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
(a) log [In⁻]/[HIn] = pKᵢₙ - pH
(b) log [HIn]/[In⁻] = pKᵢₙ - pH
(c) log [HIn]/[In⁻] = pH - pKᵢₙ
(d) log [In⁻]/[HIn] = pH - pKᵢₙ
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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,
log [In⁻]/[HIn] = pH - pKln
Explanation:
The equilibrium reaction of acidic indicator is
Hln → ln⁻ + H⁺
In term of dissociation constant:
Ka = [H⁺][ln⁻] / [Hln]
The expression for pH indicator:
pH = pKln +log[ln⁻]/[Hln]
Thus,
log[ln⁻]/[Hln] = pH - pKln
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