Equivalence point in acid-base titration

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Discussion Overview

The discussion revolves around the concept of the equivalence point in acid-base titration, exploring the relationship between the nature of the acid and base involved and the resulting pH at the equivalence point. Participants examine various scenarios including strong acids with strong bases, weak acids with strong bases, strong acids with weak bases, and weak acids with weak bases, focusing on the implications for pH and the use of indicators.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant suggests that if the analyte is neutralized, it must always be at pH 7, questioning the use of indicators with different ranges.
  • Another participant points out that a pH meter can be used to determine the endpoint, noting that a pH of 7 is specific to titrations involving strong acids and strong bases.
  • Some participants clarify that the equivalence point pH varies depending on the strength of the acids and bases involved, with strong acid and strong base yielding a pH of 7, while weak acids with strong bases result in a pH greater than 7 due to hydrolysis of the salt formed.
  • It is noted that a strong acid with a weak base leads to a pH less than 7 at the equivalence point due to the hydrolysis of the cation from the salt formed.
  • Participants discuss that for weak acid and weak base titrations, the pH at the equivalence point is influenced by the relative strengths of the acid and base, determined by their equilibrium constants.

Areas of Agreement / Disagreement

Participants express disagreement regarding the assertion that neutralization always results in a pH of 7, with multiple perspectives on how the nature of the reactants affects the equivalence point pH. The discussion remains unresolved on the implications of these varying conditions.

Contextual Notes

Participants reference specific examples and calculations to illustrate their points, indicating that assumptions about the behavior of the resulting solutions depend on the specific acids and bases involved, as well as their concentrations.

Tony Stark
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In acid base titration, we try to neutralize the analyte by adding sufficient titrant and observing the color change in the indicator.
But if the analyte is neutralized, then it must always be at ph 7. Why is it that we use indicator of different ranges to check equivalence point?
 
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We can and many laboratory practitioners do use a pH meter to watch for the endpoint.

pH 7 endpoint would be for titrating strong acid or base, with strong base or acid.

Why to pH 7?
Look at the water dissociation constant.
(H+)(OH-)=1.0*10-14
 
For titration of ...
Strong Acid + Strong Base => Equiv Pt pH = 7 b/c the ions of the salt formed do not hydrolyze. Example: HCl + NaOH => NaCl + H2O ... NaCl => Na+ + Cl- ... Na+ + H2O => No Rxn and Cl- + H2 => No Rxn. Consequently the pH depends solely upon the autoionization of water;i.e. H2O => H3O+ + OH- and Kw = [H3O+][OH-]. [H3O+] = [OH-] = 1 X 10-7M => Kw = [10-7][10-7] = 1x10-14 ... and, pH = -log[H3+] = -log(10-7) = 7

A Weak Acid + Strong Base => Equiv Pt pH > 7 b/c the anion of the salt formed hydrolyzes leaving an excess OH- in solution at equivalence point. Example: HOAc + NaOH => NaOAc + H2O ... NaOAc => Na+ + OAc- ... Na+ + H2O => no rxn ... OAc- + 2H2O => HOAc + OH- ... The excess OH- leaves the pH at equivalence point ~ 8.90 for a mix of equal volumes of a 0.10M HOAc + 0.10M NaOH reaction.

A Strong Acid + Weak Base => Equiv Pt pH < 7 b/c the cation of the salt formed hydrolyzes leaving an excess of H3O+ ions in solution at the equivalence point. Example: NH4OH + HCl => NH4Cl + H2O ... NH4Cl => NH4+ + Cl- ... NH4+ + 2H2O => NH4OH + H3O+ ... The excess H3O+ => pH ~ 5.0 for a mix of equal volumes of 0.10M NH4OH + 0.10M HCl.

A Weak Acid + Weak Base at Equiv Pt is dominated by the electrolyte with the larger Keq value. That is, if Kb > Ka => pH > 7 at Equiv Pt, and if Kb < Ka => pH < 7 at Equiv Pt.
 

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