Well, I'm not sure! The standard reactions involves adding ferric ion in the presence of fluoride ion in order to get the dark-blue color. Here are the steps:
(1) Sodium fusion
high temperature
Organic sample containing C, H, O, N, S, Cl + Na –––––––––––�?gt; NaCN, NaCl, Na2S, NaOH, etc.
(2) Reaction of cyanide with ferrous sulfate (alkaline solution)
2 NaCN (aq) + FeSO4 (aq) ––�?gt; Fe(CN)2 (aq) + Na2SO4 (aq)
4 NaCN (aq) + Fe(CN)2 (aq) ––�?gt; Na4[Fe(CN)6] (aq)
sodium ferrocyanide
(3) Reaction with ferric ion (in acidic solution to avoid precipitation of Fe(OH)3)
Na4[Fe(CN)6] (aq) + FeCl3 (aq) ––�?gt; NaFe[Fe(CN)6] (s) + 3 NaCl (aq)
Prussian blue
To get the dark blue color, you need a mixture of Fe2+ and Fe3+ in the Prussian blue complex commonly formulated as MFe[Fe(CN)6] where M+ is usually Na+ or K+. Here are the reactions of ferrocyanide ion with Fe2+ and Fe3+ ion:
(a) 2 Fe2+ (aq) + Na4[Fe(CN)6] (aq) ––�?gt; Fe2[Fe(CN)6] (s) + 4 Na+ (aq)
sodium ferrocyanide "ferrous ferrocyanide"
(blue-white)
(b) Fe3+ (aq) + Na4[Fe(CN)6] (aq) ––�?gt; NaFe[Fe(CN)6] (s) + 3 Na+ (aq)
sodium ferrocyanide "ferric ferrocyanide"
(dark blue)
which says that the fluoride ion "intensifies" the blue color. It may be that fluoride "ties up" Fe2+ ions more than Fe3+ ions, favoring reaction (b) over reaction (a) above.
Good question!
Steve
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