when the absorption wavelengths of its acidic and basic forms overlap. Participants explore various methods and equations related to absorbance measurements, including the use of Beer's Law and the Henderson Hasselbalch equation, as well as the concept of isosbestic points.

Discussion Character

  • Homework-related
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant suggests using Beer's Law to find concentrations when distinct wavelengths are available, but questions how to proceed when wavelengths overlap.
  • Another participant proposes measuring absorbance at two separate, non-overlapping wavelengths to create two equations for the acid and base forms.
  • Some participants discuss the importance of knowing the total concentration of both forms and how this relates to absorbance measurements.
  • A participant mentions the utility of the isosbestic point for calculating the ratio of protonated and deprotonated species, contingent on the existence of such a point for the indicator.
  • There is a suggestion that having two equations with two unknowns could help solve for the concentrations of the acid and base forms.
  • One participant expresses uncertainty about their understanding and seeks clarification on the necessity of using two different wavelengths.

Areas of Agreement / Disagreement

Participants express differing views on the necessity of using two distinct wavelengths versus relying on the isosbestic point. While some agree on the importance of total concentration and absorbance, there is no consensus on the best approach to determine the acid/base ratio under overlapping absorption conditions.

Contextual Notes

Participants acknowledge limitations in their approaches, such as the need for specific conditions like the existence of an isosbestic point and the implications of overlapping absorption wavelengths on measurements.

pyridine
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Homework Statement


OK, so if your indicator has 2 clearly different wavelengths of absorption for acidic and basic forms, you can measure the absorbance of buffered samples at a wavelength which only absorbs, for instance, the acidic form. Then you can use beer's law to find the concentration of the absorbing form A=elc (e=molar absorptivity, l= path length (1cm) and c=concentration of the thing which is absorbing). Then you can find by difference the concentration of the non absorbing form and then the pKa with the Henderson Hasselbalch equation.

BUT!

I want to know, how you find the concentration of the absorbing and non absorbing forms if there isn't a wavelength which only one absorbs. How would you figure out the ratio of acid/base?


Homework Equations


Beer's Law: A=elc

Henderson Hasselbalch equation: pKa = pH+ log(acid/base)



The Attempt at a Solution



I know Beer's Law would then be A(total)=A(base)+A(acid), and hence A(total)= ebc(base) +ebc(acid) but that's as far as i know.

Someone please help! I'm so close and this lab report has taken me nearly 2 weeks to do :(
 
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You want separate scans of wavelengths for the acid form and for the alkaline form of this indicator. Look for nonoverlapping wavelengths, significantly separated, in which both forms of the indicator absorb. When you measure your sample solution, you must use the two separate wavelengths. This way, you can have two equations; one for one wavelength for the acid form, and the other for the wavelength for the alkaline form. See your sample may be expected to have the two, differently colored forms of the indicator present, the concentration of each being unknown; but you then have two equations - each based on the absorbance at different wavelengths. I am not sure if I misunderstood your question, but I say that you want two different wavelengths which are not close to each other, and that one wavelength absorbs significantly the acidic form and the other wavelength is absorbed significantly by the alkaline form.
 
Would it help if you will know sum of concentrations of both forms?

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Symbolipoint: Even if you scan an acid wavelength and a basic wavelength like you said, if the wavelengths overlap, all of the absorption wouldn't be related to a single form. Do you think that'd matter?

Borek: You'd know the sum of the concentrations of both, it's just the total concentration of your sample.
 
Awright...
I've inched further forward. I know the total concentration and the total absorbance. The total absorbance is made up of x concentration of base and y concentration of acid. What would be a way to work out the ratio of acid/base, if you know the absorbance of a purely acid sample as well as a purely base sample at the same wavelength?
 
I find the http://en.wikipedia.org/wiki/Isosbestic_point" to me most useful for these problems. You will need the molar absorptivity at the isosbestic wavelength for it to be useful. After that you can calculate the ratio of both the protonated and deprotonated species (in the case of an acid/base indicator).
 
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pyridine said:
I know the total concentration and the total absorbance.

Doesn't it give you two equations in two unknowns?

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I still believe that the member named 'pyridine' needs two different wavelengths. Borek, what am I missing?
 
I have already proved today (in other life) that I am an idiot, so I don't trust myself too seriously, but

[HA] + [A-] = Ctotal

kHA[HA] + kA[A-] = Atotal

If kHA<>kA there are two independent equations in two unknowns. Note that the second equation is for one measurement, at one wavelength.

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  • #10
2 equations, 2 unknowns..of course. Thanks!
 
  • #11
If you adjust the pH to the isosbestic pH and then choose the wavelength for this isosbestic point, you could then directly measure the total concentration of the indicator compound. This would require an isosbestic point to exist for this compound. Pyridine and Borek, is that what you wanted to do?
 
  • #12
I suppose total concentration was already given.

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