Reduction of Hexacyanoferrate(III) Ion with Sulfite Ion in the Presence of Zinc lont

Abstract

In order to remove [Fe(CN)₆]^3- and [Fe(CN)₆]^4- from waste water, the reduction of [Fe(CN)₆]^3- with sulfite ion has been investigated in the presence of zinc ion which finall y precipitate the iron cyanide complexes as zinc hexacyanoferrate(II).
The reduction of [Fe(CN)₆]^3- was acceleraterl in the presence of zinc ion and the reaction rate was found to be the highest in the vicinity of neutrality. In this reaction, the coexistence of relatively large amounts of alkali metal or alkaline earth metal salts led to a retardation in the reaction rate in the presence of zinc ion. The remarkable effect of zinc ion can be explained by the transfer of electron between sulfite ion and [Fe(CN)₆]^3- in a transition state such as [SO₃-Zn-Fe^III₆]^3- bridged by zinc ion and the resultant formation of⋅SO₃^- radical. In the atmosphere, a pronounced autoxidation of sulfite ion took place simultaneously with the reduction of [Fe(CN)₆]^3-. This phenomenon was considered to support the formation of ⋅SO₃^- radical as an intermediate in the reduction of [Fe(CN)₆]^3-.
For the reduction of [Fe(CN)₆]^3- in the atmosphere, a combined use of sulfite and thiosulfate ions was recommended which brought about a great synergistic effect.