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Grade: 12
        How do we differentiate between Fe3+  and Cr3+  in group III?
6 years ago

Answers : (1)

Aarti Gupta
askIITians Faculty
300 Points
							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.
FeCl3 + 3K4Fe(CN)6 -----------> Fe4[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
6 years ago
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