THE RUDOLF REPORT

GERMAR RUDOLF · THE RUDOLF REPORTFor the formation of Iron Blue, therefore, a part of this iron mustbe reduced to bivalent form (Fe 2+ ). The subsequent combination ofthese different iron ions with CN – to Iron Blue occurs spontaneouslyand completely. 336 The most probable mechanism 337 is one in whichthe cyanide ion itself acts as a reducing agent. The starting point in sodoing is an Fe 3+ ion, largely surrounded (complexed) by CN – ions:[Fe(CN) 4-6 ] (1-3)- . A slightly alkaline environment is favorable to the finalreduction of the iron(III)-ion to iron(II). 338The pigment formation in the case under consideration here is thenorganized in 5 steps:a) Ad-/absorption of hydrogen cyanide (HCN); 313b) Ionic splitting (electrolytic dissociation) 339 of hydrogen cyanidein water to the cyanide ion, which alone can form complexes336 F. Krleza, M. Avlijas, G. Dokovic, Glap. Hem. Tehnol. Bosne Hercegovine, 23-24 (1977, Vol.Date 1976), pp. 7-13.337 Photolytic decomposition of the [Fe III (CN) 6 ] 3– by means of UV radiation is also conceivable asan alternative. Since the interior walls of the rooms in question are not exposed to any UV radition,this mechanism is ignored here. See also G. Stochel, Z. Stasicka, Polyhedron 4(11)(1985), pp. 1887-1890; T. Ozeki, K. Matsumoto, S. Hikime, Anal. Chem. 56 (14) (1984), pp.2819-2822; L. Moggi, F. Bolletta, V. Balzani, F. Scandola, J. Inorg. Nucl. Chem. 28 (1966),pp. 2589-2598.338 pH value of 9-10 according to M.A. Alich, D.T. Haworth, M.F. Johnson, J. Inorg. Nucl.Chem. 29 (1967), pp. 1637-1642. Spectroscopic studies of the reaction of hexacyanoferrate(III)in water and ethanol. 3.3×10 -4 M Fe(NO 3 ) 3 were exposed with a cyanide excessof likewise 3.3×10 -4 mol l -1 . With pH values of approximately 10, all the Fe 2 [Fe(CN) 6 ]was converted into Iron Blue within 48 hours. Cyanate, the anticipated product of the oxidationof the CN – , could not, however, be proven. Perhaps this is further oxidized directly intoCO 2 . If this mechanism is assumed, the result, purely stoichiometrically, is that an alkalineenvironment must be favorable. This finding is supported by the known fact that hexacyanoferrate(III)is a strong oxidation agent in alkaline medium and is even able to oxidize trivalentchrome to hexavalent, therefore, that is, CN – ions must have oxidized very quickly: J.C.Bailar, Comprehensive Inorganic Chemistry, Vol. 3, Pergamon Press, Oxford 1973, p. 1047.An overly alkaline environment would, however, disturb the complexing of the Fe 3+ - ion bycyanide, which is then displaced by OH – (Fe(OH) 3 then occurs as a by-product) and/or the lattercan hardly be displaced from the iron.The driving force in the reduction of the Fe 3+ is the considerably more favorable energeticalsituation of the hexacyanoferrate(II) as compared to hexacyanoferrate(III); see, in this regard,R.M. Izatt, G.D. Watt, C.H. Bartholomew, J.J. Christensen, Inorg. Chem. 9 (1970), pp.2019ff. Calorimetric measurements relating to the formation enthalpies of Iron Blue from respectiveeducts (in brackets) were as follows:∆H(Fe 2+ + [Fe(CN) 6 ] 3- )= -66.128 kJ mol -1 ; ∆H(Fe 3+ + [Fe(CN) 6 ] 4- )= 2.197 kJ mol -1 .For this reason, a direct reduction of uncomplexed Fe 3+ , i.e., not surrounded by cyanide, hasan energy disadvantage and is therefore negligible.339 Dissociation: is the splitting of a compound, in this case into two differently charged ions(heterolytic) in aqueous medium (electrolysis):HCN + H 2 O CN – + H 3 O +160