Question

The rapid change of pH near the stocichiometric point of an acid-base titration is the basis of indicator detrection. pH of the solution is related to the ratio of concentration of conjugate acid (HIn) to the concentration of base `(In^(-))` froms of the indicator by the expression
A. `"log"([In^(-)])/([HIn])=pK_(In)-pH`
B. `"log"([HIn])/([In^(-)])=pK_(In)-pH`
C. `"log"([HIn])/([In^(-)])=pH-pK_(In)`
D. `"log"([In^(-)])/([HIn])=pH-pK_(In)`

Answer 1

Correct Answer - D
Consider the equilibrium expression
`HIn hArr H^(+)+In^(-)`
`K_(In)=([H^(+)][In^(-)])/([HIn])=[H^(+)] ([In^(-)])/([HIn])`
Taking negative log of both sides, we have
`-log K_(In)= -log[H^(+)]- log.([In^(-)])/([H In])`
`pK_(In)=pH-log. ([In^(-)])/([H In])`
or `log. ([In^(-)])/([H In]) = pH-pK_(In)`