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The rapid change of pH near the stoichiometric point of an acid-base titration is the basis of indicator detection. pH of the solution is related to ratio of the concentrations of the conjugate acid (HIn) and base (In⁻) forms of the indicator by the expression:

1. log [In⁻]/[HIn] = pKₐ − pH
2. log [HIn]/[In⁻] = pKₐ − pH
3. log [In⁻]/[HIn] = pH − pKₐ
4. log [HIn]/[In⁻] = pH − pKₐ

Correct answer: log [HIn]/[In⁻] = pH − pKₐ

Solution

The Henderson-Hasselbalch equation is pH = pKa + log([In⁻]/[HIn]). Rearranging gives log([HIn]/[In⁻]) = pH − pKa.

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