CSIR UGC NET Dec15 Q.7

Identify the complex ions in the sequential order when ferroin is used as an indicator in the titration of iron(II) with potassium dichromate. (phen = 1, 10- phenathroline) 
A. [Fe(phen)3] 2+ and [Fe(phen)3] 3+ 
B. [Fe(phen)3] 3+ and [Fe(phen)3] 2+ 
C. [Fe(CN)6] 4- and [Fe(CN)6] 3- 
D. [Fe(CN)6] 3- and [Fe(CN)6] 4- 

Soln: A. [Fe(phen)3] 2+ and [Fe(phen)3] 3+ 

Expln:
Ferroin is the chemical compound with the formula [Fe(o-phen)₃]SO₄, where o-phen is an abbreviation for 1,10-phenanthroline, a bidentate ligand.

The most important application of dichromate is in its reaction with iron(II) in which it is often preferred to permanganate. 
The relevant half reaction is :
Fe²⁺ → Fe³⁺ + e^-     E° = -0.77 V
and the total reaction is:
Cr₂O₇^2- + 6 Fe²⁺ + 14H⁺ → 2Cr³⁺ + 6 Fe³⁺ + 7H₂O     E° = 0.56 V 
Therefore, a sharp colour change from red to blue occurs on oxidation of the complex. 
[Ph3Fe]2+ → [Ph3Fe]3+

The indicator is prepared by mixing equivalent quantities of iron(II) sulphate and 1, 10- phananthroline. The resulting Fe(II) complex sulphate is called Ferroin. The Fe(III) complex sulphate is called ferrin. The color of ferroin is so much more intense than that of ferrin.

(Reference: Harris, D. C. Quantitative Chemical Analysis (4th ed.). New York, NY: W. H. Freeman. ISBN 0-7167-2508-8.)
http://wwwchem.uwimona.edu.jm/lab_manuals/c10expt31.html