THE RUDOLF REPORT

GERMAR RUDOLF · THE RUDOLF REPORT6.4. Composition of Iron Blue6.4.1. OverviewThe stoichiometric composition of an ideal Iron Blue crystal is:Fe 4 [Fe(CN) 6 ] 3It is characteristic that the iron in this compound is present in twodifferent oxidation states: Fe 2+ (here in square brackets) and Fe 3+ (hereon the outer left). The interaction between these two different iron ionsalso gives rise to the blue color of this compound (Charge-Transfer-Complex). The actual composition can be quite variable, depending onthe stoichiometry on formation and the presence of impurities, inwhich case the color varies between dark blue and greenish-blue tones.6.4.2. ExcursusIt was with support of the Mösbauer spectroscopy 327 that a longlastingargument could be decided: 328,329 Turnbull’s Blue,Fe 3 [Fe(CN) 6 ] 2 , is actually the same as Berlin Blue, Fe 4 [Fe(CN) 6 ] 3 , evenif the summation formulas suggest they are different. As a matter offact, the summation formula of Berlin Blue is closest to the reality: Inthe ideal Iron Blue crystal, 16 molecules of coordination water are included:Fe 4 [Fe(CN) 6 ] 3 · x H 2 O (x=14 to 16)Today it is known that the ‘soluble’ Iron Blue which frequently isreferred to in older literature, is mainly a substance with the compositionMeFe III [Fe II (CN 6 )] · x H 2 O, where Me is the counter ion to the oppositecyanoferrate, [Fe(CN) 6 ] 3-/4- , mostly potassium (K + ) or ammonium(NH + 4 ).According to Buser, 329 ‘soluble’ Iron Blue is formed mainly duringquick formation and precipitation of the pigments, leading to theinclusion of large amounts of water and potassium or ammonium ions327 Impulseless resonance absorption of γ-quants (gamma radiation) from a radioactive isotope,here Cobalt: 57 Co 57 Fe + γ (main quant: 122 keV; quant used for spectroscopy has a differentenergy)328 E. Fluck, W. Kerler, W. Neuwirth, Z. anorg. allg. Chem. 333 (1964), pp. 235-247; J.F. Duncan,J. Chem. Soc. 1963, pp. 1120-1125.329 H.J. Buser, D. Schwarzenbach, W. Peter, A. Ludi, Inorg. Chem. 16 (1977), pp. 2704-2710.Iron Blue single crystals of high purity and homogeneity were obtained by slow oxidation of asolution of Fe[Fe II (CN) 6 ] in concentrated (!) HCl aq. in air. If in the presence of molar amountsof Kalium only some 2% inclusions were observed.158