Identify the complex ions in sequential order when ferroin is used as an indicator in the titration of iron(II) with potassium dichromate, (phen = 1, 10-phenathroline)
(a)$\left[\mathrm{Fe}(\text { phen })_{3}\right]^{2+}$ and $\left[\mathrm{Fe}(\text { phen })_{3}\right]^{3+}$
(b)$\left[\mathrm{Fe}(\mathrm{phen})_{3}\right]^{3+}$ and $\left[\mathrm{Fe}(\mathrm{phen})_{3}\right]^{2+}$
(c)$\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]^{4-}$ and $\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]^{3-}$
(d)$\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]^{3-}$ and $\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]^{4-}$
Answer
Answer: A ✓ checked by 4AB · confidence high
Explanation
Ferroin is [Fe(phen)₃]²⁺ (deep red); at the end point excess dichromate oxidises it to [Fe(phen)₃]³⁺ (pale blue).