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

• A
  $\log \frac{[HIn]}{[In^{-}]} = pH - pK_{In}$
• B
  $\log \frac{[In^{-}]}{[HIn]} = pH - pK_{In}$
• C
  $\log \frac{[In^{-}]}{[HIn]} = pK_{In} - pH$
• D
  $\log \frac{[HIn]}{[In^{-}]} = pK_{In} - pH$