Mos Def
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#1
Report Thread starter 11 years ago
#1
It says on my M/S, if you add NaOH to a green solution containing Fe(II) ions, that it will form a green ppt, is the ppt green because of the solution colour? And are any of you aware of this test, is it well recognised?
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Creative
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#2
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If you add NaOH the hydrated Fe2+ ion it's deprotonated (of 2 water ligands), resulting in a neutral green species which is insoluble.. however when you leave this to stand, aerial oxidation of the Fe2+ to Fe3+ occurs and the precipitate turns red brown.

The ppt is green due to the initial soln colour yes, cuz you've got Fe2+ in both the ppt and the solution, so the same d-d electron transition will occur so the same wavelengths of light will be absorbed from the visible region.
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Mos Def
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#3
Report Thread starter 11 years ago
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(Original post by Creative)
If you add NaOH the hydrated Fe2+ ion it's deprotonated (of 2 water ligands), resulting in a neutral green species which is insoluble.. however when you leave this to stand, aerial oxidation of the Fe2+ to Fe3+ occurs and the precipitate turns red brown.

The ppt is green due to the initial soln colour yes, cuz you've got Fe2+ in both the ppt and the solution, so the same d-d electron transition will occur so the same wavelengths of light will be absorbed from the visible region.
So Fe(H2O)2+ + 2NaOH ==> Fe(H2O)4(OH-)2 + ... ?
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Creative
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#4
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[Fe(H2O)6]2+ + 2OH- --> [Fe(H2O)4(OH)2] + 2H2O
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Mos Def
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#5
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(Original post by Creative)
[Fe(H2O)6]2+ + 2OH- --> [Fe(H2O)4(OH)2] + 2H2O
and that complex ion on the RHS is insoluble you say?
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Creative
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#6
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Yep, it's neutral. 2OH- and Fe2+ results in zero charge, so it won't dissolve in water (not attracted to the partially positive or negative atoms in the H2O mols).
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