THE RUDOLF REPORT

6. FORMATION AND STABILITY OF IRON BLUEwith iron;c) Complexing of trivalent iron (Fe 3+ ) to the complex iron(III)-cyanide,340 that is, the displacement of oxygen and/or OH – ions inrust by cyanide ions;b) Reduction of iron(III)-cyanide to iron(II)-cyanide;e) Precipitation of iron(II)-cyanide with trivalent iron as Iron Blue.The velocity of formation of the pigment can be influenced byvarious factors, which will be considered:1. Water content of the reaction medium;2. Reactivity of the iron;3. Temperature;4. Acid content.6.5.2. Water Content6.5.2.1. OverviewThe formation of cyanide through absorption and subsequent dissociationof hydrogen cyanide in water is the necessary preconditionfor a reaction with iron compounds, since the hydrogen cyanide itselfexhibits only a low reactivity. All reactions listed in chapter 6.5.1. undera)-e) occur almost exclusively in water. Water furthermore ensuresthat the reaction partners—all salts capable of being dissolved in water—cometogether in the first place. Finally, the moisture contained inthe building material also acts as a hydrogen cyanide trap, since hydrogencyanide dissolves eagerly in water. A relatively high water contentin the masonry will therefore considerably increase the speed of reaction.6.5.2.2. ExcursusThe reason for the low reactivity of HCN compared to the freecyanide ion is because HCN is less nulceophilic than the free ion. 341Aside from the dissociation of hydrogen cyanide in water, the process340 Correct: hexacyanoferrate(III).341 nucleophilic (gr.: core/nucleus loving) is the tendency of a particle to react with positivelycharged particles. For this, at least a partial negative charge of the nucleophilic particle is required.In this case, cyanide is, due to its negative charge (CN – ), much more nucleophilic towardsthe positively charge iron (Fe 3+ ) than the formally uncharged (though polar) hydrogencyanide.161