THE RUDOLF REPORT

6. FORMATION AND STABILITY OF IRON BLUEIron Blue. 337 As far as hexacyanoferrate(II) is concerned, quantum efficiencies376 of 0.1 to 0.4 are reported for wave lengths of 365 nm. 377Lately, it has been discussed whether complex cyanides can beremoved from industrial waste waters by ultraviolet radiation. The unboundcyanide will be oxidized and destroyed by hydroxyl radicalsoriginating from the parallelly occurring photolysis of water. 378 However,results are not unequivocal. 379As for Iron Blue, one knows of the bleaching effect under strong,perpetual sun radiation and the ensuing re-darkening during thenight. 380 Here also, the liberation of CN – is responsible, which reducesparts of the Fe 3+ ions to Fe 2+ ions. The latter process, however, willreverse during the night under the influence of oxygen and moisture.The Iron Blue concentration will eventually be reduced by the loss ofthe released CN – , either by evaporation of hydrogen cyanide, by washingout as CN – , or by oxidation through Fe 3+ /atmospheric oxygen orfrom hydroxyl radicals from the natural photolysis of water. The latterprocess is minute and can therefore be omitted. At any rate, most of thecyanide released by photolysis will again be complex bound to iron.6.6.5. Long-Term TestThe best long-term test available to us consists of disinfestationbuildings BW 5a and 5b in Birkenau, which have defied the wind andweather of the strongly corrosive climate in the industrial region ofUpper Silesia for over 50 years, and which are still colored blue, bothinside and out, exhibiting a high cyanide content. These findings arealso supported by two other long-term tests.The color durability of Iron Blue, in addition to other pigments,376 Quantum efficiency is that part of the absorbed light quants which leads to photo reactionsunder scrutiny, here from 10 to 40%.377 L. Moggi, et al., op. cit. (note 337); V. Carassiti, V. Balzani, Ann. Chim. 50 (1960), pp. 782-789.378 Photolysis of water leads to the splitting of water into uncharged parts with unpaired electrons(formation of radicals through homolytic splitting (homolysis); see also dissociation, note339):2 H 2 O + hν H 3 O· + OH·(hν = photo quant)hydroxyl radical379 M.D. Gurol, J.H. Woodman, Hazard. Ind. Waste 21 (1989), pp. 282-290; S.A. Zaidi, J. Carey,in: Proceedings of the Conference on Cyanide and the Environment, Colorado State University,1984, pp. 363-377.380 Deutsche Chemische Gesellschaft (ed.), Gmelins Handbuch, op. cit. (note 335); UllmannsEncyklopädie, op. cit. (note 374); L. Müller-Focken, op. cit. (note 374).177