Equivalence point in acid-base titration

  • Thread starter Tony Stark
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  • #1
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Main Question or Discussion Point

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?
 

Answers and Replies

  • #2
symbolipoint
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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
 
  • #4
James Pelezo
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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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