Concentration of both buffers is the same, only the pH is different

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Discussion Overview

The discussion revolves around the behavior of two phosphate buffers with the same concentration but different pH levels. Participants explore the implications of using the Henderson-Hasselbalch equation and the buffering capacity of the solutions, particularly in relation to the pKa of phosphoric acid.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant applies the Henderson-Hasselbalch equation but finds it confusing due to the cancellation of terms when both buffers have the same pKa and log ratio.
  • Another participant notes the complexity of phosphate buffers due to phosphoric acid being tribasic, suggesting that only certain equilibria need to be considered at pH 7.
  • A participant questions how to determine which buffer will resist changes in pH better, given that both buffers have the same pKa.
  • One participant references a resource on buffer systems, potentially offering additional context or information.
  • Another participant mentions that buffering power is maximized at pH = pKa and discusses the relevance of the distance from pKa in determining buffering effectiveness.

Areas of Agreement / Disagreement

Participants express uncertainty regarding the buffering capacity of the two solutions and how to assess their effectiveness, indicating that multiple views and questions remain unresolved.

Contextual Notes

Participants highlight the complexity of phosphate equilibria and the potential impact of pH on the individual species present, which may affect the buffering capacity. There is also a lack of consensus on how to evaluate the buffering effectiveness given the same pKa.

Who May Find This Useful

Individuals interested in buffer chemistry, particularly those studying or working with phosphate buffers in biochemical or laboratory settings, may find this discussion relevant.

Sayantan21
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Homework Statement
You are given two buffers [a) 0.1 M phosphate buffer of pH 7.7 and (b) 0.1 M phosphatebuffer at pH 6.71 . If acid is to be added to the buffers, which of them, do you think, will resist the pH changes better
Relevant Equations
Henderson Hesselbach Equation
pH = pKa + log([ conjugate base] /[ acid])
Here I used Henderson Hesselbach equation in both the cases , I.e
pH= pKa + log( [(PO4)3-]/ [H3PO4]) here pKa of phosphoric acid is 2.16 but the problem Is in both the cases the log part is same and Pka is also same so both will cancel out, and how can we solve? It is confusing.
 
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Phosphate is complicated because phosphoric acid is tribasic, so there are several equilibria. (See e.g. Wikipedia article on phosphoric acid.) In the region of pH 7, you only really need to consider the equilibrium between H2PO4- and HPO42-. 0.1M is the total concentration of phosphate species; the individual species will change with pH.
 
So how will I know which will resist the pH better? Both are same buffers, with same Pka value
 
You do know buffering power is maximum at pH = pK. ?
So that might be an easy question, which buffer is at the pH nearer to the pK?
However if I am not mistaken they are both equally distant.
So it is a question of in which direction are you going - and is adding acid taking you into a region of greater buffering or less?
 

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