Ferrous Ferricyanide: Pyrolysis Preparation

Abstract The previously reported synthesis of ferrous ferricyanide involved pyrolysis of Prussian Blue which contained both the soluble KFeFe(CN)6 and the insoluble Fe4 [Fe(CN)6]3 forms. The soluble and insoluble Prussian Blues, and the ferrous ferricyanide compounds which derive from this vacuum pyrolysis at 300–500°C, have been examined by 57Fe Mössbauer spectroscopy. The extra ferric ion present in the cages in insoluble P.B., which is replaced by potassium in soluble P.B., becomes ferrous on pyrolysis and gives a very small quadrupole splitting. Since it absorbs in an otherwise clear region of the spectrum its presence forms an analytical indicator for determining the soluble/insoluble ratio of P.B.'s. The spectra from the high and low spin irons overlap severely and isotopic enrichment was used to clarify the ferricyanide region. Unexpectedly, the isotopic enrichment was gradually lost on pyrolysis and reconstruction. This suggests that the cyanide ligands rotate readily above 300°C. It also suggests that ferrous ferricyanide prepared by pyrolysis is not simply an interchange of valency, but involves intermediate iron sites such as Fe(CN)x(NC)6-x.