Question

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 the ratio of the concentrations of the conjugate acid \[\left( HIn \right)\] and base \[\left( In \right)\] forms of the indicator by the expression-
A. \[\log \dfrac{\left[ I{{n}^{-}} \right]}{\left[ HIn \right]}=p{{K}_{In}}-pH\]
B. \[\log \dfrac{\left[ I{{n}^{-}} \right]}{\left[ HIn \right]}=pH-p{{K}_{In}}\]
C. \[\log \dfrac{\left[ HIn \right]}{\left[ I{{n}^{-}} \right]}=pH-p{{K}_{In}}\]
D. None of these.

Answer 1

Hint: We know that the titration is the method employed in the quantitative analysis of any substance. This analysis usually tells us the concentration and strength in terms of molarity or molality of any chemical compound.

Complete answer:
As we know that the indicators are compounds that are added to the analyte solutions in which titrant is added. Titration is the quantitative analysis of any sample. The titration can be acid base type usually, but it can also be redox or other type. An indicator is used in the process of titration. This is added in the analyte, which is added in the conical flask, and to this the titrant from the burette is added. Indicator is the substance that changes or imparts certain color at the stoichiometric end point of the titration. This means when a certain concentration of the acid or a base is completely neutralized, then at the end point we observe a color change.
For example, an indicator \[\left( HIn \right)\] can dissociate in the following way.
\[HIn\rightleftharpoons {{H}^{+}}+I{{n}^{-}}\] thus, \[{{E}_{EquilibriumConstant}}\], \[{{K}_{In}}=\dfrac{[{{H}^{+}}][I{{n}^{-}}]}{[{{H}_{In}}]}.\]
On further solving we get; \[p{{K}_{In}}=pH-\log \dfrac{\left[ I{{n}^{-}} \right]}{\left[ HIn \right]}\Rightarrow \log \dfrac{\left[ I{{n}^{-}} \right]}{\left[ HIn \right]}=pH-p{{K}_{In}}\]

So, the correct answer is “Option B”.

Note:
Remember that the neutralization between NaOH and HCl is the basis of titration. Here the equivalence point or the end point will be when all the number of moles of an acid is neutralized or added to the base. Then, the pH will be at \[7.0,\] like for this strong acid and strong base titration.