Question

In the titration of Fe²⁺ ions versus Cr2O7^2– ions using diphenylamine as the internal indicator, phosphoric acid is added in the solution containing Fe²⁺ ions. The role of phosphoric acid is to

• (A) lower the value of E^o (Fe³⁺, Fe²⁺|Pt)
• (B) raise the value of E^o (Fe³⁺, Fe²⁺|Pt)
• (C) form complex with ferrous ions
• (D) change the reduction potential involving the indicator

Answer 1

The Correct Option is A

Explanation:
The reduction reaction to be considered is ${\mathrm{Fe}}^{3+}(\mathrm{aq})+{\mathrm{e}}^{-}⟶{\mathrm{Fe}}^{2+}(\mathrm{aq})$Its Nernst equation is:$E=E^0-\frac{RT}{F}\ln \frac{\left[F{e}^{2+}\right]}{\left[F{e}^{3+}\right]}$The addition of phosphoric acid forms the complex with ferric ion and thus lowers its concentration. Hence, the reduction potential becomes more negative.