Hi Chemtalkers,
I'm preparing the Year 12 assesment task and I'm having trouble with Fe2+ and Fe3+.
Fe2+ has a greenish precipitate with NaOH and Fe3+ has a brown precipitate - as they should.
But they react the same in the confirmation tests. ie they both turn dark blue with potassium ferricyanide and dark brown with potassium thiocyanide. I'm using acidified ferrous ammonium sulfate for the Fe2+ ions.
Carol
Cations and Anions
Re: Cations and Anions
Yes, that one is tricky.
Make sure you have very fresh Fe2+ .
Use potassium ferrocyanide for Fe3+
and potassium ferricyanide for Fe2+ test.
Good luck
Make sure you have very fresh Fe2+ .
Use potassium ferrocyanide for Fe3+
and potassium ferricyanide for Fe2+ test.
Good luck
Re: Cations and Anions
Just another thought.
Maybe confirmation tests give you positive results for both Fe ions but NaOH test will separate Fe2+ from Fe3?
Maybe confirmation tests give you positive results for both Fe ions but NaOH test will separate Fe2+ from Fe3?
Re: Cations and Anions
Well I opened a new bottle of ferrous ammonium sulfate and made up fresh solutions. Now it works. They both turn blue with potassium ferricyanide, but only the Fe3+ goes brown with potassium thiocyanide.
I guess I'll have to make it up fresh on Tuesday ...............and hope the assessment task is not postponed..
I guess I'll have to make it up fresh on Tuesday ...............and hope the assessment task is not postponed..
Re: Cations and Anions
It is a pain, but I have to make up Fe2+ on the day or the day before. It oxidizes to Fe3+
Re: Cations and Anions
From Wiki
Prussian blue is produced by oxidation of ferrous ferrocyanide salts. These white solids have the formula M2Fe[Fe(CN)6] where M+ = Na+ or K+. The iron in this material is all ferrous, hence the absence of deep color associated with the mixed valency. Oxidation of this white solid with hydrogen peroxide or sodium chlorate produces ferricyanide and affords Prussian Blue.[12]
A "soluble" form of PB, K[FeIIIFeII(CN)6], which is really colloidal, can be made from potassium ferrocyanide and iron(III):
K+ + Fe3+ + [FeII(CN)6]4- → KFeIII[FeII(CN)6]
The similar reaction of potassium ferricyanide and iron(II) results in the same colloidal solution, because [FeIII(CN)6]3- is converted into ferrocyanide.
"Insoluble" Prussian blue is produced if in the reactions above an excess of Fe3+ or Fe2+, respectively, is added. In the first case:
4Fe3+ + 3[FeII(CN)6]4- → FeIII[FeIIIFeII(CN)6]3 [13]
"Turnbull's blue"
Ferricyanide ion, used to make 'Turnbull's blue'.
In former times, it was thought that addition of Fe(II) salts to a solution of ferricyanide affords a material different from Prussian blue. The product was traditionally named "Turnbull's Blue" (TB). It has been shown, however, by means of X-ray diffraction and electron diffraction methods, that the structures of PB and TB are identical.[14][15] The differences in the colors for TB and PB reflect subtle differences in the method of precipitation, which strongly affects particle size and impurity content.
Prussian blue is produced by oxidation of ferrous ferrocyanide salts. These white solids have the formula M2Fe[Fe(CN)6] where M+ = Na+ or K+. The iron in this material is all ferrous, hence the absence of deep color associated with the mixed valency. Oxidation of this white solid with hydrogen peroxide or sodium chlorate produces ferricyanide and affords Prussian Blue.[12]
A "soluble" form of PB, K[FeIIIFeII(CN)6], which is really colloidal, can be made from potassium ferrocyanide and iron(III):
K+ + Fe3+ + [FeII(CN)6]4- → KFeIII[FeII(CN)6]
The similar reaction of potassium ferricyanide and iron(II) results in the same colloidal solution, because [FeIII(CN)6]3- is converted into ferrocyanide.
"Insoluble" Prussian blue is produced if in the reactions above an excess of Fe3+ or Fe2+, respectively, is added. In the first case:
4Fe3+ + 3[FeII(CN)6]4- → FeIII[FeIIIFeII(CN)6]3 [13]
"Turnbull's blue"
Ferricyanide ion, used to make 'Turnbull's blue'.
In former times, it was thought that addition of Fe(II) salts to a solution of ferricyanide affords a material different from Prussian blue. The product was traditionally named "Turnbull's Blue" (TB). It has been shown, however, by means of X-ray diffraction and electron diffraction methods, that the structures of PB and TB are identical.[14][15] The differences in the colors for TB and PB reflect subtle differences in the method of precipitation, which strongly affects particle size and impurity content.
Cheers Jazz
