Anonymous asked in Science & MathematicsChemistry · 1 month ago

Titration question?

Would methyl red be a good pH indicator for a NaOH-acetic acid titration? If not, what pH indicator should I use instead?

1 Answer

  • 1 month ago

    No, methyl red is NOT a good pH indicator for an NaOH - acetic acid titration. 

    At the equivalence point of the titration, all the acid and the alkali completely react to form sodium acetate.  As acetate ion is the conjugate base of acetic acid which is a weak acid, at equivalence point the acetate ions in the solution would undergo hydrolysis to form acetic acid molecules and hydroxide ions to make the solution slightly alkaline.

    CH₃COO⁻(aq) + H₂O(ℓ) → CH₃COOH(aq) + OH⁻(aq)

    Methyl red is a pH indicator which change color at slightly acidic solution of pH 4.4-6.2, and thus methyl red does not change color at the equivalence point of the titration.  This causes an error.

    For such titration of a strong alkali against a weak acid, the indicator employed should change its color at a pH of slightly alkaline.  Hence, phenolphthalein could be used as the pH indicator instead as phenolphthalein changes color at pH 8.2 - 10.0.

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