How do you differentiate between fe+3 and cr+3 iin group 3?
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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)3dissolves 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
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)3dissolves 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
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