THE RUDOLF REPORT

GERMAR RUDOLF · THE RUDOLF REPORTthe water supplied are decisive in determining the actual velocity ofdissolution of a substance. Iron Blue formed in masonry will be presentin a finely crystalline form and adsorbed by capillary effects, inwhich case the former favors dissolution, while the latter is extremelydetrimental to dissolution. Water almost or entirely saturated withiron(III)-ions is no longer capable of dissolving further iron. Furthermore,water permeation through finely porous solid material like masonryis extremely low even at high water tables; the iron saturationconcentration is quickly attained, which, in addition, as remarkedabove, is generated by the slightly more soluble iron oxides of the solidbody rather than by the Iron Blue having once arisen. It is furthermorevery well known that mortar and concrete permeated with paints practicallycannot be rendered colorless. 364 It cannot, therefore, be anticipatedthat the Iron Blue content once having arisen in walls can be perceptiblyreduced by dissolution in water. Water running down the exteriorsurfaces is considerably more aggressive, exerting, in particular, anerosive effect, i.e., damaging the masonry as such.6.6.2.2. ExcursusTananaev et al. 365 examined the solubility of metal hexacyanoferrate(II)and discovered a solubility product 366 of 3 · 10 -41 (pK S =40.5) forthe solubility product of Iron Blue, without mentioning the unit used.Assuming they used the summation formula of Fe 4 [Fe(CN) 6 ] 3(unit being mol 7 l -7 ), one attains a solubility of 0.5 mg per liter water.Thus, it would be 14 times less soluble than the nearly insoluble calciumcarbonate (CaCO 3 , 7.1 mg per liter water, K S = 4.95 · 10 -10 mol 2 l -2 ). 367 Later publications support these findings, 336 although attentionmust be paid to deviations in the stoichiometry (composition) of IronBlue with impurities, leading to an increased solubility.Tananaev et al. precipitated the complex metal cyanoferrate from364 See also, in this regard, the remarks of a company dealing in colored cements and concretes:Davis Colors, 3700 East Olympics Blvd., Los Angeles, CA 90023,www.coloredconcrete.com/davis/Tech/03470.html.365 I.V. Tananaev, M.A. Glushkova, G.B. Seifer, J. Inorg. Chem. USSR, 1 (1956), pp. 72ff.366 The solubility product of a compound is defined as the product of the entire ionic concentrationof the totally dissociated compound: Fe 4 [Fe(CN) 6 ] 3 → 4 Fe 3+ + 3 [Fe(CN) 6 ] 4- ;K L (Fe 4 [Fe(CN) 6 ] 3 ) = c(Fe 3+ )·c(Fe 3+ )·c(Fe 3+ )·c(Fe 3+ )·c([Fe(CN) 6 ] 4- )·c([Fe(CN) 6 ] 4- )·c([Fe(CN) 6 ] 4- )= c 4 (Fe 3+ )·c 3 ([Fe(CN) 6 ] 4- ).The pK S value correlates to the negative decimal logarithm of the product of solubility.367 R.C. Weast (ed.), op. cit., (note 322), p. B 222.172