THE RUDOLF REPORT

6. FORMATION AND STABILITY OF IRON BLUEThe literature contains authenticated cases of studies with iron atpH values of 9 and 10, in which it is still stable. 360 The pH rangearound 10 to 11 can be considered the critical limit for the stability ofIron Blue. Based on the alkaline behavior of fresh mortar and concrete(in this regard, see also chapter 6.7.2), Iron Blue is only used to paintthese surfaces to a limited extent. 3616.6.2. Solubility6.6.2.1. OverviewIron Blue is considered one of the least soluble cyanide compounds,which is the precondition for its widely-varied application as apigment. 362 The literature flatly refers to Iron Blue as “insoluble”. 363Concrete, reliable values on the solubility of Iron Blue are not recordedin the scientific literature. Based on comparative calculationsbetween the known solubility of Fe(OH) 3 on the one hand, and thelimit value of the pH stability of Iron Blue on the other hand (pH 10),the approximate solubility of Iron Blue in water can, however, be calculated(see chapter 6.6.2.2.). It amounts to approximately 10 -24 g IronBlue per liter of water, this means that 0.000000000000000000000001g Iron Blue dissolve in 1,000 g of water.In addition to a compound’s solubility in water, its condition(crudely or finely crystalline, superficially adherent or adsorbed bycapillary effects) as well as, in particular, the condition and quantity ofthe Fe 3+ is, initially, merely withdrawn; the base-resistant [Fe(CN) 6 ] 4- remains intact; see note344.360 See the studies by M.A. Alich et al., op. cit. (note 338).361 J.A. Sistino, op. cit. (note 332); H. Beakes, Paint Ind. Mag. 69(11) (1954), pp. 33f. Mixturesof Iron Blue and phtalocyanine blue generally find application, since both, alone, lack sufficientlong-term stability; Degussa describes the lime fastness of Iron Blue as “not good” (H.Ferch, H. Schäfer, op. cit. (note 333)); however, Degussa is referring to its fastness on still uncarbonated,alkaline plasters and concretes: H. Winkler, Degussa AG, letter to this author,June 18, 1991. My own experiments with the dissolution of fresh Iron Blue precipitations resultedin a limit value of pH 10-11 for the stability of Iron Blue.362 This property is used in Russian industry, for example, for the passivation of steel pipesagainst aggressive waste waters, since CN – contained in waste waters coats the interior ofpipes with an insoluble protective layer of Iron Blue: N.G. Chen, J. Appl. Chem. USSR,74(1)(1974), pp. 139-142. But it should be noted that this borders on criminal negligence,since toxic cyanides simply do not belong in waste waters.363 DIN Safety Data Sheet VOSSEN-Blau ® , in: Schriftenreihe Pigmente Nr. 50, Degussa AG,Frankfurt 1985; see also H. Ferch, H. Schäfer, op. cit. (note 333). Last but not least, pigments,by definition, are coloring agents practically insoluble in dissolvents and binding agents (DIN55,943 and 55,945).171