Posted by Nicola on Friday, November 21, 2008 at 1:23pm.
The ppt probably is Fe(OH)3 and that is soluble in HCl.
Remember (Fe^+3)(SCN^-) = Fe(SCN)^+2
and Fe(SCN)^+2/(Fe^+3)(SCN^-)= Kformation so a complex FeSCN^+2 forms when Fe^+3 and SCN^- are high enough. If NaOH is added, then we are raising (OH^-) from the NaOH to a high level so that Ksp for Fe(OH)3 is exceeded and a ppt of Fe(OH)3 forms.
Remember (Fe^+3)(OH^-)^3 = Ksp.
Probably the red colored FeSCN^+3 complex disappears, also, because the pptn of Fe(OH)3 removes so much Fe^+3.
Adding HCl dissolves the ppt because H^+ is large (from the HCl) which reduces the OH^- (because of the
(H^+)(OH^-) = Kw and if the OH^- is reduced enough, Ksp for Fe(OH)3 is no longer exceeded and it dissolves.
Thanks a lot!!!
I think I got it!!!
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