How do we differentiate between Fe3+and Cr3+in group III?

The group reagent for group III is NH₄OH in presence of NH₄Cl,which leads to formation of their respective hydroxides- Fe(OH)₃ and Cr(OH)₃. Now minm. quantity of dil. HCl is added and then excess of NaOH is added boil and filter.The filtrate contain sodium aluminate which separates Al³⁺ ion another memeber of this group,and residue is then tested for Fe3+ and Cr3+.
Test for Fe³⁺:-
1.Dissolve a part of residue in dil.HCl and solution is divided into 2 parts :-
a).To 1 part add potassium ferrocyanide, a prussian blue colour ppt. of ferri-ferrocyanide is obtained.
FeCl_{3 +} 3K₄Fe(CN)₆ -----------> Fe_4[Fe(CN)₆]₃ + 12 KCl
Prussian blue
Test for Cr³⁺:-
To a portion of residue contianing Fe(OH)₃ ,Cr(OH)₃ is heated with NaOH and bromine water,Cr(OH)₃ dissolves forming sodium chromate,and Fe(OH)₃ remains as residue,when acetic acid and lead acetate is added to above solution,a yellow ppt. of lead chromate is obtained.
Na₂CrO₄ + Pb(CH3COO)_{2 -}--------> PbCrO₄ + 2CH₃COONa
Yellow ppt.
Thanks & regard
Aarti gupta
akiitians Faculty